TABLE OF CONTENT 1.0 INTRODUCTION 1 The concept of sound 1 2.0 GENERAL PROPERTIES OF SOUND 3 2.1 frequency 3 2.2 Wavelength 4 2.3 Amplitude and intensity 5 2.4 Speed of sound in various media 6 3.0 DECIBEL LEVELS 7 4.0 SOUND QUALITY 8 5.0 BEHAVIOR OF SOUND WAVES 9 5.1 Reflection 9 5.2 Refraction 9 5.3 Diffraction 10 5.4 Echo 10 5.5 Reverberation 11 6.0 PRODUCTION OF SOUND WAVES 12 6.1 Musical instruments 12 6.2 Electronic amplification 12 6.3 The human voice 12 7.0 TYPES OF ORDINARY SOUND 13 7.1 Speech 13 7.2 Noise 13 7.3 Music 13 8.0 CONCLUSION 13 9.0 REFERENCES 13 Analysis of Sound ii 1.0 INTRODUCTION THE CONCEPT OF SOUND Sound is a disturbance of mechanical energy that propagates through matter as a longitudinal wave; it is a mechanical wave because it is characterized by these three attributes. First, there is a medium which carries the disturbance from one location to another. Typically, this medium is air; though it could be any material such as water or steel. The medium is simply a series of interconnected and interacting particles. Second, there is an original source of the wave, some vibrating object capable of disturbing the first particle of the medium. The vibrating object which creates the disturbance could be the vocal chords of a person, the vibrating string and sound board of a guitar or violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a radio speaker. Third, the sound wave is transported from one location to another by means of the particle interaction. If the sound wave is moving through air, then as one air particle is displaced from its equilibrium position, it exerts a push or pull on its nearest neighbors, causing them to be displaced from their equilibrium position. This particle interaction continues throughout the entire medium, with each particle interacting and causing a disturbance of its nearest neighbors. Humans perceive sound by the sense of hearing. By sound, we commonly mean the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider definition of sound that includes low and high frequency vibrations in air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases, liquids and solids. The scientific study of sound is called acoustics. Analysis of Sound 1 The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air. Regardless of the source of the sound wave - whether it be a vibrating string or the vibrating tines of a tuning fork - sound is a longitudinal wave. And the essential characteristic of a longitudinal wave which distinguishes it from other types of waves is that the particles of the medium move in a direction parallel to the direction of energy transport. A propagating sound wave consists of alternating compressions and rarefactions which are detected by a receiver as changes in pressure. Structures in our ears, and also most man-made receptors, are sensitive to these changes in sound pressure (Richardson et al.1995, Gordon and Moscrop 1996). Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave. Sound is characterized by the properties of sound waves, which are frequency, wavelength, period, amplitude, and speed. If a detector, whether it be the human ear or a man-made instrument is used to detect a sound wave, it would detect fluctuations in pressure as the sound wave impinges upon the detecting device. At one instant in time, the detector would detect a high pressure; this would correspond to the arrival of a compression at the detector site. At the next instant in time, the detector might detect normal pressure. And then finally a low pressure would be detected, corresponding to the arrival of a rarefaction at the detector site. Since the fluctuations in pressure as detected by the detector occur at periodic and regular time intervals, a plot of pressure vs. time would appear as a sine curve. The crests of the sine curve correspond to compressions; the troughs correspond to rarefactions; and the "zero point" corresponds to the pressure which the air would have if there were no disturbance moving through it (see figure 3) Analysis of Sound 2 2.0 GENERAL PROPERTIES OF SOUND Sound as a longitudinal wave is characterized by several features which are esoteric to longitudinal waves. Any simple sound, such as a musical note, may be completely described by specifying three perceptual characteristics: pitch, loudness (or intensity), and quality (or timbre). These characteristics correspond exactly to three physical characteristics: frequency, amplitude, and harmonic constitution, or waveform, respectively. 2.1 FREQUENCY. The frequency of a sound wave is the rate of oscillation or vibration of the wave particles (i.e. the rate amplitude cycles from high to low to high, etc.) The hertz (Hz) is a unit of frequency equaling one vibration or cycle per second (1 Hertz = 1 vibration/second). The Audible frequency range: the human ear is capable of detecting sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound. Such vibrations reach the inner ear when they are transmitted through air. The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through the medium. This frequency is measured as the number of complete back-and-forth vibrations of a particle of the medium per unit of time. If a particle of air undergoes 1000 longitudinal vibrations in 2 seconds, then the frequency of the wave would be 500 vibrations per second. As a sound wave moves through a medium, each particle of the medium vibrates at the same frequency. Subsequently, a guitar string vibrating at 500 Hz will set the air particles in the room vibrating at the same frequency of 500 Hz which carries a sound signal to the ear of a listener which is detected as a 500 Hz sound wave. Analysis of Sound 3 Sound Frequency and Pitch: The sensations of these frequencies are commonly referred to as the pitch of a sound. A high pitch sound corresponds to a high frequency and a low pitch sound corresponds to a low frequency. Amazingly, many people, especially those who have been musically trained, are capable of detecting a difference in frequency between two separate sounds which is as little as 2 Hz. When two sounds with a frequency difference of greater than 7 Hz are played simultaneously, most people are capable of detecting the presence of a complex wave pattern resulting from the interference and superposition of the two sound waves. Certain sound waves when played (and heard) simultaneously will produce a particularly pleasant sensation when heard, they are said to be consonant. 2.2 WAVELENGTH Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave (see figure 4). A wave of very high frequency would produce a corresponding short wave length while a wave of low frequency would produce a large wavelength. Thus, a frequency of 20 Hz, at the bottom end of human audibility, has a very large wavelength: 56 ft (17 m). The top end frequency of 20,000 Hz is only 0.67 inches (17 mm). The wavelengths of sounds in the range of human audibility are comparable to the size of ordinary objects. To block out a sound wave, one needs something of much greater dimensions—width, height, and depth—than a mere cloth curtain. A thick concrete wall, for instance, may be enough to block out the Analysis of Sound 4 waves. Better still would be the use of materials that absorb sound, such as cork, or even the use of machines that produce sound waves which destructively interfere with the offending sound. 2.3 AMPLITUDE AND INTENSITY Amplitude is critical to the understanding of sound, though it is mathematically independent from the parameters so far discussed. Amplitude is defined as the maximum displacement of a vibrating material from its mean or rest position, amplitude is the "size" of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity, commonly known as "volume," which is the rate at which a wave moves energy per unit of a cross-sectional area. The greater the amplitude of the wave, the harder the molecules strikes the eardrum and the louder the sound that is perceived. The amplitude of a sound wave can be expressed in terms of absolute units by measuring the actual distance of displacement of the air molecules, the changes in pressure as the wave passes, or the energy contained in the wave. Intensity of a sound wave is the amount of energy which is transported past a given area of the medium per unit of time. The greater the amplitude of vibrations of the particles of the medium, the greater the rate at which energy is transported through it, and the more intense that the sound wave is.