Sound and the Ear Chapter 2

Chapter© Jones & Bartlett 2 Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Sound and the Ear

© Jones Karen & J. Kushla,Bartlett ScD, Learning, CCC-A, FAAA LLC © Jones & Bartlett Learning, LLC Lecturer NOT School FOR of SALE Communication OR DISTRIBUTION Disorders and Deafness NOT FOR SALE OR DISTRIBUTION Kean University

© Jones & Bartlett Key Learning, Terms LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR Acceleration DISTRIBUTION Incus NOT FOR SALE OR Saccule DISTRIBUTION Acoustics Inertia Scala media Auditory labyrinth Inner hair cells Scala tympani Basilar membrane Linear scale Scala vestibuli Bel Logarithmic scale Semicircular canals Boyle’s law Malleus Sensorineural hearing loss Broca’s area © Jones & Bartlett Mass Learning, LLC Simple harmonic© Jones motion (SHM) & Bartlett Learning, LLC Brownian motion Membranous labyrinth Sound Cochlea NOT FOR SALE OR Mixed DISTRIBUTION hearing loss Stapedius muscleNOT FOR SALE OR DISTRIBUTION Compression Organ of Corti Stapes Condensation Osseous labyrinth Tectorial membrane Conductive hearing loss Ossicular chain Tensor tympani muscle Decibel (dB) Ossicles Tonotopic organization © Jones Decibel & hearing Bartlett level (dB Learning, HL) LLC Outer ear © Jones Transducer & Bartlett Learning, LLC Decibel sensation level (dB SL) Outer hair cells Traveling wave theory NOT Decibel FOR sound SALE pressure OR level DISTRIBUTION (dB SPL) Oval window NOT TympanicFOR SALE membrane OR DISTRIBUTION Displacement Pars flaccida Uniform circular motion Elastic Pars tensa Utricle Endocochlear electrical potential Pascal (Pa) Vector Endolymph Perilymph Vestibular labyrinth Equilibrium Pinna Vestibular membrane © Jones & Bartlett Eustachian Learning, tube LLC Pressure wave© Jones & Bartlett Vestibule Learning, LLC NOT FOR SALE OR External DISTRIBUTION auditory meatus Propagation NOT FOR SALE OR Wavelength DISTRIBUTION Force Pure tone Wernicke’s area Helicotrema Rarefaction Impedance-matching transformer Round window

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC ObjectivesNOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION • Describe the characteristics of sound. • Define the concept of simple harmonic motion and its relationship to periodic sounds. • Summarize the physical characteristics of sound. © Jones• Identify the anatomy of the auditory system and trace the transmission of sound throughout. & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT• FOR Differentiate the types of hearing loss an abnormality in the auditory system can cause. SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

Chapter opener image: © Filip Fuxa/Shutterstock © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 9781284132793_CH02_015_036.indd 15 30/06/17 10:20 am © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

16 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC IntroductionNOT FOR SALE OR DISTRIBUTIONwith the medium of transmissionNOT we FORcall air. SALE Air OR DISTRIBUTION molecules are not static; in fact, they are moving For the speech-language pathologist to work within constantly in random fashion. This random move- his or her scope of practice with individuals with ment at high speeds is called Brownian motion, hearing loss, interpret audiograms, and screen for named for Robert Brown (1773–1858), a Scottish auditory disorders, one must have a firm under- © Jones & Bartlett Learning, LLC botanist who ©described Jones this & motion,Bartlett which Learning, results LLC standing of what, how, and why we hear. The inten- from the impact of molecules found within a gas NOTtion FOR of this SALE chapter ORis to provideDISTRIBUTION an overview of the NOT FOR SALE OR DISTRIBUTION or liquid. Brownian motion causes these air mol- characteristics of sound, sound transmission, and ecules to collide with each other and with whatever the path sound takes as it is transmitted through the is in their path—walls, furniture, or people. These auditory system. molecules are elastic—that is, the objects exhibit a As a supplement to exhaustive coursework © Jones & Bartlett Learning, LLC © Jonestendency & to Bartlett resist deformity Learning, and return LLC to their rest required by the American Speech-Language- NOT FOR SALE OR DISTRIBUTION NOTposition—so FOR SALE there isOR no changeDISTRIBUTION in their shape when Hearing Association, the intention of the following they bump into each other and/or other objects. information is to provide the reader with a sum- mary of acoustics and anatomy and physiology of These collisions produce pressure. Although we the auditory system to reference within this text, may not be able to feel that pressure, it is there. You rather than to ©take Jones the place & of Bartlett that coursework. Learning,feel LLC this pressure whenever air ©is Jonesset into motion, & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTIONsuch as on a windy day or when NOTwe speak. FOR SALE OR DISTRIBUTION A source of energy, such as a force, is the next General Characteristics prerequisite. Force is a push or a pull on an object, of Sound and is a vector that has both magnitude (some Sound is all around us, although it may be too faint amount greater than zero) and direction. Force is © Jonesfor us to hear& Bartlett or too intense Learning, for us to listen LLC to for any mathematically© determinedJones & Bartlettto be the productLearning, of LLC NOTlength FOR of time. SALE In the OR 1700s, DISTRIBUTION the British philosopher mass times accelerationNOT FOR (F SALE= ma). ORAir moleculesDISTRIBUTION George Berkeley asked the question, “If a tree falls have mass (the quantity of matter present). Mass is in the forest and no one is around to hear it, does it not identical to weight because weight is affected by make a sound?” Of course it does—unless it falls on gravitational forces; however, for our purposes, mass another planet with little to no gaseous atmosphere, and weight are the same. Because air molecules have © Jones & Bartlettin which Learning, case there LLCis no sound. © Jonesmass, they & obeyBartlett laws of Learning, motion set forth LLC by the great NOT FOR SALE ORThe DISTRIBUTION study of sound is a branch of physics calledNOT English FOR scientist SALE Sir OR Isaac DISTRIBUTION Newton (1643–1727), the acoustics. Sound itself is a physical phenomenon first of which states that all bodies remain at rest that is described as the movement or propagation or in a state of uniform motion unless other forces of a disturbance (i.e., a vibration) through an elas- act in opposition. (This property is called inertia.) tic medium (e.g.,© airJones molecules) & Bartlett without permanent Learning, The LLC amount of inertia an object© (e.g., Jones an air & mol Bartlett- Learning, LLC ecule) has is directly proportional to its mass: The displacement ofNOT the particles. FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION There are three prerequisites for production of greater an object’s mass, the greater its inertia. An sound: (1) a source of energy (e.g., a force), (2) a vibrat- outside force must be applied to change this ten- ing object that generates an audible pressure wave, dency. Acceleration is the speed (distance traveled and (3) a medium of transmission (e.g., air). However, per unit time) of an object per unit time, which is represented mathematically as length . When a force © Jonesa receiver & of Bartlett these prerequisites Learning, of sound LLC production © Jones & Bartlett(time)2 Learning, LLC NOTis optional; FOR SALE that is, a ORlistener DISTRIBUTION is not required. is applied to theNOT air particles FOR SALE by a moving OR object, DISTRIBUTION the As human beings, we produce sound primarily in air particles will travel in the direction of the force. air, so let’s begin our discussion of the prerequisites The amount of this distance is proportional to the

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General Characteristics of Sound 17

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC magnitude ofNOT the appliedFOR SALEforce—a OR large DISTRIBUTION force will Elasticity in the tuning NOTfork tine FOR allows SALE for OR DISTRIBUTION cause the object to travel much further than a small this displacement, but also generates a restor- force. Therefore, the greater the force applied to the ing force that momentarily stops the movement object, the greater the distance the object travels by at the point of maximum amplitude away from that force; in addition, the restoring force is pro- the rest position. The restoring force pushes the © Jonesportional & Bartlettto the displacement Learning, (i.e., theLLC object obeys tuning fork© tines Jones back &to Bartletttheir rest position,Learning, but LLC NOTHooke’s FOR law,SALE named OR for DISTRIBUTION Robert Hooke [1635–1703], inertia carriesNOT the FORtines pastSALE the restOR position. DISTRIBUTION By an English experimental philosopher who first overshooting the rest position, the tines then are described this action). pushed toward the opposite maximal position, at Finally, we need an object that is capable of which point the restoring force builds up in the vibrating. Air molecules happen to vibrate quite other direction and the tuning fork tines return © Jones & Bartlettwell, Learning, and can be setLLC into vibration easily to produce© Jones to the & rest Bartlett position Learning, once again. TheLLC tuning fork NOT FOR SALE ORa pressure DISTRIBUTION wave. For example, if we strike a tuningNOT tinesFOR overshoot SALE ORthe DISTRIBUTIONrest position, the restoring fork on a hard surface to set its tines into vibration, force builds up again, and the pattern repeats; this the air molecules surrounding the tuning fork tines alternating pattern of inertia and elasticity creates are also set into vibration, creating this pressure one full cycle of vibration. As you can see, inertia wave. This initial© Jones impact &starts Bartlett movement Learning, of the air LLCand the restoring forces vary© continuouslyJones & Bartlett dur- Learning, LLC molecules (displacementNOT FOR) SALEin the same OR direction DISTRIBUTION of ing each cycle: Inertia is strongNOT when FOR the SALErestor- OR DISTRIBUTION the force. This pressure wave displaces air molecules ing force is weak, and vice versa. This interplay near the tuning fork tines; these displaced air mol- between the two forces enables the vibration to ecules further displace other air molecules adjacent persist until other external forces (for example, to the pressure wave, which displace adjacent air friction, which causes a gradual decay in vibratory © Jonesmolecules, & Bartlett and so on. Learning, Therefore, the LLC wave motion amplitude) ©overcome Jones the & Bartletttuning fork Learning, tines’ mass LLC NOTis FOR propagated, SALE or ORtransferred, DISTRIBUTION through the air to the and elasticityNOT and FORthe energy SALE dissipates. OR DISTRIBUTION Although human ear. the air molecules are displaced from their rest When the air molecules reach the maximum position at various points throughout the cycles point of displacement, their motion is momentar- of vibration, they continue to vibrate at the same ily halted because of inertia (i.e., air molecules fol- frequency as the tuning fork. © Jones & Bartlettlow Learning, Newton’s first LLC law of motion, which states that© Jones As &air Bartlett molecules Learning,vibrate, waves LLCof pressure fluc- NOT FOR SALE ORobjects DISTRIBUTION at rest will remain at rest unless acted uponNOT tuationsFOR SALE are created OR and DISTRIBUTION travel through the elastic by a force). Once the force is removed, the restoring medium. (However, the molecules themselves move force of elasticity returns the displaced air molecule only a short distance.) As this vibratory disturbance to a resting state called equilibrium. When the air (and not the air molecules themselves) propagates through the air, the atmosphere goes through alter- molecules return© Jones to their & resting Bartlett state, Learning,the void left LLC © Jones & Bartlett Learning, LLC by their former positions is filled by the adjacent nating periods of increased and decreased air par- air molecules,NOT which FOR then SALE displace OR the DISTRIBUTION adjacent ticle density and, consequently,NOT of FORhigh and SALE low OR DISTRIBUTION air molecules in the opposite direction (i.e., the air pressure. Because air molecules can flow easily, they molecules follow Newton’s third law, which states flow from regions of higher pressure to regions of that for any action there is an equal and opposite lower pressure. The density (concentration) of these © Jonesreaction). & Bartlett The elastic Learning, medium is not LLC displaced over air particles© alternately Jones increases& Bartlett and decreases Learning, rela- LLC NOTan FOR appreciable SALE distance; OR DISTRIBUTION rather, the air molecules tive to theirNOT conditions FOR at SALErest (i.e., ORwhen DISTRIBUTION there is no vibrate to and fro about their average equilibrium vibration and the molecules are in equilibrium). For positions away from the source of energy. a fixed volume of vibrating air molecules, increased

