ME570 Chapter 2 Principles of Acoustics, Sound and Noise
Presented By John G. Cherng Mechanical Engineering Department University of Michigan Dearborn Dearborn, MI 48128
THE UNIVERSITY OF MICHIGAN-DEARBORN 1 Introduction to Sound and Noise
1. The nature of Sound
2. Human Hearing Response
3. Description of Sound Field
4. Measurement and Analysis of Sound
THE UNIVERSITY OF MICHIGAN-DEARBORN 2 1.1 Definitions
• Acoustics is the study of sound. • Sound is the air-borne wave phenomena that gives rise to the sensation of hearing. • Sound waves are generated by disturbances that propagates through an elastic medium at a speed that is mainly depending on the medium and the temperature of the medium. • Sound waves are compressive waves that compress and expand the molecules of the medium. Therefore, it can not travel in a vacuum. • In air and at standard conditions, the speed of sound is approximate at 1130 ft/sec or 343 m/sec with the following relationship λ=c/f Where λ=wave length, m c=speed of sound, m/s f =frequency, Hz • With the human audible range(20-20kHz), the speed of sound is independent of the frequency of the acoustic wave. Therefore, the wave length of a sound wave can be determined by knowing its frequency or vice versa • Noise: Unwanted Sound (very subjective)
THE UNIVERSITY OF MICHIGAN-DEARBORN 3 1.2 Temperature and Sound Speed
• Temperature is the most dominant factor for the speed of sound to vary. Under the perfect gas theorem, the speed of sound can be calculated by :
c RT
Where: C=speed of sound, m/sec γ=ratio of specific heats=1.4 for air R=gas constant=287m2/sec2-ok T=absolute temperature=273+0C,0k Example: Determine the speed of sound in air to (a) cold climate condition-40oC(b) hot exhaust gas at 5300C
THE UNIVERSITY OF MICHIGAN-DEARBORN 4 Example: Determine the speed of sound in air to (a) cold climate condition-40oC(b) hot exhaust gas at 5300C
• Solution:
1. At – 40 oC,
C = [1.4x287x (273-40)]1/2 = 307.94 m/sec
2. At 530 oC
C = [1.4x287x (273+530)]1/2 = 568.02 m/sec
3. Percent increase
(568.02-307.94)/307.94 =84.5%
THE UNIVERSITY OF MICHIGAN-DEARBORN 5 1.2 Generation and Propagation of Sound
1. Generated by compressive waves, disturbance by a dynamic force, or mechanical vibration.
2. Propagation– depend on the characteristic of the medium
*Indoors *Outdoor
THE UNIVERSITY OF MICHIGAN-DEARBORN 6 1.3. Wave Nature of Sound (a) Interaction with obstacle
L/λ>1: Large obstacle, strong interaction L/λ<1: Small obstacle, weak interaction
THE UNIVERSITY OF MICHIGAN-DEARBORN 7 (b) Interaction with Absorbers
L/λ>1/4: Thick absorber, large pressure drop
L/λ<<1/4: Thin absorber, small pressure drop
THE UNIVERSITY OF MICHIGAN-DEARBORN 8 1.4. Definition of Boundary
Boundary: The interface between two mediums
Air to Water Air to wall Confined Air to open Air
THE UNIVERSITY OF MICHIGAN-DEARBORN 9 1.5.Transmission,Reflection and Absorption of Sound at Boundary
Conservation of Energy
EI=ER+EA+ET
Where EI: Incident Energy ER: Reflected Energy EA: Absorbed Energy ET: Transmitted Energy τ=Transmission Coefficient=ET/EI α=Absorption Coefficient=(EA+ET)/EI=(EI-ER)/EI
