available in two or three "classes," depending on
Turning vanes in the 90 square turn reduce the in-
attenuation. The mufflers of the higher insertion-
sertion loss values. If turning vanes are used, only
l0SS class typically have only about 25% to 35%
one-half the insertion loss values of table 39 may
open area, with the remainder of the space filled
be used for the 63- through 500-Hz bands and only
with absorption material. The lower insertion-loss
one-fourth the values for the 1000- through
classes have about 50% open area. The mufflers
8000-Hz bands. When a muffler is used at the turn,
with the larger open area have less pressure drop
full attenuation of the muffler is realized as well as
and are known as "low-pressure-drop units. " The
the additional loss due to the lined turn.
mufflers with the smaller open area are known as
c. Ventilation-duct mufflers. For ducted air-
"high-pressure-drop units. " When ordering special-
handling, ventilation, or air-conditioning systems,
purpose mufflers, one should state the speed and
packaged duct mufflers can be purchased directly
the temperature of the air or gas flow, as these
from reputable acoustical products suppliers. Their
may require special surface protection and special
catalogs show the available dimensions and inser-
acoustic filler materials. The approximate insertion
tion losses provided in their standard rectangular
losses of a representative group of ventilation-duct
and circular cross-section mufflers. These pack-
mufflers are given in table 310. Individual suppli-
aged duct mufflers are sold by manufacturers in
ers can give data for their specific products.
3-ft., 5-ft., and 7-ft. lengths. They are also usually
For hot exhausts, the exhaust gas is of lower den-
There is no precise schedule of self-noise as a func-
sity and consequently has a higher total volume
tion of exit speed for large mufflers, but the follow-
flow for a given mass flow than would exist at nor-
ing rules-of-thumb for exhaust stacks of turbine en-
mal ambient temperature. The manufacturers of
gines are offered. For installations in relatively
31 1
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