On the length scale and Strouhal numbers for sound transmission across coupled duct cavities at low Mach number

Abstract

The sound transmission across two coupled cavities along a rectangular duct in the presence of a low Mach number flow is examined experimentally in the present study. Effort is also made for deeper understanding on how the flow, the excitation sound frequency and the excitation level influence the sound transmission loss. Results confirm that the high sound transmission loss across the cavities is associated with the strong out-of-phase pressure fluctuations within the cavities. The sound transmission loss deteriorates significantly once the flow speed exceeds a threshold value. A new length scale is proposed. This length scale, together with the threshold flow speed and the peak sound transmission loss frequency, gives a Strouhal number which is basically independent of the cavity offset for a fixed cavity length. The present finding extends the previous effort of the authors, enabling the prediction of the flow speed limit and the operating frequency of coupled cavities for duct silencing at low Mach number.