flers (different baffle dimensions) may be stacked
ations are known as "dissipative" mufflers. As the
in series to provide maximum insertion loss over a
name implies, dissipative mufflers are made up of
broad frequency range.
various arrangements of sound absorbent material,
which actually absorbs sound energy out of the
(1) When large amounts of loss are required,
moving air or exhaust stream. The most popular
baffles are installed at close spacings with perhaps
configuration is an array of "parallel baffles" placed
only 30 to 50 percent open air passage through the
in the air stream. The baffles may range from 2-in.
total muffler cross section. This, in turn, produces
to 16-in. thick, and are filled with glass fiber or
a high pressure drop in the flow, so the final muf-
mineral wool. Under severe uses, the muffler ma-
fler design represents a compromise of cost, area,
terial must be able to withstand the operating tem-
length, pressure drop, and frequency response.
perature of the air or gas flow, and it must have
Pressure drop of flow through the muffler can usu-
adequate internal construction and surface protec-
ally be reduced by fitting a rounded or pointed end
tion to resist the destruction and erosion of high-
cap to the entrance and exits ends of a baffle.
speed, turbulent flow. These mufflers should be ob-
(2) The side walls of the chamber that contains
from an
tained
experienced,
reputable
the muffler must not permit sound escape greater
manufacturer to insure proper quality of materials,
than that which passes through the muffler itself.
design, workmanship, and ultimately, long life and
Thus, the side walls at the noisy end of the muffler
durability of the unit. Dissipative mufflers are di-
should have a TL at least 10 dB greater than the
vided here into two groups: the special custom-
insertion loss of the muffler for each frequency
designed and constructed mufflers for gas turbine
band. At the quiet end of the muffler, the TL of the
engines and other heavy-duty applications, and
side walls can be reduced to about 10 dB greater
ventilation-duct mufflers that are stock items man-
than one-half the total insertion loss of the muffler.
ufactured and available from several companies.
(3) In the contract specifications, the amount
of insertion loss that is expected of a muffler should
of a gas turbine is usually strong in the high-
be stated so that the muffler manufacturer may be
frequency region and is caused by the blade pas-
held to an agreed-upon value. It is more important
sage frequencies of the first one or two compressor
to specify the insertion loss than the dimension and
stages of the turbine. Thin parallel baffles of ap-
composition of the muffler because different manu-
proximately 4-in. thickness, with 4-in. to 6-in. air
facturers may have different, but equally accepta-
spaces between baffles, are quite effective in
ble, fabrication methods for achieving the values.
reducing high-frequency sound. The discharge
(4) Operating temperature should also be stat-
noise of a gas turbine engine, on the other hand, is
ed. When dissipative mufflers carry air or gas at
strong in the low-frequency region. Mufflers must
elevated temperatures, the wavelength of sound is
have large dimensions to be effective in the low-
longer, so the mufflers appear shorter in length
frequency region, where wavelength dimensions
(compared to the wavelength) and therefore less
are large (para 26b of the N&V manual). Thus,
effective acoustically (para 2-6b of the N&V
these baffles may be 6-in. to 18-in. thick, with 8-in.
manual).
to 16-in. air spaces between baffles, and have rug-
ged construction to withstand the high tempera-
performance, tables 33 through 38 give the ap-
ture and turbulent flow of the engine discharge.
proximate insertion loss values to be expected of a
Depending on the seriousness of the noise prob-
number of muffler arrangements. Values may vary
lems, mufflers may range from 8 ft. to 20 ft. in
from one manufacturer to another, depending on
length, and for very critical problems (i. e., very
materials and designs.
close neighbors), two different 12- to 18-ft. muf-
3-5
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