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18 Chapter 2 Sound and the Ear

Pressure (P2)

Wavelength

Distance © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Figure 2.2 Wavelength. NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Volume (V1) Volume (V2)

Figure 2.1 Boyle’s Law illustrated. length (e.g., meters) and is represented by the Greek letter lambda (λ). © Jones & Bartlett Learning, LLCSound in air moves in the ©same Jones (or opposite) & Bartlett Learning, LLC concentration NOT(density) FOR of airSALE particles OR results DISTRIBUTION in direction of the force; in other words,NOT thisFOR pressure SALE OR DISTRIBUTION increased air pressure; this is called Boyle’s law wave moves longitudinally. In a longitudinal wave, (after Robert Boyle [1627–1691], British physicist air molecules approach and recede from each other and chemist), which states that the pressure and to create variations in pressure so that the wave move- volume of a gas are inversely proportional if kept at ment is parallel to the force. The air molecules do not © Jonesa constant & temperature Bartlett Learning,(see Figure 2.1 LLC). move far from© their Jones rest positions; & Bartlett instead, Learning, they move LLC NOT FORBecause SALE the initial OR force DISTRIBUTION is a vector, it causes an a short distanceNOT in either FOR direction SALE from OR rest, DISTRIBUTION but do outward movement of the tuning fork tines toward not move forward with the wave itself. To demonstrate a positive displacement, which causes the surround- longitudinal waves at home, have a friend hold one ing air molecules to be crowded together. The force end of a Slinky (the metal ones work best) while you of displacement is passed from molecule to mole- hold the other end. Pinch a few of the coils together © Jones & Bartlettcule; Learning,this displacement LLC creates areas of increased© Jonesand then & release Bartlett them. TheLearning, energy released LLC will travel NOT FOR SALEpressure OR DISTRIBUTION and density of air molecules that are calledNOT down FOR the SlinkySALE toward OR the DISTRIBUTION other end and then return condensation (also known as compression). When to you until the energy is overcome by friction and the tines return toward equilibrium because of dies. In a transverse wave, on the other hand, the air elasticity, the force on the surrounding medium is molecules vibrate at right angles to the direction of relieved, and the© Jonesair molecules & Bartlett also return Learning, toward wave LLC propagation. To demonstrate© transverseJones &waves, Bartlett Learning, LLC their position of equilibrium. This “thinning” of air NOT FOR SALE OR DISTRIBUTIONfill a deep, wide bowl with water,NOT and place FOR a feather SALE OR DISTRIBUTION molecules creates areas of decreased air pressure (or float a cork) on the water surface. (The fluid ten- and density (rarefaction). The distance between sion will keep the feather or the cork floating on the two successive condensations (i.e., from a point water’s surface because the water has greater density on one wave to the same point on the next cycle than either the feather or the cork.) Drop a small © Jonesof the wave) & Bartlett is called theLearning, wavelength LLC of the sound object (e.g., a ©pebble Jones or a penny)& Bartlett into the Learning, bowl; the LLC NOTwave. FOR The SALEwavelength OR represents DISTRIBUTION the length of the feather or corkNOT will bob FOR up andSALE down, OR but DISTRIBUTIONnot move disturbance created by the wave in a medium (see very far from where it is floating. This movement is Figure 2.2 ). Wavelength is measured in units of perpendicular to the direction of wave propagation.

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General Characteristics of Sound 19

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Simple HarmonicNOT FOR MotionSALE OR DISTRIBUTIONperiodic. A period (p) is a physicalNOT FOR characteristic SALE OR DISTRIBUTION and Sound that describes the amount of time it takes to complete one full cycle of vibration, and is measured in In acoustics, when air particles are set into motion units of time (usually seconds [s] or milliseconds by a force to produce changes in pressure, areas [ms]). Frequency (f) is the inverse of the pure tone’s © Jonesof condensation & Bartlett and Learning,rarefaction alternate. LLC If these period and is© a Jonesphysical characteristic& Bartlett thatLearning, describes LLC areas of alternating condensation and rarefaction NOT FOR SALE OR DISTRIBUTION the numberNOT of complete FOR SALEcycles ofOR vibration DISTRIBUTION that occur at a steady rate of change, the resultant pres- occur per unit time ( Figure 2.4 ). Frequency is sure wave is said to be a pure tone (e.g., those little measured in units called Hertz (Hz), in honor of beeps you hear during a hearing test), which moves Heinrich Hertz (1857–1894), a German physicist in simple harmonic motion (SHM) and is repre- who contributed to the field of electromagnetism © Jones & Bartlettsented Learning, graphically LLC by a sine wave. Although pure© Jones through & hisBartlett description Learning, of wave movement.LLC Only NOT FOR SALE ORtones DISTRIBUTION rarely occur in nature, they result when soundNOT oneFOR frequency SALE isOR described DISTRIBUTION in a pure tone (e.g., waves are propagated through an elastic medium 1000 Hz). and complete the same number of complete cycles Pitch and frequency are not synonymous. of vibration per unit time. Examples of pure tones Because frequency is a physical characteristic, it include tuning forks and pendulums, both of depends on the mass of the vibrating object, its which produce© Jones vibrations & thatBartlett move inLearning, SHM (see LLCoverall size, and so on; in general,© Jones the larger& Bartlett the Learning, LLC Figure 2.3 ).NOT FOR SALE OR DISTRIBUTIONvibrating object, the more slowlyNOT that FOR object SALE will OR DISTRIBUTION Characteristics of SHM vibrate. Pitch, on the other hand, is a percept (a psychological correlate) and is related to the listener’s The basic attributes of a sound wave—period, perceptual response to frequency. We might also © Jonesfrequency, & Bartlett amplitude, Learning, and phase—are LLC explained think of pitch© Jonesas a relative & Bartlettterm; that Learning,is, if you ask LLC through SHM. When pure tones move in SHM, NOT FOR SALE OR DISTRIBUTION whether a certainNOT soundFOR isSALE high pitch OR or DISTRIBUTION low pitch, they take the same amount of time to complete each the question that would arise is: Higher or lower cycle of vibration. In other words, pure tones are than what? Pitch is measured in Mels. The Mel

S © Jones & Bartlett Learning, LLC Rigid support © Jones & Bartlett Learning,Rigid support LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

L Long thread © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Bob (metal ball) C B Extreme position Extreme position A A Mean position © Jones & BartlettSimple pendulum Learning, LLC © Jones & Bartlett Learning, LLC or NOT FOR SALE OR DISTRIBUTION NOTCenter FOR position SALE OR DISTRIBUTION Figure 2.3 Example of simple harmonic motion.

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20 Chapter 2 Sound and the Ear

Low-frequency wave © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Period NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Figure 2.4 High- and low-frequency waves. The waveform at the top has twice as many cycles and its period is half as long as the wave at the bottom; therefore, the upper wave is the octave of the bottom wave. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION scale is a psycho-physical scale of pitch perception; 5000 1000 Mels is the pitch equal to a 1000-Hz tone at 4500 a specific intensity. Figure 2.5 shows the relation- 4000 ship between pitch and frequency. 3500 © Jones & Bartlett Learning, LLC © Jones3000 & Bartlett Learning, LLC As a sound wave travels through an elastic 2500 NOT FOR SALEmedium OR DISTRIBUTION like air, we can calculate how far it travNOT- FOR2000 SALE OR DISTRIBUTION

els through one complete cycle of vibration. This Pitch (Mels) 1500 is called the wavelength (λ), and is measured in 1000 500 units of length (e.g., meters). We can also deter- 0 mine the speed© (velocity) Jones of& theBartlett sound waveLearning, if we LLC 0.5 11.5 2 2.5 3© 3.5Jones 4 4.5 & 5 Bartlett Learning, LLC know how far NOTit travels FOR per unitSALE time. OR The DISTRIBUTIONvelocity FrequencyNOT (cps) FOR SALE OR DISTRIBUTION of air at standard room temperature and pressure (20 degrees Celsius at sea level) is approximately Figure 2.5 This graph shows the relationship 344 m/s. How fast the sound wave moves depends between frequency (x-axis, in units of cycles on the density and elastic properties of the medium per second [cps]) and pitch (y-axis, in units of © Jonesthrough &which Bartlett it is moving, Learning, and is LLC independent Mels). At lower© Jonesfrequencies, & Bartlett frequency Learning, (dashed LLC NOTof pressureFOR SALE as long OR as air DISTRIBUTION temperature is constant. line) and pitchNOT (solid FOR line) SALEhave nearly OR aDISTRIBUTION 1:1 (In gases like air, temperature plays an important relationship, but at higher frequencies, pitch part in how fast sound travels. Sounds travel faster differs from frequency.

