ABSTRACT
A sound wave compresses and dilates the elastic medium it passes through, generating associated pressure fluctuations. The minimum fluctuation, to which the ear responds, is extremely small; e.g., at a frequency of 1000 Hz, the just noticeable effective air pressure amplitude, or in other words the root mean square (RMS) value, is approximately 20 u,Pa, i.e., less than 10~9 of the standard atmospheric pressure (equal to ~1000 hPa=105 Pa). The limit of danger followed by the threshold of pain corresponds to effective air pressure amplitude one million (106) times larger, but still less than onethousandth of the atmospheric pressure [20],
Because of this wide range of acoustic pressure amplitudes, it has become conventional to specify the sound pressure level (SPL), Lp, in terms of a decimal logarithm with the (dimensionless) unit of the decibel (dB)
Lp = 20log,0-£- (11.2) Po
where po = 20 jxPa. Another quantity, which is often used, is the sound intensity level, L/, defined in decibel as
I*=10l0g IO! (11.3) K>
The reference in this case is the sound intensity /0 = 10" l2 W/m2. For a free progressive acoustic wave in
air, SPL and sound intensity level are approximately equal.
