Index of content:
Volume 103, Issue 2, February 1998
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
A numerical and experimental investigation of the performance of sound intensity probes at high frequencies103(1998); http://dx.doi.org/10.1121/1.421212View Description Hide Description
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 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.
103(1998); http://dx.doi.org/10.1121/1.421213View Description Hide Description
Random acoustic fields and their energetic quantities (acoustic active and reactive intensities, potential and kinetic energy densities) are described in terms of the mutual coherences between sources. Conditions to correctly construct the coherence matrix of sources in a multivariate random process are given. It is shown that the description of a sound field using the coherence matrix of source is equivalent to the superposition of a number of independent coherent fields, which do not correspond to the original localized sources. A method based on processing the principal components of the coherence matrix of sources is given to reduce the number of necessary fields. The coherence function between acoustic pressure and particle velocity and the curl of active intensity are proposed as two indicators for estimating the degree of coherence and the polarization of acoustic fields. These indicators are analyzed theoretically and experimentally. The description of the structure of partially coherent fields is generalized by the definition of the field matrix whose rank is an indicator of the local complexity of an acoustic field.