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General Characteristics of Sound 21

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC through liquidsNOT and FOR fastest SALE along solids OR becauseDISTRIBUTION the around the circumference of thatNOT circle FOR at a constant SALE OR DISTRIBUTION greater elasticity and density of these media increase rate, we could describe that movement as projected the velocity of conduction.) Therefore, a faint sound uniform circular motion. The air molecule’s dis- travels at the same velocity as a loud sound. We placement along that circumference varies with the can calculate the wavelength l of a 1000-Hz sound passage of time in the same way during a cycle of © Joneswave very & Bartlett easily if we Learning, know the velocity LLC of sound; movement if© the Jones frequency & Bartlett of the sine Learning,wave is con- LLC NOTbecause FOR velocitySALE dividedOR DISTRIBUTION by frequency equals wave- stant. This NOTbrings FORus to ourSALE last ORcharacteristic DISTRIBUTION of 344 m/s SHM: phase. Phase is that portion of a cycle that has length, 1000 cycles/s = 0.344 m (approximately 1 foot). (Note: Do not confuse the velocity of sound wave elapsed at any instant in time, relative to some arbi- propagation with the velocity of particle movement; trary starting point—that is, the relative timing of particles vibrating in SHM constantly change veloc- compressions and rarefactions of an object moving © Jones & Bartlettity, Learning,moving with LLCmaximum velocity over their rest© Jones in SHM. & BecauseBartlett of thisLearning, relationship LLC between SHM NOT FOR SALE ORposition.) DISTRIBUTION NOT andFOR projected SALE uniform OR DISTRIBUTION circular motion, phase is Amplitude (A) is another vector quantity that measured in degrees (from 0° to 360°). Figure 2.6 describes both magnitude and direction of wave and Figure 2.7 depict the relationship between displacement from rest. Amplitude is a derived unit these concepts. of measurement that describes the distance from Why is phase important? If two sound waves © Jones & Bartlett Learning, LLCof the exact same frequency ©are Jones exactly &in Bartlettphase, Learning, LLC an object’s restNOT position FOR by SALE a vibrating OR body DISTRIBUTION or the NOT FOR SALE OR DISTRIBUTION magnitude of pressure change that occurs by that their amplitudes add together and result in a dou- object’s motion. The greater the distance caused by bling of intensity; if these sound waves are slightly vibration is from the point of rest, the greater the out of phase, their amplitudes add together, but the amplitude. In general, the greater the amplitude, the resultant intensity ranges from not quite doubled © Joneslouder & the Bartlett pure tone soundsLearning, to a listener. LLC Amplitude to almost zero.© Jones If two sound & Bartlett waves are Learning, exactly out LLC NOTcan FOR be describedSALE OR by both DISTRIBUTION physical parameters and of phase, theirNOT amplitudes FOR SALE add together OR DISTRIBUTION to cancel; psychophysical percepts. Loudness is the percept of no sound is produced because there is no change intensity and depends on how our inner ears (spe- in sound pressure. This is how noise-cancellation cifically, the cochlea) interpret how much sound headphones work: A sound wave exactly opposite pressure is presented over our tympanic membranes in phase from the generating wave is produced so that their amplitudes cancel. © Jones & Bartlett(eardrums). Learning, The humanLLC ear happens to be very sen© -Jones & Bartlett Learning, LLC NOT FOR SALE ORsitive DISTRIBUTION to changes in sound pressure, so small changesNOT TheFOR Decibel: SALE OR Measure DISTRIBUTION of Relative in pressure (i.e., intensity) will result in either an Intensity increase or a decrease in loudness sensation. Inten- sity is a derived unit of measurement that describes What Is a Decibel? the amount ©of Jonesacoustic &energy Bartlett (i.e., Learning,sound) that LLCEarlier we noted that intensity© is Jones the physical & Bartlett mea- Learning, LLC passes through a unit of area in a given time span. A NOT FOR SALE OR DISTRIBUTIONsure of what we perceive as theNOT loudness FOR of a SALE sound. OR DISTRIBUTION pure tone’s intensity is measured by the amplitude of A sound’s intensity is measured in acoustic (sound) its sine wave, and varies with time. The human ear is pressure. Pressure is created when a force is distrib- capable of hearing a wide range of sound intensities. uted over an area; mathematically, pressure = force. area SHM is usually depicted as a sine wave, with (Force is the product of mass and acceleration; its © Jonespeaks &(i.e., Bartlett compressions) Learning, and troughs LLC (i.e., rar- unit of measurement© Jones is &the Bartlett dyne.) When Learning, we mea- LLC NOTefactions). FOR SALE If we ORwere DISTRIBUTIONto cut that sine wave in half sure sound NOTintensity, FOR we areSALE measuring OR DISTRIBUTIONthe force of and move the trough directly beneath the peak, we that sound wave’s vibration over a given unit of area: would form a circle. If an air molecule were to move The greater the change in air pressure, the greater

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22 Chapter 2 Sound and the Ear

0.375 sec/135° 0.125 sec/45°

Figure 2.6 Relationship among simple harmonic motion, © Jonesprojected & Bartlett uniform circular Learning, motion, LLC phase, and degrees in a © Jones & Bartlett Learning, LLC NOTsine FOR wa ve.SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

the intensity of sound. The unit of measurement When we describe sound pressure using Pa, we that describes sound pressure is the Pascal (Pa), are using what is called a linear (or integral) mea- named in honor of Blaise Pascal (1623–1662), suring scale (also known as an absolute scale)— © Jonesa French & mathematician; Bartlett Learning, 1 Pa = 10LLC dynes/cm2. there is a true© zero Jones point, & each Bartlett increment Learning, on the LLC NOTNor FORmal human-hearing SALE OR DISTRIBUTION sensitivity ranges from scale is equal toNOT every FOR other increment,SALE OR and DISTRIBUTION you can 0.0002 dynes/cm2 to 2000 dynes/cm2. Although it sum incremental units by addition. An example of is possible to measure sound pressure in units of a linear scale is a ruler like a yardstick; you cannot dynes/cm2, it would force us to use very large num- have a negative distance, and each increment (e.g., bers to describe a person’s hearing sensitivity (e.g., 1 inch) is equivalent. A better measurement scale © Jones & Bartlettan intensity Learning, range LLCof 10,000,000,000,000 between© Jonesto use for & intensity, Bartlett however, Learning, is a logarithmic LLC (ratio) NOT FOR SALEsoftest OR DISTRIBUTIONsounds and threshold of pain). NOTscale. FOR A logarithmic SALE OR scale DISTRIBUTION is a relative scale where

B = 90 B = 90°

Ring Displacement

D = 270 D = 270° © Jones & Bartlett Learning, LLC © JonesTime & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Figure 2.7 Relationship among simple harmonic motion, phase, and projected uniform circular motion.

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General Characteristics of Sound 23

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Linear scale NOT FOR SALE OR DISTRIBUTIONscale of the Bel has been compressedNOT FOR so much SALE that OR DISTRIBUTION 0100 200 300 400 500 600 700 800 900 1000 fractions must be used to reflect the desired accu- racy of measurement of intensity (e.g., an intensity of 4.5 Bels). To minimize the use of fractions and Logarithmic scale decimals, we can use a smaller unit of measure- © Jones01 & Bartlett10 Learning,100 000 LLC ment, the decibel© Jones (literally, & Bartlettone-tenth ofLearning, a Bel). The LLC NOT FOR SALE OR DISTRIBUTION decibel (dB)NOT is a muchFOR more SALE user-friendly OR DISTRIBUTION unit of Figure 2.8 Relationship between linear and measurement of intensity, and the range of human logarithmic scales. hearing on the decibel scale becomes whole numbers that range between 0 dB and 140 dB. The decibel expresses a logarithmic ratio there is no zero point (you must define what zero © Jones & Bartlett Learning, LLC © Jonesbetween & Bartlettthe measured Learning, sound pressure LLC and a rela- is), the zero point does not represent the absence of NOT FOR SALE OR DISTRIBUTION NOT tiveFOR sound SALE pressure OR (defined DISTRIBUTION at 0.0002 dynes/cm2, what is being measured, and each successive unit which happens to be the softest sound a person with is larger than the one preceding it; therefore, each increment is not equal and represents increasingly normal hearing sensitivity can hear). In its sim- large numerical differences. A logarithmic scale plest form, a logarithm is the same as an exponent, compresses the© Jonespotentially & veryBartlett large numbers Learning, used LLCwhich indicates how many times© Jones a number & is Bartlett mul- Learning, LLC × in a linear scaleNOT into FOR much SALE more manageable OR DISTRIBUTION incre- tiplied by itself. Take the equationNOT 10FOR 10 SALE = 100; OR DISTRIBUTION ments to use. See Figure 2.8 , which illustrates the the number 10 is multiplied by itself. We can also 2 incremental differences between linear and loga- express this equation as 10 = 100; in this case, the rithmic scales. number 10 is the base and the number 2 is the expo- Why do we use a logarithmic scale for inten- nent. If we wanted to express the second equation © Jonessity? It & has Bartlett been known Learning, since the 19th LLC century that logarithmically,© Jones we can &also Bartlett say log10 100Learning, = 2. The LLC NOTthe FOR logarithmic SALE ORscale DISTRIBUTIONcorresponds nicely to how number 10 NOTis still theFOR base, SALE the number OR 2DISTRIBUTION is still the intensity differences are perceived in the human exponent, and the number 100 is still the product of ear. Equal increases in sensation (in this case, loud- the multiplication of 10 × 10, but we just rearranged ness) are obtained by multiplying the stimulus by a how we expressed the multiplication problem using constant factor. Although this doesn’t work for all logarithms. To multiply logarithms with the same © Jones & Bartlettintensities Learning, to which LLC the ear is sensitive, it is accurate© Jones base number,& Bartlett you add Learning, their exponents; LLC to divide NOT FOR SALE ORenough DISTRIBUTION to be practical. NOT logarithmsFOR SALE with theOR same DISTRIBUTION base number, you subtract The unit of measurement used to describe their exponents. human intensity differences is the Bel (named in We also use decibels to denote intensity for honor of Alexander Graham Bell [1847–1922], the another reason: We can describe intensity either in Scottish-American© Jones inventor & Bartlett and teacher Learning, of oral LLCunits of power (used in acoustics)© Jones or sound & pressure Bartlett Learning, LLC speech to individuals who are deaf). The Bel is a (decibel sound pressure level [dB SPL], used in NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION relative measurement of intensity that expresses the measurement of hearing sensitivity). (Because the ratio of a measured sound intensity to a relative we are primarily interested in changes in sound sound intensity. In other words, this very large range pressure—e.g., running speech—this discussion of human hearing (on the order of 1014 dynes/cm2) will be limited to audiometric applications.) In audi- © Jonesis compressed & Bartlett so that Learning, smaller numbers LLC are used. By ology, intensity© Jones level refers & Bartlettto the changes Learning, in sound LLC NOTusing FOR the SALE Bel we bringOR DISTRIBUTIONthe range of intensities heard pressure level,NOT as measuredFOR SALE in dynes/cm OR DISTRIBUTION2. Because from 1014 units to a range of 0 to 14. However, this decibels are based on relative differences in intensi- is so far to the other extreme that it is absurd! The ties, a reference value (standard) must be provided,

