b. "Lined" and "unlined" bends in turbine
adding a thick lining of acoustic absorption materi-
stacks. When a long duct or passageway contains a
al at the end of the turn (facing the oncoming sound
square-ended 90 turn, there is a tendency for
wave), extending into the duct past the turn for a
sound traveling in that duct to be reflected back to-
length of one or two times the average width of the
ward the direction from which it came. Because
duct. A long muffler, located immediately past the
high-frequency sound is more "directional" (be-
turn, also serves to simulate a lined bend. Table
haves more nearly as a beam of light), it is more
39 gives the estimated insertion loss of unlined
readily reflected back by the end wall of the 90
and lined bends, and figure 31 shows schematical-
turn and less sound is transmitted around the cor-
ly the bend configurations. The orientation of the
ner. Low-frequency sound "bends" around the turn
parallel baffles of a muffler located just past a turn
more readily, so this reflection effect is less pro-
should be as shown in figure 31 to achieve the
nounced. The attenuation provided by a square-
Class 1 and Class 2 lined bend effects.
ended 90 turn can be increased noticeably by
3-9
Previous Page
