Influence of sound pressure and grazing flow on the acoustic resistance of resonator orifice

Abstract

Acoustic impedance of a rectangular orifice of a resonator on a side wall of a duct was measured. The measurement was based on two-microphone-method, in which the acoustic impedance was calculated from the transfer function of sound pressures at the end of the resonator and at the outlet of the orifice. The acoustic impedance, especially the resistance θ_p at resonance frequency f_p, depends on the particle velocity mach number undulating through the orifice M₀ and the flow speed mach number in the duct grazing over the orifice M. When the grazing flow speed mach number M is small, the resistance θ_p mainly increases with the particle velocity mach number M₀. On the other hand, when M is large, θ_p increases with M and is almost independent of M₀. In the latter state, the sound pressure at the resonator end decreases with M while the sound pressure at the orifice slightly increases with M, which results in the increase in θ_p. Those two states are roughly classified by use of the ratio M/M₀; M/M₀ is lower or higher than around 3.