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24 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC which is the thresholdNOT FOR of human SALE hearing OR (equalDISTRIBUTION to between loudness and intensityNOT is not FOR linear. SALE At OR DISTRIBUTION 0.0002 dynes/cm2 in units of sound pressure). We a given intensity, loudness perception varies with can calculate sound intensity in decibels using the sound frequency because the human auditory following formula: system is designed to receive the middle frequencies with much less intensity than is needed for dB SPL = 20 log (P /P ) © Jones & Bartlett Learning,10 o r LLC extremely high© andJones low frequencies.& Bartlett Just Learning, as fre- LLC where P = measured sound pressure and P = NOT FORo SALE OR DISTRIBUTION r quency has a perceptualNOT FOR correlate SALE (pitch), OR DISTRIBUTIONintensity recognized reference point (0.0002 dynes/cm2). also has perceptual correlates: the phon (a unit of To illustrate how we use this equation, let’s say equal loudness) and the sone (an arbitrary unit

that our measured sound pressure (Po) is equivalent of loudness). The phon level roughly matches 2 to our reference pressure (Pr = 0.0002 dynes/cm ). intensity (in dB SPL) at a frequency of 1000 Hz. © Jones & BartlettWe can Learning, then substitute LLC these values into the equa©- JonesFrequencies & Bartlett in the range Learning, of 1000 Hz LLC to 6000 Hz NOT FOR SALEtion OR to DISTRIBUTIONget: NOTare FOR detected SALE at the OR lowest DISTRIBUTION sound pressure levels, whereas very low and very high frequencies require dB SPL = 20 log (0.0002 dynes/cm2 10 greater sound pressure levels to pass the threshold ÷ 0.0002 dynes/cm2) = 20 log (1) = 0 10 of hearing. Figure 2.9 shows how equal loudness To what power do we raise 10 to equal 1? The changes over a range of frequencies—that is, the answer is zero (0)© Jonesbecause 10 &0 =Bartlett 1, and anything Learning, mul- minimum LLC audibility needed at© eachJones frequency. & Bartlett Learning, LLC tiplied by 0 is equalNOT to FOR0. Therefore, SALE a sound OR DISTRIBUTIONstimulus (However, lower frequencies spanNOT the FOR range SALE of OR DISTRIBUTION that is minimally audible has an intensity of 0 dB SPL. Table 2.1 As you can see from , a tenfold increase Loudness level (phons)

in sound pressure (a linear measure) yields a 20-dB )

2 130 increase in intensity (a logarithmic measure). 120 © Jones & Bartlett Learning, LLC 120 © Jones & Bartlett Learning, LLC

N/m 110 110 NOTIntensity FOR SALE versus OR Loudness DISTRIBUTION NOT FOR SALE100 OR DISTRIBUTION 100 90 2 × 10 Intensity, like frequency, is a physical property of 90 80 an acoustic signal. The loudness—the subjective, 80 70 psychological sensation of intensity—of a signal is 70 60 60 related to its intensity; however, this relationship 50 © Jones & Bartlett Learning, LLC © Jones50 & Bartlett Learning,40 LLC NOT FOR SALE OR DISTRIBUTION NOT FOR40 SALE OR DISTRIBUTION30 Table 2.1 Relationship of Measured Pressure 30 20 to Intensity 20 Threshold 10 of audibility Measured Pressure 10 Phon 0 2 (dB re level Sound pressure (dynes/cm ) © Jones Intensity& Bartlett (dB SPL)Learning, LLC 20 40 60 100 200 500 1000© 2000 Jones 500010 &k Bartlett20k Learning, LLC 0.0002NOT FOR0 (mSALEinimum auORdible DISTRIBUTION sound) Frequency NOT(Hz) FOR SALE OR DISTRIBUTION 0.002 20 Figure 2.9 The heavy line on a phon curve 0.02 40 also represents the 0 dB HL line on an audio- 0.2 60 gram. This is also known as a Fletcher–Munson 2.0 80 curve, named for the researchers (H. Fletcher © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC 20.0 100 and W.A. Munson) who developed the scale. NOT FOR200.0 SALE OR DISTRIBUTION120 Reprinted with permission NOTfrom Fletcher, FORH., & Munson, SALE W.A. (1933). Loudness OR of aDISTRIBUTION Complex Tone, Its Definition, Measurement and Calculation. Journal of the Acoustical Society of America, 5(65), 82–108. 2000.0 140 Copyright 1933, Acoustic Society of America.

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Anatomy and Physiology of Hearing 25

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Table 2.2 Relative Scales Used in Acoustics NOT FOR SALE OR DISTRIBUTIONTherefore, at each frequency,NOT the FORaverage SALE of the OR DISTRIBUTION Name Unit softest intensity heard by young adults is denoted as Physical Frequency Hertz (Hz) 0 dB HL (also known as audiometric zero), to which Properties we can compare an individual’s auditory sensitiv- Intensity Decibel (dB) ity. We denote these comparisons on a graph called © JonesPsychological & Bartlett Pitch Learning, MelLLC (scaling) an audiogram,© Jones which plots & Bartlett the intensity Learning, (in units LLC NOT PropertiesFOR SALE OR DISTRIBUTION of dB HL) forNOT each FOR test frequency SALE OR(in units DISTRIBUTION of Hz). Loudness Sone (scaling) Another common reference that is used audiomet- Phon (equal) rically is the individual’s auditory threshold for a stimulus. A threshold is defined as the level at which a stimulus (e.g., a pure tone or speech) is so soft that © Jones & Bartlettloudness Learning, with a smallerLLC range of perceptual inten© -Jonesit is perceived & Bartlett 50% ofLearning, the time it isLLC presented. The NOT FOR SALE ORsities DISTRIBUTION than do the higher frequencies.) The sone,NOT intensityFOR SALE in decibels OR above DISTRIBUTION an individual’s threshold on the other hand, is defined as the loudness of a is called the sensation level (SL), and is known as the 1000-Hz tone set at 40 dB above threshold. The decibel sensation level (dB SL). We often use dB SL sensation of loudness increases more slowly than when denoting speech audiometric testing; just as the actual increase in intensity for normal auditory we can determine speech intensity (in dB HL), we © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC systems; in pathologic systems, the abnormally can also test speech understanding at intensity levels rapid growthNOT of loudness FOR SALE is called OR recruitment. DISTRIBUTION above threshold (in dB SL). NOT FOR SALE OR DISTRIBUTION This phenomenon usually occurs in those individuals who have sensorineural (especially cochlear) Anatomy and Physiology hearing loss. The scales used in acoustics are of Hearing shown in Table 2.2 . © Jones & Bartlett Learning, LLC Sound is audible© Jones to us only& Bartlett if we have Learning, an auditory LLC NOT FOR SALE OR DISTRIBUTION system thatNOT can utilize FOR the SALE physical OR characteristics DISTRIBUTION Which Decibel Should I Use? of sound—that is, a sound’s frequency (or frequen- The decibel symbol is often qualified with a suffix cies), intensity, and phase(s)—to understand the that indicates which reference quantity has been world around us. Our hearing is sensitive enough used. We have seen how the decibel (in units of to hear very faint sounds (e.g., leaves rustling on © Jones & Bartlettsound Learning, pressure level, LLC dB SPL) expresses a ratio© of Jones the ground & Bartlett from a gentle Learning, breeze), yet LLC can appreciate NOT FOR SALE ORmeasured DISTRIBUTION sound pressure to a reference sound presNOT- andFOR identify SALE the ORdifferent DISTRIBUTION instruments comprising sure. Indeed, we use dB SPL to indicate the intensity a symphony orchestra at much higher intensities. of a sound stimulus. However, when we measure This section will describe the different parts of the an individual’s auditory sensitivity, it is more use- ear—the outer, middle, and inner ear—to see how ful to compare© thatJones threshold & Bartlett intensity toLearning, the softest LLCsound waves travel from the ©ambient Jones air & into Bartlett the Learning, LLC intensity theNOT average FOR person SALE with ORnormal DISTRIBUTION hearing outer ear and then are funneledNOT through FOR the SALEmiddle OR DISTRIBUTION sensitivity can hear. Therefore, we use a different and inner ears up to the brain. decibel—in terms of hearing level—to show this The ear itself is described as a transducer—it deviation from what is considered to be “normal changes one form of energy (in this case, acoustic hearing.” The decibel hearing level (dB HL) is energy) to another form (fluid/electrical) via mechan- © Jonesused audiometrically& Bartlett Learning, to show the LLCdegree of hear- ical energy ©of Jonesthe middle & ear.Bartlett This transduction Learning, of LLC NOTing FOR impairment, SALE ORand itsDISTRIBUTION reference level varies with sound enablesNOT the earFOR to analyze SALE the OR various DISTRIBUTION physical frequency according to a minimum audibility curve parameters (frequency, intensity, phase, and duration) (as was shown in the discussion of phon). to perceive in the brain what the ear has heard.

