Influence of grazing flow and bias 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. Three types of resonator orifices were used: right-angled edge on the duct side (NN), round on the downstream side (NR), and round on the downstream and upstream sides (RR). 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, depends on the sound pressure and the flow speed. When the flow speed in the duct grazing over the orifice U_g is low, the resistance θ_p mainly increases with the sound pressure. On the other hand, when U_g is high, θ_p increases with . The higher the bias flow speed U_b through orifice from resonator to duct, the higher the resistance θ_p. The orifice shape hardly changes the resistance θ_p of NN and NR, but the RR has a lower θ_p.