THE UNIVERSITY OF MICHIGAN-DEARBORN 10 1.6 Acoustic Materials
A. Transmitter =100% transmission-thin membrane
B. Barrier=100% reflection-rigid, massive wall, non-porous
C. Absorber=100% absorption- porous, light weight
D. Elastic, mass, dissipative-combination of the three-standard materials.
THE UNIVERSITY OF MICHIGAN-DEARBORN 11 Cinder Block with Fiber Glass
THE UNIVERSITY OF MICHIGAN-DEARBORN 12 1.7. Physical & Mathematical descriptions of Sound
Consider a pulsation sphere RA P( r , t )0 sin[ k ( r R ) t ] r 0
1 1 ( )2 u(,)(,) r tkr P r t 0c
Where: Ro: Radius of the sphere A: Amplitude of the pressure K: Wave number=(ω/c) r: Distance from the source φ: Phase angle=tan-1(c/ωr) ―Pressure amplitude decreases in direct proportion to the distance from the center of the source‖
THE UNIVERSITY OF MICHIGAN-DEARBORN 13 Plane Wave Propagation
1.7 Plane Wave kr>>1, u & p are in phase
PP u or0 c Characteristic Impedance 0cu 415 Rayls
* Wave in a Duct P( x , t ) A sin ( t ) c Atsin( ) c Asin( t k )
THE UNIVERSITY OF MICHIGAN-DEARBORN 14 *Sound Intensity: Sound Power Per Unit Area
()P dA u I Pu dA 1 p(,)(,) r t u r t dt 0 1 p ( r , t ) p(,) r t dt 0 c 0 P2 I rms () vector 0c *Sound Power ------Total Energy from the source per unit time
2 P 2 W IA rms 4 r ( scalar ) 0c
THE UNIVERSITY OF MICHIGAN-DEARBORN 15 1.10 Total Intensity (when far from the source)
Total=I1+I2+······In Total Sound Pressure (a) Same frequency (correlated) sources
2 2 2 Pt P1 P 2 2 PP 1 2 cos( 1 2 ) ( 1 , 2 Phase angle )
(b) Broad Band (Non correlated) sources
2 2 2 2 PPPPtn12
THE UNIVERSITY OF MICHIGAN-DEARBORN 16 1.11 Levels and decibel ---A logarithmic circuit
2 2 Lp: Sound pressure level=10log(P rms/P ref) LI: Sound Intensity level=10log(I/Iref) Lw: Sound Power level=10log(W/Wref)
International conversion references:
-5 2 Pref=2 x 10 N/m -12 2 Iref=1.0 x 10 Watt/m -12 Wref=1.0 x 10 watt
In air, standard condition, i.e. 20oC, 1 atm,
Lp=LI
THE UNIVERSITY OF MICHIGAN-DEARBORN 17 THE UNIVERSITY OF MICHIGAN-DEARBORN 18
Typical Sound Pressure Levels and Sources
THE UNIVERSITY OF MICHIGAN-DEARBORN 19 Exercises
62 1.Threshold of Hearing Prms 20 10 N / m 6 2 6 2 LP 10log(2 10 ) /(2 10 ) 10log1 0 dB
22 2.Threshold of Pain Prms 2 10 N / m 2 6 2 7 LP 10log(2 10 / 20 10 ) 20log10 140 dB
2 3.Jet take off at 60 m Prms 60 N / m 62 6 LP 10log(60 / 20 10 ) 20log(3 10 ) 129.5dB
THE UNIVERSITY OF MICHIGAN-DEARBORN 20 1.12 Addition and Subtracts of Sound Sources *dB is a Log scale, can not be added or subtracted directly * All units are energy related
THE UNIVERSITY OF MICHIGAN-DEARBORN 21 THE UNIVERSITY OF MICHIGAN-DEARBORN 22 Adding and Subtraction of Sound Pressure Levels
THE UNIVERSITY OF MICHIGAN-DEARBORN 23 Example 1: Calculate the intensity and SPL(sound pressure level) at a distance of 10m from a uniform radiating source 1-Watt power
THE UNIVERSITY OF MICHIGAN-DEARBORN 24 Example 2: The noise level from a power station with 10 identical transformers measured near some residential property line was found to be 54 dB. The maximum permitted in this area is 50 dB at night. How many transformer could be used during the Night?