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26 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC The OuterNOT Ear FOR SALE OR DISTRIBUTIONto the eardrum (tympanic membraneNOT). FOR It is approx SALE- OR DISTRIBUTION imately 6 mm in diameter and about 23–29 mm long We most often think of our ears as just what is visible, in adults, is lined with epithelium (skin) and tiny the outer ear. The outer ear ( Figure 2.10 ) comprises hairs (cilia), and contains glands in the cartilaginous two structures, the pinna (or auricle) and the exter- portion that produce earwax (cerumen). Cerumen nal auditory meatus (ear canal). The pinna is the is waxy and somewhat sticky, which helps to keep © Jonesvisible part & ofBartlett the auditory Learning, system and LLC is shaped like © Jones & Bartlett Learning, LLC the ear canal moisturized and clean of debris that NOTa funnel; FOR itSALE is composed OR DISTRIBUTION of skin overlaying stiffer NOT FOR SALE OR DISTRIBUTION cartilage along with a fleshier lobe, and is attached could accumulate. The external auditory canal has to the cranium by ligaments. The pinna has sev- two main functions: to protect the delicate middle eral landmarks, such as the concha (depression in and inner ears from foreign bodies that could damage these structures and, with the concha, to boost © Jones & Bartlettthe lower Learning, center of LLCthe pinna that forms the exter©- Jones & Bartlett Learning, LLC nal auditory meatus), the helix (auricular rim), the (that is, increase) the amplitude of high-frequency NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION antihelix (ridge just inside the helix), the scaphoid sounds. The concha and external auditory meatus fossa (which lies between the helix and antihelix, each have a natural resonant frequency to which at the lateral aspect of the pinna), and the triangu- they respond best, and each structure increases lar fossa (which lies superior to the scaphoid fossa the sound pressure at its resonant frequency by between the helix© Jones and antihelix). & Bartlett Other landmarksLearning, approximately LLC 10 to 15 dB for© frequencies Jones & rang Bartlett- Learning, LLC ing from 2000 Hz through 5000 Hz. This increase include the tragusNOT (small FOR flap SALE of cartilage OR DISTRIBUTIONanterior NOT FOR SALE OR DISTRIBUTION to the opening of the external auditory meatus), the in amplitude is helpful in discriminating fricative antitragus (lies just opposite the tragus and forms consonants such as s, z, f, and sh, all of which have the inferior boundary of the concha), and the highly acoustic energy above 2000 Hz. This boost of high- vascular lobe, which is inferior to the external audi- frequency sounds also enables us to localize the © Jonestory meatus. & Bartlett The funnel-like Learning, shape ofLLC the concha source of sounds,© Jones because & high-frequency Bartlett Learning, sounds LLC NOTgives FOR rise to SALE the pinna’s OR basic DISTRIBUTION function: to collect and have short wavelengthsNOT FOR that SALE cannot ORtravel DISTRIBUTION around send sound waves through the ear canal. The pinna the head. (In contrast, low-frequency sounds have also assists in sound localization and helps to pro- longer wavelengths, which enable them to travel tect the entrance to the external auditory canal. around the head.) Differences in sound wavelengths The external auditory meatus is a somewhat help to create timing differences between the ears © Jones & Bartlettirregularly Learning, S-shaped LLC tube that runs from the pinna© Jonesand give &us Bartlettcues to where Learning, sounds are LLClocated. NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Tr iangular fossa The Middle Ear The external auditory meatus terminates medially Concha at the tympanic membrane, which acts as the ana- Helix © Jones & Bartlett Learning,tomic LLC boundary between the outer© Jones and middle & Bartlettear. Learning, LLC The tympanic membrane is a thin, concave, elastic, NOT FOR SALETr ORagus DISTRIBUTIONpearly gray to whitish translucentNOT membrane FOR that SALE is OR DISTRIBUTION made up of multiple layers of tissue—epit helial tis- Antihelix Intertragal notch sue (lateral layer), fibrous middle layer, and medial membranous layer—that are both concentric and Antitragus radial, i.e., they fan out from a central point in a circu- © Jones & Bartlett Learning, LLCLobe © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION lar fashion. TheNOT membranous FOR SALE layer of OR the tympanicDISTRIBUTION membrane is contiguous with the membranous Figure 2.10 The outer ear. lining of the external auditory meatus. The fibrous

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Anatomy and Physiology of Hearing 27

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC layer maintainsNOT compliance FOR SALE of the membraneOR DISTRIBUTION itself by five ligaments, which permitNOT movement FOR SALE of the OR DISTRIBUTION so that it can vibrate. The inner, membranous layer ossicles. is contiguous with the mucous membrane lining of The ossicular chain acts like an impedance- the middle ear space, a small cavity that links the matching transformer. The middle ear compen- outer ear to the fluid-filled inner ear. The tympanic sates for loss of sound energy when going from © Jonesmembrane & Bartlett has a more Learning, compliant, LLCsmaller section low-impedance,© Jones air-filled & Bartlettmedium Learning,to a high- LLC NOTcalled FOR the SALE pars flaccida OR DISTRIBUTION, which is located superiorly, impedance, NOTfluid-filled FOR medium SALE through OR DISTRIBUTION three pri- and a stiffer, larger section called the pars tensa, mary mechanisms: the difference in area between located inferiorly. the tympanic membrane and the oval window (the It is within the petrous portion of the tempo- tympanic membrane is about 17 times the size ral bone that we find the middle ear cavity, which © Jones & Bartlett Learning, LLC © Jonesof the & oval Bartlett window); Learning, the incudomalleolar LLC joint houses the ossicles, muscles, and ligaments of the between the malleus and long process of the incus, NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION middle ear. This air-filled cavity is medial to the tym- which forms a complex lever system that helps to panic membrane and contains three very tiny bones amplify sounds traveling through the middle ear (in fact, the smallest and hardest-working bones in space to the inner ear; and the tympanic mem- the body)—the malleus, incus, and stapes—all of brane buckling effect ( Figure 2.11 ). Sound vibra- which can fit easily on a dime with room to spare. © Jones & Bartlett Learning, LLCtions hitting the proportionally© Jones larger surface & Bartlett of Learning, LLC The ossicles are suspended in the middle ear cavity NOT FOR SALE OR DISTRIBUTIONthe tympanic membrane mustNOT be communicated FOR SALE to OR DISTRIBUTION by ligaments, which permit the ossicular chain to the much smaller area of the oval window, which move like a piston to push the sound waves through concentrates the energy (because it takes more the middle ear to the inner ear fluids, and help the energy—about 30 dB—to push against fluid than inner ear from being overdriven by excessively to push against air). This area difference between © Jonesstrong &sound Bartlett vibrations. Learning, The malleus LLC, less than a © Jones & Bartlett Learning, LLC the tympanic membrane and oval window recov- centimeter in length, is embedded slightly into the NOT FOR SALE OR DISTRIBUTION ers almost NOT25 dB ofFOR sound SALE energy. OR The DISTRIBUTION difference fibrous and mucous membrane layers of the tym- in length between the malleus and long process of panic membrane at its manubrium (handle); as the tympanic membrane vibrates from sound energy the incus is called the “step-up function” and adds impinging on it, the malleus (and incus, with which another 2 dB of sound energy. Finally, the tympanic © Jones & Bartlettit articulatesLearning, and LLCwith which forms a unit) moves© Jones membrane & Bartlett buckles inLearning, response to LLC sound, but the NOT FOR SALE ORat the DISTRIBUTION same vibratory speed. The incus is the midNOT- surfaceFOR SALEof the membrane OR DISTRIBUTION moves a greater distance dle bone, attaching to both the malleus head (at the than the malleus, reducing displacement velocity incudomalleolar articulation) and the stapes. It is of the malleus and adding about 5 dB to the sound less than a centimeter in length and has two pro- intensity. Together, approximately 30 dB of sound energy are added to the system to compensate for cesses: the short© Jones crus, which & Bartlett fits into a recessLearning, in the LLC © Jones & Bartlett Learning, LLC wall of the tympanic membrane, and the long crus, the impedance mismatch between the air-filled which is parallelNOT to FOR the manubrium SALE OR of the DISTRIBUTION malleus middle ear and the fluid-filledNOT inner FOR ear. However, SALE OR DISTRIBUTION and attaches to the head of the stapes at the incu- the ability of the middle ear system to amplify the dostapedial joint. The smallest of the three bones, sound pressure depends on the signal’s frequency; the stapes looks like a stirrup, with two crura (arms) little amplification occurs for frequencies below © Jonesand a footplate,& Bartlett which Learning, fits very neatly LLC over the oval 100 Hz or above© Jones 2500 Hz.& BartlettThe outer Learning,ear, however, LLC NOTwindow FOR SALEof the cochlear OR DISTRIBUTION wall. The stapedial footplate amplifies soundNOT energy FOR by SALE about 20 OR dB forDISTRIBUTION frequen- helps to push the acoustic energy into the inner ear. cies between 2000 Hz and 5000 Hz. Taken together, The ossicles are suspended in the middle ear cavity this range of frequencies corresponds to the range of

