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.nFour resonator-type mufflers of different attenuation capabilities were designed arid 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.
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