THE UNIVERSITY OF MICHIGAN-DEARBORN 25 2. Human Hearing Response 2.1 Loudness Level—Phons 2.2 Subjective Loudness—Sones 2.3 Loudness of Broad band noise 2.4 Annoyance 2.5 Sound Quality
THE UNIVERSITY OF MICHIGAN-DEARBORN 26 2.1 Human Hearing Mechanism
Three major components: • Outer Ear • Middle Ear • Inner Ear
THE UNIVERSITY OF MICHIGAN-DEARBORN 27 Main Function of Each Component
• Outer Ear: Consists of Pinna, Concha ,Lobe, Auditory Canal and Tympanic Membrane (Eardrum)
• Both to locate sound sources and enhances some frequencies with respect to others. As a whole, the outer ear serves to modify the frequency response of incoming sound due to resonance effects, primarily of the auditory canal(25mm to 35mm)which has a resonant frequency in the region around 4kHz
THE UNIVERSITY OF MICHIGAN-DEARBORN 28 • Main Function of Each Component 1. Middle Ear: Consists of three bones known as ossicles, comprising the Malleus, Incus and Stapes-more commonly known as the Hammer, Anvil and Stirrup 2. Two Main Functions: 1. To transmit the movements from the eardrum to the fluid which fills the cochlea without significant loss of energy. 2.To protect the hearing system to some extent from the effects of loud sound.
THE UNIVERSITY OF MICHIGAN-DEARBORN 29 • Middle Ear Pressure Amplification PAL2 1 1 BF PAL1 2 2 where
AL11 13, 1.3 AL22 BF BucklingFactor 2 *Buckling Effect(Pickles,1982) ―Twofold increase in the force applied to the Malleus‖ • Total Pressure Amplification: 13x1.3x2=33.8 times
THE UNIVERSITY OF MICHIGAN-DEARBORN 30 Example 3: Express the pressure ratio between the stapes footplate and the tympanic membrane in decibels
Solution: The pressure ration is 33.8, the difference in dB will be:
dB 20log P 2/ P 1 20log(33.8) 30.6 dB
i.e. the sound pressure level will increase by approximate 31dB through the middle ear
THE UNIVERSITY OF MICHIGAN-DEARBORN 31 • Middle Ear Hearing Protection By the action of two muscles in the middle ear: the Tensor Tympani and the Stapedius Muscle. These Muscles contract automatically in response to sounds with levels greater than approximately 78dB(SPL) and they have the effect of increasing the impedance of the middle ear by stiffening the ossicular chain. It is called: ‖Acoustic reflex‖
Approximately 12 to 14 dB of attenuation can be achieved for the frequencies below 1 KHz
It takes about 60ms to 120 ms for the muscles to contract in response to a loud sound
THE UNIVERSITY OF MICHIGAN-DEARBORN 32 • Main Function of Each Component (Inner Ear) Inner Ear: Consists of snail-like structure known as the cochlea. It is to convert mechanical vibrations into nerve firings to be processed eventually by the brain The basilar membrane responds best to high frequencies where it is narrow and thin (at the base), and to low frequencies where it is wide and thick (at the apex)
THE UNIVERSITY OF MICHIGAN-DEARBORN 33 2.2 Critical Bands • How well the hearing system can discriminate between individual frequency components of an input sound. The band width at which subjective responses rather abruptly change. Scharf(1970) • Moore and Glasber(1983) Equivalent Rectangular Bandwidth (ERB) Equation
6 2 3 ERB{[6.23 10 fcc ] [93.39 10 f ] 28.52} Hz Example5.2 calculate the critical bandwidth at 200 Hz and 2000 Hz [6.23 106 200 2 ] [93.39 10 3 200] 28.52 47.5Hz [6.23 106 2000 2 ] [93.39 10 3 2000] 28.52 240Hz
THE UNIVERSITY OF MICHIGAN-DEARBORN 34 • Critical Bands Effects Beating: Difference within 12.5 Hz up to 15 Hz Rough: Difference from 15Hz to CB Smooth: Difference above CB
THE UNIVERSITY OF MICHIGAN-DEARBORN 35 2.3 Articulation Index-Speech Intelligibility ----A predicator of speech intelligibility in the presence of noise. (French and Steinbery,1947,ANSI 1969,Keyter, 1970 and Johnson, 1980) • Add 12dB to the speech level, minus the background noise (limited to 0-30dB), multiply the weighting factors, then divided by normalization constant: 10000. • Consider Octave band between 250 Hz to 4000 Hz or one-third Octave band between 200 Hz to 5000 Hz • The index is normalized in the range of 0 to 1, the higher articulation index, the better conversation environment.
