On the relationship between sound intensity, pressure, particle velocity, and measurement accuracy

Abstract

Sound intensity measurements are sensitive to instrumentation inaccuracies in a manner that varies with the sound field in which the meaurements are made. For this reason a number of “quality indicators” have been proposed to allow evaluation of the uncertainty of a measured intensity, the most widely used indicator being the “pressure‐intensity index.” Intensity is usually calculated from measurements of sound pressure and either pressure gradient (i.e., two‐microphone, p‐p devices) or particle velocity (p‐u devices). The interpretation and use of the pressure‐intensity index is explored for such devices, with emphasis on practical use. It is shown that this indicator is a “phase index” for p‐p devices, and an “impedance index” for p‐u devices. It is shown also that proper interpretation of the indicator varies with the manner in which the measurement device is realized. For example, p‐p devices that sum and difference before filtering require interpretation different from those that filter first. It is concluded that universal limits to ensure a given measurement accuracy cannot be established independent of the measuring device.