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164 Gas Turbine Handbook: Principles and Practices
Figure 10-7. Typical Duct Wall Cross Section
There are no published algorithms or simple equations for
calculating the noise reduction double wall designs can achieve but
there are a limited number of papers 16 that address the design issues
and offer limited design tools; reference [17] provides excellent
resources as well. In most cases the candidate wall system is built
and tested to verify performance 18 and most OEMs and Noise Control
Engineers working in the field have extensive databases on duct
wall performance. And the orientation of the wall does not affect TL
performance; the thinner liner sheet can be on the outside or the
inside depending on design latitudes or requirements. In acoustics,
this is referred to as the Law of Reciprocity.
Certain mass properties of the shell plate must also be carefully
evaluated on the basis of their application. A well-known phenomenon
occurs when the stiffness-bending wavelength of the shell
plate is coincident with an excitation or forcing frequency such as
the inlet bpf. This coincidence frequency (fc) results in a dramatic
reduction in the duct wall transmission loss at that frequency because
of the matching of the two wavelengths—the airborne to the
structure where the wall becomes nearly “transparent” acoustically.
This frequency may be estimated by the following equation 17
fc = c o 2
2É • ph B
1/2
Hz
(10-8)
and
B =
Eh 3
12 1–ν 2 (10-9)