THE UNIVERSITY OF MICHIGAN-DEARBORN 36 Example: Find the articulation index in the presence of a pink noise with a noise level contribution of 45 dB in each octave band. Assume a male speaker at typical voice level,1.0m from the listener Solution:
THE UNIVERSITY OF MICHIGAN-DEARBORN 37 2.3.1 AI Vs. ERPM at 2nd Gear WOT
THE UNIVERSITY OF MICHIGAN-DEARBORN 38
2.3.2 AI vs. ERPM at 1st Gear POT
THE UNIVERSITY OF MICHIGAN-DEARBORN 39
THE UNIVERSITY OF MICHIGAN-DEARBORN 40
2. Human perceived sound and noise Subjectively perceived by • Loudness • Frequency • Duration • Spectrum complexity and existence of pure tones • Amplitude and frequency of level fluctuation Objective Measures • Weighted sound pressure levels, A, C & D • Equal loudness level—Phons • Subjective loudness—Sones
• Perceived noise level(PNL)=LD+7(PNdB) • Equivalent sound level (Leq) • Preferred speech interference level (PSIL)
PSIL=(L500+L1000+L2000)/3
THE UNIVERSITY OF MICHIGAN-DEARBORN 41 Weighting Function -- A frequency bais scale
THE UNIVERSITY OF MICHIGAN-DEARBORN 42 Characteristics of Weighting Scales
A: Low Intensity Loudness, Hearing Damage Annoyance
B: Medium Intensity Loudness (obsolete)
C: High Intensity Loudness
D: Annoyance (Air-craft flyover noise)
E: Linear
THE UNIVERSITY OF MICHIGAN-DEARBORN 43 THE UNIVERSITY OF MICHIGAN-DEARBORN 44 2.1 Loudness Level---Phons
THE UNIVERSITY OF MICHIGAN-DEARBORN 45 2.2 Subjective Loudness---Sones
THE UNIVERSITY OF MICHIGAN-DEARBORN 46 2.3 Loudness of Broadband Noise ---Steven Mark VI Method
N Loudness0.7 S max 0.3 ( Si ) i1
Where Smax: Highest Loudness Index Si: Loudness Index of each octave band N: Total octave band sources
THE UNIVERSITY OF MICHIGAN-DEARBORN 47 THE UNIVERSITY OF MICHIGAN-DEARBORN 48 Exercise An Octave Band Source
Si 250 Hz 55dB 2.53
500 Hz 60dB 4.10 1000 Hz 65dB 6.60 2000 hz 72dB 11.80=(S ) max S=0.7x11.8+0.3x(2.53+4.10+6.6+11.8) =8.26+7.51 =15.80 sones
---16 times louder than a source at 40dB
THE UNIVERSITY OF MICHIGAN-DEARBORN 49 2.4 Annoyance--- Perceived Noisiness ----Based on five features 1. Spectrum content and level 2. Spectrum complexity and existence of pure tones 3. Duration 4. Amplitude and frequency of level fluctuation 5. Rise time of impulsive sound
Generate equal loudness contour
THE UNIVERSITY OF MICHIGAN-DEARBORN 50 THE UNIVERSITY OF MICHIGAN-DEARBORN 51 (b) Radiated Sound Fields ----- Five types of sound fields * Near field: R ≤ λor R≤ 2D * Far field: R>>λ, or R/λ>>(L/λ)2
* Free field: ΔL p= 6dB per doubling distance * Reverberant field: reflection exist * Diffuse field: uniform distribution in a reverberant field
THE UNIVERSITY OF MICHIGAN-DEARBORN 52 THE UNIVERSITY OF MICHIGAN-DEARBORN 53 3.2 Near and Far Field Characteristics (a) Near Field *Individual sources are distinct. *Sound intensity varies both in magnitude and direction at the distance from the source 2 W Prms IIIIII2 ,, 1 2 3 4Rc0 (b) Far Field *Individual sources merged *Radiation pattern near uniform, plannar *Intensity depends primarily on distance