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28 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Malleus to a hearing loss caused by problems with sound NOT FOR SALE ORF2 DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Tympanic conduction (i.e., a conductive hearing loss). This membrane Stapes disorder, called otitis media (middle ear infection), is S1 Incus often caused by an upper respiratory infection and/ Oval window or allergies and occurs most often in young children © Jones & Bartlett Learning, LLC S2 due to the immature© Jones angle &of theBartlett Eustachian Learning, tube in LLC NOT FOR SALE OR DISTRIBUTION comparison toNOT adults FOR (see FigureSALE 2.12 OR). Acute DISTRIBUTION otitis media is usually caused by a bacterial infection d1 d2 and often presents with an elevated temperature F1 (Rosenfeld et al., 2004). In many cases, this condi- Figure 2.11 Schematic of the ossicular lever tion goes away on its own without treatment with © Jones & Bartlettsystem Learning, and size differential LLC between the tym- © Jonesantibiotics, & Bartlettbut on occasion Learning, fluid will LLC remain in NOT FOR SALEpanic OR membraneDISTRIBUTION S1 and oval window S2. NOTthe FOR middle SALE ear space OR because DISTRIBUTION the Eustachian tube walls stick to each other and create a vacuum, which frequencies in human speech that are most impor- pulls the fluid from the skin cells lining the mid- tant for communication. dle ear. This fluid is called effusion. The presence Also found in the middle ear cavity is the Eusta- of effusion may result in a temporary loss of sound chian (auditory)© tube,Jones which & Bartlettis composed Learning, primar- intensity LLC (i.e., a conductive hearing© Jonesloss). To remedy& Bartlett Learning, LLC ily of cartilage NOTand has FOR two important SALE OR functions: DISTRIBUTION to this situation, an otolaryngologistNOT (ear–nose–throat FOR SALE OR DISTRIBUTION equalize air pressure between the middle ear cav- surgeon) may surgically insert a tympanostomy ity and the nasopharynx and to help drain fluids (pressure-equalizing) tube into the eardrum. This that might accumulate in the middle ear into the tube helps to ventilate the middle ear space, thereby © Jonesnasopharynx. & Bartlett You may Learning, be familiar with LLC the stuffed giving the dysfunctional© Jones Eustachian& Bartlett tube Learning, a chance LLC NOTfeeling FOR in yourSALE ears ORwhen DISTRIBUTION you take off or land in an to heal so thatNOT the middle FOR earSALE cavity OR is once DISTRIBUTION again airplane. The Eustachian tube is at work, equalizing aerated normally. the air pressure in the middle ear. Finally, there are two muscles found in the mid- If the Eustachian tube is not functioning well, dle ear, the stapedius muscle and the tensor tym- fluids can build up in the normally air-filled middle pani muscle. These muscles are specially designed © Jones & Bartlettear space, Learning, which compromises LLC the ossicular chain© Jonesfor efficiency. & Bartlett They are Learning, (a) very tense, LLC so that they NOT FOR SALEmovement. OR DISTRIBUTION Additional sound pressure is needed toNOT stop FOR vibrating SALE instantly OR to DISTRIBUTION limit distortion of incom- overcome this lack of ossicular movement, leading ing sound stimuli; (b) very elastic, to dampen

INFANT ADULT

Eustachian (auditory) tube Eustachian © Jones & Bartlett Learning,(auditory) tube LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

Figure 2.12 The angle difference between infant and adult Eustachian tubes.

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Anatomy and Physiology of Hearing 29

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC (reduce) vibrations;NOT FOR and (c) SALE very small, OR soDISTRIBUTION that the The Inner Ear NOT FOR SALE OR DISTRIBUTION vibratory energy does not spread. The stapedius The inner ear consists of the auditory labyrinth and muscle inserts into the posterior neck of the stapes vestibular labyrinth, which are intricate pathways and is innervated by the stapedial branch of the facial in the petrous portion of the mastoid process of each nerve (cranial nerve [CN] VII). The other muscle, temporal bone. The petrous portion of the tempo- © Jonesthe tensor & Bartlett tympani, Learning,originates from LLC the cartilagi- ral bone’s mastoid© Jones process & is Bartlett the densest Learning, bone in the LLC NOTnous FOR portion SALE of theOR Eustachian DISTRIBUTION tube and the sphe- body, protectingNOT the FOR delicate SALE organs OR of hearingDISTRIBUTION and noid facial bone and inserts into the manubrium. balance. The auditory and vestibular labyrinths are The tensor tympani muscle, which is innervated by comprised of two labyrinths: the osseous labyrinth, trigeminal nerve (CN V), assists in the function of which is a channel in the temporal bone that encases © Jones & Bartlettthe Learning, Eustachian tube. LLC When the tensor tympani con© -Jonesthe auditory & Bartlett and vestibular Learning, labyrinths, LLC and the mem- tracts, it pulls on the malleus to draw the tympanic branous labyrinth, which consists of soft-tissue, NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION membrane inward, which increases the pressure in fluid-filled channels within the osseous labyrinth the middle ear and Eustachian tube. The tensor veli containing the end-organ structures of the hear- palatini muscle then contracts to open the Eusta- ing and vestibular systems. The auditory labyrinth chian tube. is called the cochlea and is the sensory end organ Bilateral ©contraction Jones &of Bartlettthe middle Learning,ear muscles LLCof hearing; the semicircular canals© Jones are the & sensory Bartlett Learning, LLC in response NOTto high-intensity FOR SALE sounds OR stiffensDISTRIBUTION the end organs of balance. TheseNOT two endFOR organs SALE are OR DISTRIBUTION ossicular chain to reduce the intensity of sounds to connected via the vestibule, which houses two addi- the inner ear, thereby serving as a protective mech- tional organs of balance, the saccule and utricle. anism. The tensor tympani muscle contracts to pull The cochlea is a snail-shaped, spiral, fluid-filled canal within the temporal bone that, when straight- © Jonesthe malleus & Bartlett anteriorly Learning, and medially LLC and the stape- © Jones & Bartlett Learning, LLC dius muscle contracts to pull the stapes posteriorly, ened out, measures about 3.5 cm in length. Within each membranous duct are three chambers—the NOTresulting FOR SALE in attenuation OR DISTRIBUTION of sound pressure reaching NOT FOR SALE OR DISTRIBUTION scalae vestibuli (upper chamber), media (middle the inner ear. Depending on the frequency of the chamber), and tympani (lower chamber)—that sound, there is a 15- to 20-dB decrease in sound are filled with fluid. The scalae vestibuli and tym- pressure because the middle ear efficiency in trans- pani are incompletely separated by the osseous mitting sound energy is in the range of 75–120 dB © Jones & Bartlett Learning, LLC © Jonesspiral &lamina, Bartlett a bony Learning, shelf protruding LLC from the NOT FOR SALE ORSPL. DISTRIBUTIONThis reflex is consensual, so that when eitherNOT centralFOR core,SALE the ORmodiolus. DISTRIBUTION Circulating through the ear is stimulated appropriately, the muscles in both scalae vestibuli and tympani is perilymph, which is ears contract. It is also important for reducing the secreted by the epithelial lining of the osseous lab- upward spread of masking of high frequencies by yrinth and has a higher concentration of sodium low-energy sounds because this effect is greatest at + + © Jones & Bartlett Learning, LLCions (Na ) than potassium ions© Jones (K ), making & Bartlett it Learning, LLC frequencies less than 2000 Hz. This information is chemically similar to extracellular fluid. In contrast, used clinicallyNOT with FOR the electroacousticSALE OR DISTRIBUTION measure- endolymph, which has a higherNOT concentration FOR SALE of K+ OR DISTRIBUTION ment called the stapedial reflex, which measures than Na+, is chemically similar to intracellular fluid the intensity needed to cause contraction of the and is found in the scala media. This difference stapedius muscle. This contraction fatigues because in ionic concentration between endolymph and © Jonesprolonged & Bartlett exposure toLearning, high-intensity LLC environments perilymph ©gives Jones rise to & an Bartlett endocochlear Learning, electri- LLC NOTmay FOR decrease SALE the ORdegree DISTRIBUTION to which the stapedius con- cal potentialNOT (“cochlear FOR battery”) SALE ofOR about DISTRIBUTION 180 mV tracts, which lessens the effectiveness at damping (millivolts) in the scala media, which helps to con- loud sounds. duct neural transmission of sound. The “floor” of

© Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 9781284132793_CH02_015_036.indd 29 30/06/17 10:20 am © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

30 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC the cochlear ductNOT is the FOR basilar SALE membrane OR ,DISTRIBUTION whereas the nerve fibers of the eighth NOTcranial FOR nerve SALE(CN OR DISTRIBUTION the membranous roof is called the vestibular mem- VIII, auditory portion). Figure 2.13 shows the brane. Two tissue-covered openings are found on movement of the tectorial membrane in response the cochlea: The oval window (which is covered by to hair-cell polarization; Figure 2.14 depicts the the stapes footplate) is between the basilar mem- electrochemical response of hair-cell polarization © Jonesbrane and & scala Bartlett vestibuli Learning,, and the round LLC window, within the cochlea.© Jones The lengths& Bartlett of the Learning,outer hair LLC NOTwhich FOR is between SALE theOR scala DISTRIBUTION tympani and middle cells increase atNOT this point FOR of maximumSALE OR amplitude DISTRIBUTION so ear. The membranous portion is slightly smaller that a vigorous electrical response is created by the than the bony portion; the point where the scalae incoming stimulus. The overall effect of this change vestibuli and tympani communicate is called the of amplitude is a more precise analysis of stimulus helicotrema. frequency because of the different characteristic © Jones & BartlettWithin Learning, the cochlear LLC duct is the organ of Corti©, Jonesfrequencies & Bartlett of the auditory Learning, nerve fibers, LLC which are NOT FOR SALEwhich OR DISTRIBUTIONcontains the sensory cells of hearing andNOT arranged FOR tonotopically.SALE OR Near DISTRIBUTION the oval window, at the which lies on the basilar membrane. These mecha- base, the nerve fibers in the hair cells are attuned noreceptor cells are shaped like hair and are called, to higher frequencies; at the apex, toward the cen- appropriately, hair cells. The outer hair cells, of tral core of the cochlea, the hair cells are attuned to which there are about 15,000, form three rows low-frequency sounds. Outer hair cells are tuned shaped like a W© and Jones have their & Bartlett nerve fibers Learning, embed- primarily LLC to sound intensity; they© actJones like transduc & Bartlett- Learning, LLC ded into the tectorialNOT FOR membrane SALE, a gel-likeOR DISTRIBUTION mem- ers, changing fluid energy into electricalNOT FOR energy. SALE In OR DISTRIBUTION brane that forms the roof of the basilar membrane. fact, this cochlear transduction of sound is like that Because the basilar and tectorial membranes have of a microphone, which changes acoustic energy different pivot points, vibration of the basilar mem- to electrical energy, and is often referred to as the © Jonesbrane causes & Bartlett the cilia of Learning, the outer hair LLC cells to bend, cochlear microphonic.© Jones This & transductionBartlett Learning, function LLC NOTwhich FOR alternately SALE hyperpolarizesOR DISTRIBUTION and depolarizes is described asNOT the shearing FOR SALEforce and OR is applied DISTRIBUTION to

Shear force Sound-induced RESTING POSITION vibration Tectorial © Jones & Bartlett Learning,membrane LLC © Jones & Bartlett Learning, LLC Cilia NOT FOR SALE OR DISTRIBUTION (hair cells)NOT FOR SALE OR DISTRIBUTION To auditory nerve Upward phase

Shear force ©Basilar Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC membraneNOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Downward NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR phaseDISTRIBUTION Figure 2.13 Shearing action of the hair cells and movement of the basilar membrane.

