Measurement of directivity index by three‐dimensional spatial sampling

Abstract

The traditional method for finding the directivity index (DI) of a microphone by extrapolation from its two‐dimensional polar pattern may not be accurate when the microphone or its mounting surface is acoustically large. In situ hearing aid microphone DI determination requires knowledge of the full three‐dimensional directional response. Microphone DI may be measured directly by comparison of the on‐axis anechoic sensitivity to the sensitivity in a diffuse sound field, normalizing the comparison by reference to a perfect omnidirectional microphone. In practice, creating an accurately diffuse field is difficult, and is traditionally accomplished by averaging measurements taken at several locations in a reverberant room excited by multiple uncorrelated sound sources. Theoretically equivalent results can be obtained by three‐dimensional spatial sampling in an anechoic environment, inferring the full spherical response from a finite number of directional measurements. The necessary density, positioning, and positioning accuracy of measurement directions is determined by sampling theory, tests employing mathematically defined polar patterns, and practical considerations, leading to a minimum specification of 48 measurement points arranged in five horizontal zones. Non‐uniform spacing of these points necessitates employing a weighted average of the sensitivity measurements to obtain the best prediction of the actual diffuse field response.