2 prms W IIIIt 2 1 2 3 0cR4
THE UNIVERSITY OF MICHIGAN-DEARBORN 54 3.3 Free and Reverberant Field Characteristics
(a) Free Field * Dominated by the sound source * Intensity depends only on distance, -6dB per doubling the distance
(b) Reverberant Field * Dominated by the reflected sound * Standing wave may exist causing variation in positions
THE UNIVERSITY OF MICHIGAN-DEARBORN 55 THE UNIVERSITY OF MICHIGAN-DEARBORN 56 3.4 Anechoic Chamber and Reverberant Chamber
* Anechoic Chamber • Simulate a free field condition • 99% absorption coefficient by wedged form or fiber glass • Determine the directional properties, sound power of a noise source
* Reverberant Chamber • Simulates a diffuse field condition • 100% reflective by hard rigid wall • Non—parallel walls or with rotating diffusing vanes • Determine sound power, absorption properties and transmission loss
THE UNIVERSITY OF MICHIGAN-DEARBORN 57 Full Anechoic Chamber
THE UNIVERSITY OF MICHIGAN-DEARBORN 58 Full Anechoic Chamber
THE UNIVERSITY OF MICHIGAN-DEARBORN 59 Full Anechoic Chamber with open Floor
THE UNIVERSITY OF MICHIGAN-DEARBORN 60 Absorption Foam Wedges
THE UNIVERSITY OF MICHIGAN-DEARBORN 61 Reverberation (Diffuse) Room
THE UNIVERSITY OF MICHIGAN-DEARBORN 62 Reverberation Room with Cylindrical Walls
THE UNIVERSITY OF MICHIGAN-DEARBORN 63 Reverberation Room with Spherical Ceiling
THE UNIVERSITY OF MICHIGAN-DEARBORN 64 Building Wall STL Testing
THE UNIVERSITY OF MICHIGAN-DEARBORN 65 Reverberation Room with Reflectors
THE UNIVERSITY OF MICHIGAN-DEARBORN 66 3.5. Sound Intensity Analysis
3.5.1 Sound Intensity Definition Sound Intensity is a vector, which defines both magnitude and direction of flow of acoustic energy at a given position For a spherical source in a free field 2 W Prms Irr Pu 4rC2 0 where: W=Acoustic Power, Watt r = distance , m
Prms=root –mean-square pressure ρ0= density of the medium C= speed of sound P( t,r)=Acoustic pressure function u( t,r)=Acoustic velocity
THE UNIVERSITY OF MICHIGAN-DEARBORN 67 Relationship between Sound Power and Sound Intensity
THE UNIVERSITY OF MICHIGAN-DEARBORN 68 3.5.2 Sound Intensity Field • The distribution of the sound intensity
THE UNIVERSITY OF MICHIGAN-DEARBORN 69 THE UNIVERSITY OF MICHIGAN-DEARBORN 70 THE UNIVERSITY OF MICHIGAN-DEARBORN 71 3.5.3 Measuring Sound Intensity • Why Sound Intensity ―Sound intensity distinguishes between active propagating sound reactive (non-propagating) sound.‖ ―Sound intensity ignores standing waves.‖
• When to use Sound Intensity ―Sound intensity is best for investigating the causes noise and identifying sources‖ ―Sound pressure is used for investigating the effects of noise.‖
THE UNIVERSITY OF MICHIGAN-DEARBORN 72 Sound Pressure Level Method
• Using ten microphones at designated positions of a 1.0 meter radius semi- sphere. • Average all ten SPL readings to get overall average SPL. • Sound intensity level equals to SPL.
