Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 149
SPL = Lp =10Log P Pr
2
=20Log P
Pr dB (10-1)
where
P = sound pressure measured in Pascals (N/m 2 ), and
Pr = the reference pressure of 20 x 10 –6 Pa or 20μPa.
In these equations the logarithm is to the base 10.
Sound power is the rate at which acoustic energy is emitted.
Sound power level (PWL) is described by:
PWL = Lw =10Log W Wr dB (10-2)
where
W = calculated acoustic power in watts, and
Wr = the reference power of 1 x 10 -12 watts.
In general noise control applications we work strictly with the
decibel levels but one must be aware that on occasion, listed sound
power levels may use an old reference value of 1 x 10 -13 watts, and
when converting to current standard, results in a ten-decibel reduction.
Therefore, when specifying equipment sound power levels be
sure to use the correct reference level, otherwise you can have a ten
decibel deficiency in the design.
Sound power can only be calculated from a set of sound pressure
level measurements where a microphone measures the sound
pressure over a given area that completely envelops the equipment
being measured. For instance, a piece of equipment setting on the
ground, measures 1m x 1m x 1m (5 square meters exposed surface
area) but measuring the sound pressure level on a rectangular grid
one meter away results in an area of 33 square meters. If the area
averaged adjusted sound pressure level is 85 dB, then the sound
power is:
Lw = Lp + 10 LOG (Area) = 85 + 10 LOG(33) = 100 dB (10-3)
This is a simplified demonstration but shows that when a supplier
states his equipment does 85 dB at one meter that there is