Design of a Porous Pipe Microphone for the Rejection of Axial Flow Noise

Abstract

A porous pipe allows weak coupling between acoustic pressures outside and inside the pipe. A single microphone element is located at one end of the porous pipe. Only the waves traveling towards the microphone are preserved; the porous pipe includes a solid internal cone, which produces an anechoic termination for waves traveling in the opposite direction. The design yields the specific flow resistance of the porous pipe and the cone shape for the anechoic end. The axial sensitivity of the porous pipe microphone is found to be unity when the microphone element has a high acoustic impedance. The directivity follows that of a uniform line with uniform delays. The filtering action of the porous pipe microphone for the turbulent-boundary-layer noise is calculated for a simple model of flow noise. Experimental values of microphone sensitivity, directivity, and flow-noise rejection are presented. [This work was initiated and supported by the Systems Application Division of U. S. Army MERDC, Fort Belvoir, Virginia 22060.]