THE UNIVERSITY OF MICHIGAN-DEARBORN 73 • Two microphone Method Sound Intensity Probe
THE UNIVERSITY OF MICHIGAN-DEARBORN 74 Calculation of Sound Intensity
By definition of sound intensity 1 T Irr pu dt T 0 where p instantaneous pressure at a po int.
ur air particle velocity in the r direction and
tt 1p 1 ( pBA p ) ur dt rr0 If the pressure is replaced by average pressure, intensity becomes
TT 1 (pABBA p ) ( p p ) Ir dt) Tr002 where mass density of air r spacing between points A and B
PAB and P instantaneous acoustic pressure at points A and B T averaging time
THE UNIVERSITY OF MICHIGAN-DEARBORN 75 Sound Intensity Measurement
THE UNIVERSITY OF MICHIGAN-DEARBORN 76 Sound Intensity Probe
THE UNIVERSITY OF MICHIGAN-DEARBORN 77 Sound Intensity Mapping
526e-9
2
m
/ W
0.00 PU sound probe + calibrator
½” PU probe Real time sound intensity scanning 79 Near Field Calibrator
High Frequencies Low frequencies
80 Case Study: Over-Killed Sound Intensity Analysis Test Method and Preparation
Case Study: Over-Killed Sound Intensity Analysis Test Method and Preparation Outline
• All glass windows of the vehicle were covered by EVA heavy layer (5.0 kg/m2) and thick felt (25 mm) to block the flank paths into the interior. • Thick light weight foam (1.5m x2.5mx15cm) was used to build a free field inside the vehicle. • Mark grid points evenly distributed on the surface to be measure sound intensity. • Measure sound intensity at constant rpm. • Display overall average sound intensity mapping from 400 to 5000 Hz of each key surface . • Analyze the sound intensity map to determine the weak points of each surface.
Front Dash Grids
Case Study Over-Killed Sound Intensity Analysis Inner Dash Sound Intensity Mapping
A-Pillar Symmetric plane Case Study : Over-Killed Sound Intensity Analysis Inner Dash Sound Intensity Mapping
• The strong sound intensity is distributed in the lower portion of the dash, especially in the lower left corner. • Provide better seals in A-pillars and some blockages under the IP panel to reduce the noise transmission are recommended.
Back Door Grid Case Study: Over-Killed Sound Intensity Analysis Back Door Sound Intensity Mapping 3.5.4 Sound Intensity Applications
• Sound power Determination • Source Location -source ranking -source mapping • Transmission Loss • Absorption • Radiation Efficiency
THE UNIVERSITY OF MICHIGAN-DEARBORN 88 Acoustics Modes in an Enclosure
THE UNIVERSITY OF MICHIGAN-DEARBORN 89 Acoustic Modes of a Rectangular Box
THE UNIVERSITY OF MICHIGAN-DEARBORN 90 Acoustic Modes of a Full Size Van
THE UNIVERSITY OF MICHIGAN-DEARBORN 91
4. Measurement and Analysis of Sound
4.1 Measuring and displaying acoustical data
4.2 Microphones
4.3 Sound pressure level meter, weight scales
4.4 Correlation between vibration & sound
THE UNIVERSITY OF MICHIGAN-DEARBORN 93 4.1 Measuring and Displaying Acoustic Data
*Time Domain
*Frequency Domain -- Narrow Band -- Octave Band
THE UNIVERSITY OF MICHIGAN-DEARBORN 94 A. Octave (constant percentage) Band Spectrum • The bandwidth of each spectral line is a fraction (percentage) of central frequency • When upper limit frequency of the frequency band is equal to twice of the lower limit frequency is called 1 octave
1 octave: f2=2f1
• And B=f2-f1=0.707f0 ff 2 20 ff /2 10 • Poor resolution in the high frequency range
THE UNIVERSITY OF MICHIGAN-DEARBORN 95 B. Constant Band Spectra
• The bandwidth of every spectral line is a constant
B X Hz MaxFreq ,n spectral Lines n
• Same resolution across the entire spectrum
