AN IMPROVED METHOD FOR DESIGN OF EXPANSION-CHAMBER MUFFLERS WITH APPLICATION TO

摘要

An improved method for the design of expansion-chamber mufflers is described and applied to the task of reducing exhaust noise generated by a helicopter. The method is an improvement of standard transmission-line theory in that it accounts for the effect of the mean exhaust-gas flow on the acoustic-transmission properties of a muffler system, including the termination boundary condition. The method has been computerized, and the computer program includes an optimization procedure that adjusts muffler component lengths to achieve a minimum specified desired transmission loss over a specified frequency range. A printout of the program is included together with a user-oriented description.nA field test of a muffler designed with the aid of this method was conducted on a heli¬copter with a known exhaust-noise problem. When the exhaust noises of the helicopter with a standard exhaust system and a similar helicopter with a muffler system installed were com¬pared for hover flight conditions, the muffler system was found to reduce the exhaust noise by approximately 11 dB(A). No significant degradation in the engine performance was observed.