A numerical and experimental investigation of the performance of sound intensity probes at high frequencies

Abstract

The influence of scattering and diffraction on the performance of sound intensity probes has been examined using a boundary element model of an axisymmetric two-microphone probe with the microphones in the usual face-to-face arrangement. On the basis of calculations for a variety of sound field conditions and probe geometries it is concluded that the optimum length of the spacer between the microphones is about one microphone diameter; with this geometry the effect of diffraction and the finite difference error almost counterbalance each other up to about an octave above the frequency limit determined by the finite difference approximation. This seems to be valid under virtually any sound field condition that could be of practical importance in sound power determination. The upper frequency limit corresponds to about 10 kHz for an intensity probe with 12-in. microphones, which means that it should be possible to cover most of the audible frequency range, say, from 50 Hz to 10 kHz, with a single probe configuration. The numerical results have been confirmed by a series of experiments.

A comparison of two different sound intensity measurement principles

“…10 Much later it was discovered that the effect of scattering and diffraction not only tends to counterbalance the finite-difference error but in fact for a certain length of the spacer almost perfectly cancels it under virtually any sound field condition encountered in practice. 11 A practical consequence is that the upper frequency limit of a sound intensity probe based on two 1 / 2 in. microphones separated by a 12 mm spacer in the face-to-face arrangement is about 10 kHz, which is about an octave higher than the frequency limit determined by the finite-difference approximation.…”

“…microphones separated by a 12 mm spacer in the face-to-face arrangement is about 10 kHz, which is about an octave higher than the frequency limit determined by the finite-difference approximation. 11 The combination of 1 / 2 in. microphones and a 12 mm spacer is now regarded as optimal, and longer spacers are only used when the focus is exclusively on low frequencies.…”

A numerical investigation of the influence of windscreens on measurement of sound intensity

“…11. However, since windscreens have no influence on sound intensity measurements except at fairly low frequencies 4 where the probe does not change the sound field at all, 11 the intensity is simply calculated from the pressure at two points that represent the two microphones, that is, from the expression…”

A note on measurement of sound pressure with intensity probes

“…2 In 1998 it was found that the effect of scattering and diffraction tends to counterbalance the finitedifference error under virtually any sound field condition that can be encountered in practice. 3 It was shown that the practical upper frequency limit of a sound intensity probe based on two 0.5 in. microphones separated by a 12 mm spacer in the face-to-face arrangement is about 10 kHz, 3 which is about an octave higher than the frequency limit determined by the finite-difference approximation.…”

“…3 It was shown that the practical upper frequency limit of a sound intensity probe based on two 0.5 in. microphones separated by a 12 mm spacer in the face-to-face arrangement is about 10 kHz, 3 which is about an octave higher than the frequency limit determined by the finite-difference approximation.…”