Don D. Davis, Jr., George M. Stokes, Dewey Moore, George L. Stevens, Jr.
Equations are presented for the attenuation characteristics of single-chamber and multiple-chamber mufflers of both the expansion-chamber and resonator types, for tuned side-branch tubes, and for the combination of an expansion chamber with a resonator. Experimental curves of attentuation plotted against frequency are presented for 77 different mufflers with a reflection-free tailpipe termination, and the resuslts are compared with theory. The experiments were made at room temperature without flow; the sound source was a loud speaker. A method is given for including the tailpipe reflections in the calculations. Experimental attenuation curves are presented for four different muffler-tailpipe combinations, and results are compared with theory. The application of the theory to the design of engine-exhaust mufflers is discussed, and charts are included for the assistance of the designer. Noise spectrums are presented for a helicopter with each of the four muffler-tailpipe combinations installed. These spectrums are compared with the noise spectrum of the unmuffled helicopter. The results show that the overall noise level of the helicopter was reduced significantly by even the smallest of the four mufflers tested.
An Adobe Acrobat (PDF) file of the entire report: