Numerical research on the flow characteristics of confined sonic and supersonic air-modulated speakers

Abstract

Air-modulated speaker is one of the most famous high-intensity sources. Sound waves with SPL above 160 dB are generated by the modulation of sonic air flow through a time-varying valve. To overcome the source saturation at high chamber pressures, the performance and flow characteristics of a new design, in which the flow speed increases from sonic to supersonic by a converging and diverging nozzle, were investigated. Based on the analytical results from the widely used quasi-steady model, the source level improvement was verified for the same pneumatic power. The SPL increment of more than 5 dB is obtained at Mach number 2.75. The transient flow inside the source was simulated using a recent developed numerical model in which the compressible flow is governed by the Navier-Stokes equation and the valve movement is modeled by the dynamic mesh method. Compared with the ordinary sonic flow modulation, the source mechanism on the supersonic case has shown several distinct features: the effect of the flow-acoustic coupling could not be neglected; some complex phenomena such as shock wave formation and propagation arise in the transient flow inside the nozzle; the establishment of supersonic flow and the resulting improvement are related to the valve modulation frequency.