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National Advisory Committee for Aeronautics, Technical Notes - Dynamometer Stand Investigation of the Muffler Used in the Demonstration of Light Airplane Noise Reduction

As part of a general investigation directed toward the reduction of
airplane noises, tests were made on a. ground dynamometer stand of a
muffler designed for an army liaison—type airplane. The ground tests
indicated that a reduction of 10 to 15 decibels was 130' be expected from
this muffler over the range from 1650 to 2790 rpm and that an additional
reduction of 5 decibels was to be expected at some engine speeds from the
installation of a long tail pipe with an upward band. This reduction was
substantiated by flight tests. The calculated attenuation characteristics
of the muffler are also presented. The experimentally determined low—
frequency cut—off and upper pass frequencies agreed with the calculated '
values. Further reductions in over—all noise level for the engine appeared
possible only through quieting of intake and engine—clatter noises. The
back pressures of the airplane with muffler were about a third higher than
those for the unmodified airplane at 2790 rpm and were lower at lower engine
speeds. The power variations due' to these back—pressure changes were small.

As part of a. general investigation directed toward the reduction of
airplane noises, particularly those produced by the personal owner type
of airplane, atheoretical and experimental investigation is being
conducted by the Langley Laboratory of the National Advisory Committee
for Aeronautics of the methods of_muffler design. The study was under—
taken as a result of recent developments in propeller noise reduction
which make possible reduction of airplane propeller noise to a level
considerabe below that of the engine itself and as a result of the
fact that a reduction in noise level of one without a reduction in that
of the other usually does not result in any substantial reduction in
over-all noise level. Several theoretical methods, in particular those
of electrical analogy technique, can be used to design mufflers, but in
practice much difficulty has been experienced in correlating theory
with experiment on an actual engine, usually because of excessive noise
interference during testing from sources other than the engine exhaust.

lack of understanding of the reasons for poor correlations makes it
extremely difficult to design a muffler having the desired attenuation
‘characteristics without much empirical emerimantation. Also, at the
present time the lack of basic information regarding what noise level
is acceptable to the public handicaps the designer. The object of this
research is to gain an understanding of the main principles involved
and to reduce muffler design to a more rational basis.