THE UNIVERSITY OF MICHIGAN-DEARBORN 96 THE UNIVERSITY OF MICHIGAN-DEARBORN 97 THE UNIVERSITY OF MICHIGAN-DEARBORN 98 THE UNIVERSITY OF MICHIGAN-DEARBORN 99 THE UNIVERSITY OF MICHIGAN-DEARBORN 100 THE UNIVERSITY OF MICHIGAN-DEARBORN 101 THE UNIVERSITY OF MICHIGAN-DEARBORN 102 THE UNIVERSITY OF MICHIGAN-DEARBORN 103 C. Series and Parallel Process Series Process • Perform Fourier Transformation for all spectral frequency under one frequency at a time basis • e.g. FFT analyzer
Parallel Process • Perform Fourier Transformation for all octave or 1/3- octave band spectral frequencies simultaneously • e.g. octave (real-time) analyzer
THE UNIVERSITY OF MICHIGAN-DEARBORN 104 THE UNIVERSITY OF MICHIGAN-DEARBORN 105 THE UNIVERSITY OF MICHIGAN-DEARBORN 106 THE UNIVERSITY OF MICHIGAN-DEARBORN 107 4.2 Microphones • Definition: Converts sound pressure fluctuation into proportional voltage fluctuation. • Types of microphone by construction A. Capacitive microphone (condenser microphone) has both polarized (200 volt) or non-polarized (electret microphone) (using a coated plastic sheet that has a conductive coating on one side.) B. Crystal microphone uses a piezoelectric-type element generally activated by bending. C.Electrodynamic microphone uses the principle of the moving conductor in a magnetic field. The field is commonly provided by a permanent magnet.
THE UNIVERSITY OF MICHIGAN-DEARBORN 108 Schematic of a Condenser Microphone
THE UNIVERSITY OF MICHIGAN-DEARBORN 109 Schematic of Electric Dynamic Microphone
• Use the principle of the moving conductor in a magnetic field. • The field is commonly provided by a permanent magnet. • The inductive voltage generated by the moving coil which is attached to the diaphragm is proportional to the acoustic pressure.
THE UNIVERSITY OF MICHIGAN-DEARBORN 110 Types of Microphones by Applications
- Free Field (Frontal) microphone - Pressure Field microphone - Random Field microphone
* Frequency Sensitivity - Max. frequency range - Stability & reliability frequency range
THE UNIVERSITY OF MICHIGAN-DEARBORN 111 Multi-Field Microphone
THE UNIVERSITY OF MICHIGAN-DEARBORN 112 Microphone Selection Factor
An ideal microphone would have the following features: 1. Flat frequency response over the audible range, or application range. 2. Predictable, repeatable sensitivity over the complete dynamic range. 3. Low internal system noise level. 4. Minimum dimensions and weight 5. Low environmental impacts.
THE UNIVERSITY OF MICHIGAN-DEARBORN 113 Type of Microphones and Blockage of Microphone Size
THE UNIVERSITY OF MICHIGAN-DEARBORN 114 THE UNIVERSITY OF MICHIGAN-DEARBORN 115 THE UNIVERSITY OF MICHIGAN-DEARBORN 116 THE UNIVERSITY OF MICHIGAN-DEARBORN 117 4.3.1 Sound Pressure Level Principle
THE UNIVERSITY OF MICHIGAN-DEARBORN 118 THE UNIVERSITY OF MICHIGAN-DEARBORN 119 Equivalent Sound Pressure Level (Leq)
• It is obtained by averaging the mean- square sound pressure over the desired time interval and convert back to decibels.
THE UNIVERSITY OF MICHIGAN-DEARBORN 120 Sound Exposure Level (SEL)
• It is used to measure the transient noise, such as automobile pass-by, impact nise caused by forging operation. • It is also a time average of sound pressure level for T equals 1 second time period.
• Compare between Leq and SEL
• The difference could be significant, if T is large.
THE UNIVERSITY OF MICHIGAN-DEARBORN 121
4.4 Correlation between Vibration & Sound
-- Structure-borne and Air-borne Noise
-- Same resonant frequencies and dominated on the frequency spectrum
THE UNIVERSITY OF MICHIGAN-DEARBORN 122