Description:
The design of a porous tabular windscreen for in-duct fan sound-power measurements is described. Such a windscreen tends to cancel out turbulent pressure fluctuations. The windscreen also modifies the response of a microphone to acoustic waves, since it effectively converts an omnidirectional microphone into a line microphone with a pronounced directivity. This directivity can be used with advantage in both the plane-wave and cross-mode frequency regions, since the microphone sensitivity to reflected waves is reduced. However, in the cross-mode region, this directivity is a complication, since the acoustic power carried depends upon the angles at which the cross modes propagate. The ideal directivity in this region for estimation of sound power from pressure measurements is (cosθ)12. The directivity has been made to approximate this. Experimental data of the frequency response, directivity, and signal-to-noise ratio are presented. [Work done partly at Herrick Laboratories, supported by Air Conditioning and Refrigeration Institute, and partly at Carrier Corporation.]