Notes On Sound And Noise

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Sound is the variation of pressure in an elastic medium (such as air) caused by a mechanical disturbance. The movement of molecules in a transmitting medium caused by a disturbance results in pressure oscillations above (compressions) and below (rarefactions) the ambient pressure level. The oscillation of sound pressure variations is referred to as a sound wave. A sound wave will move through an ideal medium longitudinally and omnidirectionally. The oscillation frequency of a sound wave determines the pitch of a sound, which is measured in Hertz (Hz). The healthy human ear can typically discern frequencies between 20 and 20,000 Hz. The amplitude of the pressure oscillations of a sound wave describes the amount of
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Hard surfaces such as smooth wood or metal often reflect sound waves well but attenuate sound poorly. Conversely, soft, porous surfaces usually reflect sound waves poorly but attenuate sound waves well. Reflected sound waves can increase the overall SPL in an area, as the reflected wave essentially becomes an additional sound source. Sound behaves predictably in the far-field (i.e., far enough away from individual sound sources relative to the size of the sound sources) and most sound measurement equipment can only accurately measure SPLs in the far field. The measurement of sound generated by a source within a reflective chamber (reverberant-field) is more difficult because the large number of reflections increase the overall SPL if the measurement is also made within the reflective chamber. SPLs measured in the reverberant-field of the ear cannot directly be compared to SPLs in the free-field without first compensating for the higher SPLs at certain frequencies caused by the resonance of the ear canal.
Audition, or the perception of sound, is accomplished via the complex transformation from sound pressure waves to electrical nerve impulses and interpretation in the brain. The human ear is divided into three main parts; the outer ear, the middle ear, and the inner ear (see Figure 2.1). The outer ear consists of the pinna (or auricle), which is the visible portion of
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