Gas Turbine Handbook : Principles and Practices

160 Gas Turbine Handbook: Principles and Practices
• The insertion loss (dynamic if under flow conditions): IL = Lp2
– Lp4
• The noise reduction is: NR = Lp3 – Lp4 or Lp5 – Lp6
• The transmission loss (TL) is: Same as NR but assumes Lp4 is
anechoic (free from echoes and reverberations)
• Lp1 ≠ Lp3 because of the presence of the silencer
And it is known that IL ≠ NR ≠ TL (but they are usually close).
The NR is measured at convenient locations upstream and downstream
of the silencers. TL is measured in the same manner as NR
but the downstream end must have an anechoic termination; that is,
no sound reflection at the end of the duct—the sound is completely
absorbed. In measuring the NR and IL, the duct geometry and
length play an important role as they affect performance but TL is
less affected by the duct geometry since the duct termination must
be anechoic. Most all field verification measurements are NR under
no flow conditions; while measurements under flow conditions are
possible there is variability and uncertainty involved in the process.
So, when specifying silencer performance alone be sure to specify
the conditions under which the performance is to be evaluated or
verified. In certain cases smaller scaled model silencers or full scale
can be tested in a laboratory to verify acoustical and aerodynamic
performances. 14
A typical cross section of a silencer panel is shown in the following
figure. Shown are the outer perforated sheet, a screen mesh
or cloth used to retain the acoustical filler material and the center
filled with the acoustical material. The choice of the perforated sheet
open area ratio, the screen mesh or cloth, and the acoustical filler
material all effect performance.
Figure 10-6. Typical Silencer Partial Cross Section