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An axially short expansion chamber in a duct acts as a resonator type muffler whose effective frequency is determined by the duct and chamber geometry. To modulate this frequency, the temperature of the air in a very short chamber consisting of two rectangular branches located symmetrically on opposite sides of a rectangular duct was changed. The transmissibility of the sound was measured for a plane wave propagating in the duct through the chamber section. Each effective frequency (resonant frequency), at which the maximum transmission loss was obtained, was summarized and considered using a simple side branch type muffler approximation. The frequency modulates regularly according to the temperature in the chamber. In practice, though, the range of frequency modulation is small versus the change in temperature. However, this method provides a means for the fine adjustment of the effective frequency to a target frequency for noise reduction.

Modulation of Sound Attenuation Properties of a Short Expansion Chamber by Changing the Temperature in It