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Anatomy and Physiology of Hearing 31

Perilymph

Auditory Perilymph neurons [K+] << [Na+] 0 mV © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

Round window + Cochlear battery © Jones & Bartlett Learning, LLC VIN © Jones & Bartlett Learning, LLC – NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

Figure 2.14 Cross-section of the cochlear duct showing membranous structures.

© Jones & Bartlettthe Learning, cilia in response LLC to the acoustic stimulation,© Jones hair cells & Bartlettare neurologically Learning, connected LLC to the brain NOT FOR SALE ORgiving DISTRIBUTION rise to electrical (i.e., receptor) potentials.NOT viaFOR nerve SALE fibers—they OR DISTRIBUTION preferentially encode sound Fewer than 10% of the outer hair cells are neuro- clarity. logically connected to the brain, but they enhance The basilar membrane is where the cochlea the cochlear mechanical response to vibrations so begins its analysis of both frequency and intensity that we can hear© Jones lower-intensity & Bartlett sounds. Learning, Outer hair LLCof incoming sound signals; these© Jonesincoming & complex Bartlett Learning, LLC cells also generate their own vibrations, both spon- sound waves are transformed into simple sine waves taneously andNOT by using FOR an evokingSALE stimulus;OR DISTRIBUTION we can similar to Fourier analyses. NOTThe stapes FOR footplate SALE OR DISTRIBUTION measure these sounds (called otoacoustic emis- rocks back and forth in the oval window, which sions) clinically to determine cochlear function. establishes a transverse wave within the scala ves- Inner hair cells, in contrast, are far fewer in tibuli. Inward displacement of the perilymph at the © Jonesnumber & (aboutBartlett 3,500 Learning, altogether), LLCand form a row oval window© isJones matched & by Bartlett the outward Learning, displace- LLC NOTstretching FOR SALE from ORbase DISTRIBUTIONto apex in proximity of the ment of theNOT fluids FOR via the SALE round OR window DISTRIBUTION due to tectorial membrane, near the modiolus (bony core) increased pressure. This perilymph wave displaces of the cochlea. However, more than 90% of these the scala media, setting up a wave on the basilar

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32 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC membrane thatNOT moves FOR from SALE the base OR to theDISTRIBUTION apex. frequencies) and/or volley theoryNOT (which FOR describes SALE OR DISTRIBUTION The vibrations of the basilar membrane progress cooperation of neurons in neural transmission of dynamically as the incoming traveling waves move high frequencies). from the cochlear base toward the helicotrema at the apical end. The stiffness gradient of the basi- Retrocochlear Pathway and © Joneslar membrane & Bartlett is the primary Learning, physical LLC feature that Auditory Cortex© Jones & Bartlett Learning, LLC accounts for the direction in which the traveling NOT FOR SALE OR DISTRIBUTION The auditory NOTnerve fibersFOR fire SALE in an ORall-or-nothing DISTRIBUTION wave progresses—the greater stiffness in the basal fashion, needing only about 2 ms to rise to maxi- portion of the cochlea opposes displacement when mum amplitude of neural firing. They are arranged stimulated by low-frequency sound, and forces the on the basilar membrane in a tonotopic fashion— wave to travel further up the cochlea toward the nerve fibers at the apical end of the cochlea respond © Jones & Bartlettapex toLearning, a region having LLC less stiffness and less oppo©- Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOTpreferentially FOR SALE to low-frequency OR DISTRIBUTION stimuli, and high- sition to low-frequency vibration. Thus, more of the frequency sounds are encoded at the base. Simi- basilar membrane is stimulated by low-frequency larly, the auditory nerve is tonotopically arranged sounds. High-frequency sounds displace the basilar so that low-frequency sounds are found in the core membrane only near the basal end, at the oval win- of the auditory nerve and high-frequency sounds dow, and do not© travel Jones further & Bartletttoward the Learning,apex. This are LLC arranged around the periphery.© Jones Thus, the & brain Bartlett Learning, LLC basilar membraneNOT displacement FOR SALE pattern OR increasesDISTRIBUTION obtains information regardingNOT frequency FOR of SALE the OR DISTRIBUTION gradually in amplitude until the point of maximum incoming sound. In addition to frequency coding, amplitude is reached, and then decreases abruptly. the neural fibers of the auditory nerve also encode There is also a stronger mechanical/electrical intensity for sounds with frequencies less than response to low- and moderate-intensity sounds; 5000 Hz; neural firing approximates the period of © Jonesthis is called & Bartlett the cochlear Learning, amplifier. LLCAlthough we the stimulus waveform.© Jones & Bartlett Learning, LLC NOTare FORuncertain SALE how ORintensities DISTRIBUTION are encoded in the Neural firingNOT of the FOR auditory SALE portion OR of DISTRIBUTION CN VIII cochlea, it is thought that the relative rate of nerve generates action potentials; this electrical signal then impulse spikes transmits this information to the travels from the cochlea to the auditory cortex in the brain (see Zemlin, 1998, pp. 486–487). temporal lobe. Although most of these fibers travel The traveling wave theory of sound trans- up to the auditory cortex to form the ascending © Jones & Bartlettduction Learning, (proposed LLCby Georg von Bekesy [1899–© Jones(afferent) & pathways, Bartlett some Learning, neural fibers LLC travel from NOT FOR SALE1972], OR DISTRIBUTIONand for which he received the Nobel PrizeNOT either FOR the SALEbrainstem OR or auditoryDISTRIBUTION cortex to form the for Physiology in Medicine in 1961) through the descending (efferent) pathways. All auditory nerve cochlea describes how higher-frequency sounds fibers terminate at the level of the ipsilateral cochlear are analyzed (Zemlin, 1998). This theory does not nucleus, where frequency and timing information account for all© basilar Jones membrane & Bartlett mechanics, Learning, how- about LLC the auditory stimulus are© furtherJones encoded. & Bartlett Learning, LLC ever, because the membrane itself is not displaced Although some neural pathways are ipsilateral and sharply enoughNOT to FORdistinguish SALE low-frequencyOR DISTRIBUTION project into the next structureNOT along FORthe central SALE OR DISTRIBUTION sounds by place of stimulation. As noted in Zem- auditory pathway ( Figure 2.15 ), the superior oli- lin (1998), Ernest Glen Wever hypothesized in 1937 vary complex (in the medulla), most of these affer- and published in 1949 that low-frequency sounds ent pathways are contralateral (opposite side) so that © Jonesare determined & Bartlett by the Learning,number of clusters LLC of firing the nerve fibers© decussateJones &(cross Bartlett over) to Learning, the oppo- LLC NOTnerve FOR fibers SALE in synchrony OR DISTRIBUTION with the low frequency; site superior NOTolivary FORcomplex. SALE Therefore, OR DISTRIBUTIONauditory high-frequency sounds are analyzed through information from both ears is represented in each place theory (because neurons cannot fire at high ear, which enables us to localize sounds in space

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Anatomy and Physiology of Hearing 33

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTIONareas: primary, secondary, andNOT tertiary FOR cortices. SALE The OR DISTRIBUTION primary auditory cortex, the first cortical region of the auditory pathway, is tonotopically arranged in a fashion similar to that found in the cochlea and is largely responsible for discrimination of frequency © Jones & Bartlett Learning, LLC and intensity© ofJones the incoming & Bartlett auditory Learning, stimulus. LLC NOT FOR SALE OR DISTRIBUTION The locationNOT of a FORsound SALEstimulus ORin space DISTRIBUTION is also Auditory identified in the primary auditory cortex. The sec- cortex ondary and tertiary auditory cortices are largely responsible for language production, processing, Medial Medial geniculate geniculate and perception, and include Broca’s area (inferior © Jones & Bartlett Learning,body LLC body © Jonesfrontal & gyrus), Bartlett where Learning, motor production LLC of language NOT FOR SALE OR DISTRIBUTION NOT andFOR processing SALE of OR sentence DISTRIBUTION structure, grammar, and syntax are located, and Wernicke’s area (in the lower Inferior Inferior temporal lobe), where speech perception is located. colliculus colliculus In addition, other areas within the brain—the © Jones & Bartlett Learning, LLCsuperior temporal gyrus (where© Jones morphology & Bartlett and Learning, LLC NOT FOR SALE OR DISTRIBUTIONsyntactic processing occur inNOT the anterior FOR section,SALE OR DISTRIBUTION Superior Superior and integration of syntactic and semantic informa- Cochlear olivary olivary Cochlear nucleus complex complex nucleus tion in the posterior section), the inferior frontal gyrus (working memory and syntactic processing), and the middle temporal gyrus (lexical semantic © Jones &Cochlea Bartlett Learning, LLCCochlea processing)—contribute© Jones & to Bartlett language Learning,comprehen- LLC NOT FOR SALE OR DISTRIBUTION sion. In almostNOT all right-handedFOR SALE individuals, OR DISTRIBUTION the left Figure 2.15 The central auditory pathway. hemisphere is usually dominant, with bilateral activation occurring for syntactic processing; this left and to improve speech perception because ipsilat- hemisphere dominance is true for most left-handed eral fibers are excitatory and contralateral fibers are individuals also. The right hemisphere is important © Jones & Bartlettinhibitory. Learning, In addition, LLC low-frequency stimuli ©are Jones in processing & Bartlett suprasegmental Learning, features LLC like prosody NOT FOR SALE ORencoded DISTRIBUTION for differences in timing, whereas high-NOT andFOR melodic SALE contours. OR DISTRIBUTION frequency stimuli are encoded for differences in Although the retrocochlear auditory pathway latency. Other structures along the afferent auditory is primarily sensory and contains afferent path- pathway include the lateral lemniscus (at the level of ways from the cochlea up to the auditory cortex, a the pons), the© inferiorJones colliculus & Bartlett (in the Learning, midbrain, LLCcomplex efferent system is also© presentJones containing & Bartlett Learning, LLC where the second decussation occurs), and the descending neural fibers that correspond closely to NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION medial geniculate body (at the level of the thalamus), the ascending auditory fibers. These efferent fibers where all ascending fibers terminate before radiating connect the auditory cortex to the central auditory into the appropriate cortex (in this case, the auditory pathway and to the cochlea, and are thought to cortex). Tonotopic organization of frequency to inhibit neural activity along this pathway to increase © Jonesplace is& preserved Bartlett throughout Learning, the afferentLLC auditory neural activation© Jones at lower & brainBartlett centers. Learning, This inhib- LLC NOTpathway, FOR SALE which preserves OR DISTRIBUTION the redundancy of speech. itory feedbackNOT improves FOR SALE stimulus OR processing DISTRIBUTION by The auditory cortex is located in the temporal decreasing background noise that may interfere lobes of the brain and is divided into three basic with the stimulus.

