Acoustic intensity measurements in low Much number flows with moderate turbulence levels

Abstract

Experimental investigations were conducted to determine the limitations imposed when making acoustic intensity measurements with the two-microphone cross-spectral method in situations with mean flow. An anechoic wind tunnel and grids were used to provide the desired flow conditions. The acoustic intensity probe was mounted in the flow and a small broadband noise source of known acoustic intensity was mounted outside the flow. Intensity measurements of the source were compared with and without flow. Parameters studied included: microphone diameter and spacing, noise source intensity and relative direction, airflow velocity, turbulence level, and turbulence eddy size. The investigations showed that good performance is obtained using 12.7-mm-diam microphones spaced 16 mm apart. With this probe, the flow restricts acoustic intensity measurements to sources whose intensities are -5 dB or greater than the flow-induced wind noise on either one of the microphones. Therefore, this lower limit is easy to measure before intensity measurements are made in any unknown flow. An example is presented to show how acoustic intensity measurements can be made near a truck tire onroad to help in identifying and quantifying the suspected tire noise sources.