Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 165
where,
fc = coincident frequency,
c o = speed of sound in air (or exhaust gas),
B = bending stiffness,
ρ _ density of material,
h = thickness of plate,
E = Young’s modulus,
ν = poisons ratio.
The critical frequency plays an important parameter when calculating
the wall TL for inlet ducts because, as was shown in Figure
10-1, one needs to avoid blade passing frequencies. By examining this
figure it is evident that just given an overall sound level of 120.5
dB is not sufficient to adequately design an inlet duct wall, thus it
is important to know the principal frequencies of interest in critical
applications so always ask for those frequencies.
The duct wall free field sound power level, which is needed to
calculate far field noise levels per equation (10-4), is as follows:
Lw = Lw(i) – (TL + 6) + 10 Log (S/A) dB (10-10)
where:
Lw(i) = the sound power level inside the duct
TL = the transmission loss of the duct wall
+ 6 = accounts for free field acoustical radiation conditions
S = the duct exterior surface area (sq. meters)
A = the duct cross sectional gas path area (sq. meters)
The purpose of this equation is to demonstrate how duct geometry
affects the duct wall’s sound power level. The goal is to maximize
the TL of the wall and the cross area of the duct while at the same
time try and minimize the surface area. This is an iterative process
and the goal again is to balance acceptable noise emissions while
minimizing material cost. Note that the term (TL + 6) is also known
as NR, the wall noise reduction.