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34 Chapter 2 Sound and the Ear

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC OUTER EAR MIDDLE EAR INNER EAR NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Sound waves are captured by the Sound waves cause the Fluid in the Electrical impulses outer ear and are funnelled through tympanic membrane to inner ear are sent from the the external auditory canal to the vibrate. The three bones stimulates hair cells along the tympanic membrane. of the middle ear transmit nerve endings auditory nerve to and amplify the vibrations called hair cells. the brain © Jones & Bartlett Learning, LLC to the oval window of the © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTIONinner ear. NOT FOR SALE OR DISTRIBUTION

Cochlea © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

External Eustachian auditory tube canal Tympanic © Jones & Bartlett Learning, LLC membrane© Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION

Figure 2.16 An overview of the process of sound transduction through the auditory system. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE ORFigure DISTRIBUTION 2.16 provides an overview of the processNOT and FOR otosclerosis SALE (fixation OR DISTRIBUTION of the stapes footplate to of sound transduction through the auditory system. the oval window of the cochlea). Disorders affecting the outer and/or middle ear are usually amenable Hearing Loss: An Error to medical and/or surgical intervention to correct © Jones & Bartlett Learning,the LLC problem. Conductive hearing© loss Jones results & in Bartlettthe Learning, LLC of Sound Transduction decrease of sound intensity reaching the cochlea; NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Hearing loss may occur at any point along the audi- typically, clarity of speech is preserved in conduc- tory pathway. When damage occurs in the outer and/ tive hearing loss because the cochlea is usually unaf- or middle ears, a conductive hearing loss is the result. fected. However, chronic conductive hearing loss can Some examples of conductive hearing loss include also affect speech perception because of alterations © Jonesouter ear & disorders Bartlett such Learning, as microtia (small LLC or absent in the normal ©inertial Jones mechanisms & Bartlett of the Learning,middle ear, LLC NOTpinna) FOR and SALE atresia (lack OR of DISTRIBUTION external auditory meatus), which affect conductionNOT FOR of sound SALE through OR DISTRIBUTIONbone. and middle ear disorders such as otitis media with Those individuals whose hearing loss is found or without effusion (fluid in the middle ear space) in the inner ear have sensorineural hearing loss.

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Discussion Questions 35

© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC SensorineuralNOT hearing FOR loss SALE occurs OR due DISTRIBUTIONto damage fullness, and/or vestibular dysfunction).NOT FOR Sensori-SALE OR DISTRIBUTION to the cochlea and/or retrocochlear pathway, result- neural hearing loss may also result from the normal ing in alterations of perception of sound frequency aging process (presbycusis), leading to both cochlear and intensity. In addition to a decrease of sound and retrocochlear dysfunction, and which usually intensity, sensorineural hearing loss also results in results in poorer speech understanding due to dam- © Jonesa loss of& speech Bartlett clarity Learning, due to damage LLC to the neural age to the cochlea© Jones and higher & Bartlett auditory centers.Learning, LLC NOTfibers FOR located SALE in theOR cochlea. DISTRIBUTION Examples of sensori- When bothNOT conductive FOR SALE and sensorineural OR DISTRIBUTION com- neural hearing loss include acoustic trauma from ponents are present in hearing loss (e.g., an individ- noise, tumors on CN VIII, ototoxic agents like loop ual with sensorineural hearing loss develops otitis diuretics, systemic neural diseases like diabetes mel- media), a mixed hearing loss results. Mixed hear- litus, hypoxia (lack of oxygen), meningitis (both ing loss may result from complications of middle ear © Jones & Bartlettbacterial Learning, and viral, LLC leading to inflammation of ©the Jones surgery, & otosclerosis,Bartlett Learning, and the like. Medical/LLC surgical NOT FOR SALE ORmeninges DISTRIBUTION covering the brain), and Ménière’s diseaseNOT interventionFOR SALE may OR limit DISTRIBUTION the conductive portion of (which results in an increase of endolymph fluid in the hearing loss, but the sensorineural component the cochlea, leading to fluctuating hearing loss, aural of the loss is still present.

To understand hearing and the presence of hear- ear in stages—the outer ear (which collects sound), © Jonesing loss, & Bartlettone must first Learning, understand LLC what sound is. the middle ear© (whichJones acts & as Bartlett a transducer Learning, to change LLC NOT FORSound isSALE defined OR as the DISTRIBUTION movement of a disturbance acoustic energyNOT to FOR fluid SALEenergy ORvia mechanical DISTRIBUTION through an elastic medium (such as air molecules) energy), and then the inner ear (which sends fre- without permanent displacement of the particles. quency and intensity information up to the brain There are three prerequisites for production of via the central auditory pathway). Errors in sound sound: (1) a source of energy such as a force, (2) a transduction and the location of that damage will © Jones & Bartlettvibrating Learning, object LLCthat generates an audible pressure© Jones determine & Bartlett the presence Learning, and type LLCof hearing loss NOT FOR SALE ORwave, DISTRIBUTION and (3) a medium of transmission. SoundsNOT thatFOR results. SALE As we OR journey DISTRIBUTION through how a hearing may be described by their frequency, intensity, and loss is determined and resulting treatments, the phase, all of which are physical characteristics that basic understanding of sound and its transmission are measurable. Sound moves through the human is crucial as the underlying concept. © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC DiscussionNOT FOR SALE Questions OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 1. List the characteristics of sound. 6. For each part of the ear, identify the type of 2. What is simple harmonic motion? energy used for sound transduction. 3. How are the characteristics of frequency and 7. What is the primary function of the middle ear? © Jones pitch& Bartlett related? Learning, LLC 8. In the inner© Jones ear, name & Bartlettthe end organs Learning, of LLC NOT FOR4. How SALE are intensity OR DISTRIBUTION and loudness related? hearingNOT and balance. FOR SALE OR DISTRIBUTION 5. Why do we use the decibel to describe sound 9. What does the term tonotopic organization intensity? mean regarding cochlear function?

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36 Chapter 2 Sound and the Ear

References © Jones Rosenfeld, R. M , Culpepper, L. , Doyle, K. J. , et al ( 2004 ). & Bartlett Learning, LLC Zemlin, W. R. ( 1998 ). Hearing. In: © Jones & SpeechBartlett and hearing Learning, LLC Clinical practice guidelines: Otitis media with effu- science: Anatomy and physiology ( 4th ed. , NOT FORsion . Otolaryngology—HeadSALE OR DISTRIBUTION and Neck Surgery, pp. 414–511 ). Boston, MA : Allyn & Bacon . NOT FOR SALE OR DISTRIBUTION 130 ( 5 ), S95–S118 .

Recommended Readings © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC Bear, M. F. , Connors, B. W. , & Paradiso, M. A. ( 2001 ). The Hixon, T. J. , Weismer, G. , & Hoit, J. D. ( 2008 ). Acoustics. NOT FOR SALE ORauditory and vestibular system . In: DISTRIBUTION Neuroscience: Explor-NOT FORIn: Preclinical SALE speech OR science: DISTRIBUTION Anatomy, physiology, ing the brain ( 2nd ed. , pp. 351–395 ). Philadelphia, PA : acoustics, and perception (pp. 317–355 ). San Diego, Wolters Kluwer/Lippincott Williams & Wilkins . CA : Plural . Bess, F. H. , & Humes, L. E. ( 2008 ). Audiology: The funda- Krizman, J. , Skoe, E. , & Kraus, N. ( 2010 ). Stimulus rate and mentals ( 4th ed. ). Philadelphia, PA : Wolters Kluwer/ subcortical auditory processing of speech . Audiology Lippincott Williams & Wilkins . & Neurotology, 15 , 332–342 . Denes, P. B. , & Pinson, E. N. ( 1993 ). © Jones & BartlettThe speech Learning, chain: The Martin, F. N. , & Clark, J. G. ( 2015 ). LLC Introduction© Jones to & Bartlett Learning, LLC physics andNOT biology FOR of spoken SALE language OR ( 2nd ed. ). DISTRIBUTION audiology ( 12th ed. ). Boston, MA : Pearson Education . NOT FOR SALE OR DISTRIBUTION New York, NY : W. H. Freeman . Raphael, L. J. , Borden, G. J. , & Harris, K. S. ( 2011 ). Speech Deutsch, L. J. , & Richards, A. M. ( 1979 ). Elementary hearing science primer: Physiology, acoustics, and percep- science . Baltimore, MD : University Park Press . tion of speech ( 6th ed. ). Philadelphia, PA : Lippincott Ferrand, C. T. ( 2007 ). Speech science: An integrated Williams & Wilkins . approach to theory and clinical practice ( 2nd ed. ). Seikel, J. A. , Drumright, D. G. , & King, D. W. ( 2016 ). © Jones Boston, MA : Pearson/Allyn & Bacon . & Bartlett Learning, LLC Anatomy of hearing . In: © JonesAnatomy & Bartlett & physiology Learning, for LLC NOT Gelfand, S. A. ( 2016 ). Anatomy and physiology of the FOR SALE OR DISTRIBUTION speech, language,NOT and FOR hearing SALE ( 5th ed. , pp. 499–530 ). OR DISTRIBUTION auditory system . In: Essentials of audiology ( 4th ed. , Clifton Park, NY : Cengage Learning . pp. 30–69 ). New York, NY : Thieme Medical Publishers .