In a sound wave there are extremely
small periodic variations in atmospheric pressure to which our ears respond in a rather
complex manner. The minimum pressure fluctuation to which the ear can respond is less than
one-billionth (10-9) of atmospheric pressure.
Because of the wide range of pressure
stimuli, it is convenient to measure sound pressures on a logarithmic scale, called the decibel
(dB) scale. Although a decibel scale is actually a means for comparing two
sounds, we can define a decibel scale of sound level by comparing sounds to a reference
sound with a pressure amplitude Po = 2 ´ 10-5 (N/m2) assigned a sound pressure
level of 0 dB. Thus, we define sound pressure level (SPL or Lp) as:
(12)