Page 47 - Master Handbook of Acoustics

This value is called the sound intensity level (IL in decibels) and differs from intensity (I in
2
watts/m ). Using decibels is a convenience, and decibel values more closely follow the way we hear
the loudness of sounds.

Questions sometimes arise when levels other than intensity need to be expressed in decibels.
Equation (2-2) applies equally to acoustic intensity, as well as acoustic power, electric power, or
any other kind of power. For example, we can write the sound-power level as:

where PWL =
sound-power level, dB
W =
sound power, watts
W =
ref
–12
a reference power, 10 W

Sound intensity is difficult to measure. Sound pressure is usually the most accessible parameter to
measure in acoustics (just as voltage is for electronic circuits). For this reason, the sound-pressure
level (SPL) is often used. SPL is a logarithmic value of the sound pressure, in the same way that the
sound intensity level (IL) corresponds to sound intensity. SPL is approximately equal to IL; both are

often referred to as the sound level. Acoustic intensity (or power) is proportional to the square of the
acoustic pressure p. This slightly alters the defining equation that we use. When the reference
pressure is 20 μPa (micropascals), a sound pressure p measured in micropascals has an SPL of:

where SPL =
sound-pressure level, dB

p =
acoustic pressure, μPa or other
p =
ref
acoustic reference pressure, μPa or other

The tabulation of Table 2-2 shows whether Eqs. (2-2) or (2-3), or Eq. (2-4) applies.