The flow noise level at microphones in flow ducts

Abstract

When sound measurements are made in flow ducts one has to insure that the signal-to-noise ratio of the sound signal to the flow noise, which is produced by the turbulent flow at the microphone, is sufficiently high. To accomplish this, knowledge of the flow noise level under the specific given flow conditions is required. In this paper two procedures for determining the flow noise level are described. For the first method, the mean flow velocity and the turbulence level have to be known to estimate the flow noise level as a function of duct diameter and frequency. For the second method knowledge of only the mean flow velocity is required. The procedure involves two measurements: one with a microphone fitted with a conventional nose cone and one with a microphone fitted with a slit-tube. The slit-tube is a special device for reducing the flow noise level. Finally, guidelines are given for sound measurements in the presence of flow and for the use of a microphone with a slit-tube under unusual temperature conditions or in gases other than air.