Comparison of Dean's Method and Direct Pressure/Velocity Measurement for Slot Resonator Acoustic Impedance Determination

For the past approximately 30 years, measurements of acoustic impedance of resonant sound absorbers in grazing flow environments have been conducted using Dean’s two microphone method. In summary, the method employs one microphone in the “incident” sound field at the surface of the material or treatment to be measured and a second microphone in the resonator cavity. The area-average acoustical flow into the resonator is assumed to be related to the cavity sound pressure though a simple plane-wave transformation. The validity of the assumed relationship between acoustic flow and cavity sound pressure in the high speed grazing flow, high sound amplitude environment of interest to aircraft engine noise suppression designers is being investigated. Split-film anemometry is used to measure the flow field immediately outside the orifice of a two-dimensional slotorifice Helmholtz resonator spanning one wall of a Mach 0.3 wind tunnel as the resonator is exposed to high amplitude (130-150 dB) sound excitation from the opposite side of the tunnel. By integrating the normal component of the measured velocity across the slot orifice, a direct measurement of the acoustic flow into the orifice/resonator is obtained. This measured velocity is compared to that computed from cavity base pressure using plane-wave propagation between the cavity base and the slot plane. Significant differences between measured and computed velocities have been observed and are the subject of ongoing study. In addition, the time-dependent velocity field outside the orifice was plotted for comparison to CFD predictions.