Sound Waves and Perceptible Dynamic Range: What Sounds the System Can Detect
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Probably the most important concept covered today is the measurement of Sound Pressure Level (SPL).
Sound is merely the perception of pressure waves transmitted through a matter medium such as air. These waves, like any waves, have frequency, which is perceived as pitch, and amplitude, which is perceived as loudness or volume.
The amplitude or loudness of a sound is a function of the amount of energy the wave carries with each cycle.
Because sound waves are pressure waves, the energy a wave carries is related to the maximum pressure the wave reaches at a peak in its cycle (P).
As you hopefully know by now, Sound Pressure Level (SPL) is measured in units called decibels, abbreviated as dB. (For the absurdly curious, the original unit was the Bel, but this proved to be too unweildy and so folks interested in sound began to favor the tenth portion of a Bel, whence the name "decibel.")
Decibels are not absolute units like grams or centimeters. They are always used in the context of comparing one sound to another. Frequenlty, a sound may be referred to a reference pressure, P0.
Decibels are calculated on a logarithmic scale. This is the equation:
SPL = 20 * log [ P / P0 ]
Please note that the term "log" refers to the BASE 10 logarithm, and NOT to the natural log, "ln." If you know someone who tries to use the natural logarithm in this equation, please point at that person and call him or her a silly person.
If you use ln instead of log on the exam, you will get the question wrong, and little if any partial credit will be given. You have been warned.
Note that not only the Pressure, P, but also the Intensity I of a sound can be used to calculate the SPL. Here is the equation for calculating SPL from sound Intensity. Please note that it is NOT the same as the previous equation. Those who use the wro ng equation do not get fruit cup:
SPL = 10 * log [ I / I0 ]
Good rules to remember when dealing with SPLs:
log (A*B) = log (A) + log (B)
log (A/B) = log (A) - log (B)
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