Abstract
A transient excitation method is described which allows the free field frequency and impulse responses of a microphone to be measured in a reverberant environment. The pressure transient generated by exciting a loudspeaker with a narrow voltage pulse is detected at a point in the loudspeaker's far field by the test microphone. A reference microphone, whose response is assumed flat over the frequency range of interest, is then substituted in exactly the same position and its response to the transient measured. The outputs of the two microphones are accurately sampled and deconvolved using a discrete Fourier transform technique to give the magnitude and phase parts of the test microphone's frequency response. Inverse transformation then gives the test microphone's response to an impulse of acoustic pressure propagating in the free field. Computed frequency and impulse responses are presented and the practical implementation and accuracy of the method discussed. The use of the method is illustrated by measurements of the free field frequency and impulse responses of several different microphones, including a 1" instrumentation microphone; free field correction curves for this microphone are also presented. Three digital analysis techniques are discussed and applied to some of the measured microphone responses where the discrete Hilbert transform is found to provide a method of calculating the phase part of the 1" microphone's frequency response over its useful frequency range without the ambiguities due to microphone misalignment inherent in the pulse method. The method is then used to measure the pressure which occurs at the centre of the flat end-face of a long cylinder when excited by a delta function of acoustic pressure from various incidence angles. A numerical time domain model is established which allows the impulse responses of this configuration to be computed. The interference effects caused by the proximity of two closely spaced instrumentation microphones in various orientations and configurations are also presented and discussed.