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National Advisory Committee for Aeronautics, Report - Theoretical and Experimental Investigation of Mufflers with Comments in Engine Exhaust Muffler Design

Equations are presented for the attenuation characteristics of
Single—chamber and multiple—chamber mufflers of both the
erpanSim—chamber and resonator types, for tuned side-branch
tubes, and for the combination. of an expansion chamber with a
resonator. Erperimental curves of attenuation plotted against
frequency are presented for 77 difierent mufllers with a reflec—
tion-free tailpipe termination, and the results are compared
with the theory. The experiments were made at room. temper -
tore without flow; the sound source was a loud—speaker.

A. method is given for including the tailpipe reflections in the
calculations. Experimental atte-n-amtion cure-es are presented
for four different mufier—tailp-ipe com b-inatione, and the results
are compared with the theory.

The application of the theory to the design of engine~erhaust
mufilerc is discussed, and charts are included for the assistance
of the designer. ‘

Noise spectrums are presented for ahelicopter with each of the
four mafia-tailpipe combinations installed. These: spectrums
are compared with the noise spectrum of the umnufllecl helicopter.
The results Show that the overall noise level of the helicopter was
reduced significantly by even the smallest of the four mufflers
tested.

A theoretical and experimental investigation of the methods
of mufllor design has been conducted at the Langley full~
scale tunnel of the National Advisory Committee for Aero—
nautics as part of a general research program directed toward
the reduction of airplane noise. The. acoustic theory and
muffler literature. were studied with the aim of obtaining a
method of predicting muffler characteristics. The theory of
acoustic filters is discussed in reference 1. ‘53 .ctions of par—
ticular in torost in connection with muffler design are. the chap—
ters on change in area of wave front, transmission through
a conduit with an attached branch, and tho filtration of
sound, as well as the appendix which gives the branch«
transmission.