Gas Turbine Handbook : Principles and Practices

148 Gas Turbine Handbook: Principles and Practices
For example, an inlet having 35 blades spinning at 60 Hz will have
a fundamental bpf of 2,100 Hz with harmonics as illustrated.
Inlets have the advantage that usually there is some type of air
conditioning or filtration system that is necessary and can provide a
great reduction to the inlet noise, frequently up to a 20 decibel reduction.
These include, evaporative coolers, moisture separators, chiller
coils, and an inlet filter system, a barrier or self-cleaning (pulse type)
filter.
The exhaust also produces tones but because of the amount
of silencing needed to reduce the high aerodynamic turbulence and
combustion noises the tones are seldom an issue. This principally
broadband noise is the main concern and passive silencers generally
work well.
SOME FUNDAMENTALS OF ACOUSTICS
Acoustical Notations
The following are some common sound descriptors used in
acoustics.
L P or L p is the symbol used to describe a spectral sound pressure level
in decibels (dB).
L A is the symbol used to describe the overall A-weighted sound level
in decibels.
L W is the symbol used to describe the spectral sound power level in
decibels.
L WA is the symbol used to describe the overall A-weighted sound power
level in decibels.
dB(A) or dBA is not promoted in usage anymore (or any other suffixes
attached to “dB”).
Sound Pressure and Sound Power
There are two elements of sound that we work with: sound
pressure level and sound power level as measured and reported in
decibel levels. Sound pressure is what is heard and what is measured
using a sound level meter. Sound pressure is a measure of the distribution
of sound power in direction and distance from a source of
noise. Sound pressure level (SPL) is a decibel scale where: