Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 159
the panels should not exceed 60 m/s (200 fps) for exhaust applications
to limit self noise, aerodynamic turbulence, and fill loss; or, 30
m/s (100 fps) for inlet applications which is mainly for low pressure
drop and balanced air flow into the turbine compressor. The reader is
referred to reference [13] for an excellent overview on silencer design
and pressure drop.
Frequently, the specifying design agency may call for a silencer
to achieve a certain dynamic insertion loss (DIL) in decibels. By dynamic,
it is meant that the affects of self-noise are to be considered
in the design process. On occasion a specification may be written only
calling for a DIL but the flow rate is needed in order to calculate the
gas velocity between the silencer panels and the sound power level
(ahead of the silencer section) is needed to determine the system
DIL. Now, here’s the surprise. No one can measure the DIL without
performing an extensive and very expensive on-site operation that
would require the removal and re-installation of the silencers in the
duct system. The reason is the definition of ( dynamic) insertion loss.
It is the measure of the difference in sound level with and without
the silencer panels in place. Typically, what can be measured is the
relative attenuation of the silencer system as illustrated in the following
figure where Lp is the sound level at numbered locations and
the bottom duct shows the silencers in the same duct.
In Figure 10-5 the following holds (assuming the noise source
is to the left):
Figure 10-5. Silencer Acoustical Terminology