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National Advisory Committee for Aeronautics, Technical Notes - The Attenuation Characteristics of Four Specially Designed Mufflers Tested on a Practical Engine Setup

This paper presents the results of muffler tests made to evaluate
the theoretical resonator-muffler attenuation expression as applied to
practical installations. A specific design procedure for the mufflers
tested is also included.

Four resonator-type mufflers of different attenuation capabilities
were designed and tested on a laboratory cold-test setup and a helicopter
field-test setup. Good agreement was found between the laboratory
experimental data and theory. The field-test results, however, showed
that the mufflers attached to the helicopter did not lower the field
noise to the extent predicted by theory or the amount measured in the
laboratory test. This attenuation reduction is believed to be due in
part to the nonfulfillment of the basic theoretical assumptions of low
sound pressures and zero flow velocities and in part to the extraneous
noise level.

Although the theoretically predicted attenuation may not be obtained
in practical installations, the results show that the resonator equation
can be very useful in the design and development of mufflers.

With the power—increase developments made by the light or private-
aircraft industry in the past few years and with the rapid expansion of
the suburban—home areas, there has been an increasing aircraft-noise
disturbance problem in and around airports close to congested areas.
This noise problem was brought to the attention of the National Advisory
Committee for Aeronautics for special consideration. As a result, a ,
theoretical and experimental muffler investigation at the Langley full-
scale tunnel was undertaken, as well as propeller—noise investigations
by other NACA research facilities.

Certain phases of the investigation covering dynamometer—stand
muffler tests and propeller quieting have been completed and the results
are published in references 1 to 3. The theoretical work of reference h
presents design curves and an equation for predicting the attenuation
characteristics of several types of mufflers. One of the muffler types
considered in the investigation of reference h embodies the principle
of chamber resonance. This resonant-chamber muffler appeared to be
worthy of additional study because of its large attenuation for a given
size and for its lowaback-pressure possibilities.