Powerful tool used for locating sound sources, order ranking them and determining the sound power emitted
The technique of intensity measurements is a powerful tool used for locating sound sources, order ranking them and determining the sound power emitted. The method is based on the simultaneous determination of sound pressure and particle velocity using two closely spaced, face-to-face microphones. A sound-intensity probe must maintain a well-defined acoustical spacing between the microphones with a minimal of disturbance to the sound field.
Generally speaking, the technique of intensity measurements involves determining the direction of a sound wave by detecting differences in arrival time at two closely-spaced microphone positions.
If the sound wave arrives first at microphone A then, a little later, at microphone B, the sound wave must be travelling in the direction from A to B. On the other hand, if it arrives first at microphone B, then it must be travelling in the opposite direction. In the case where it arrives at the two microphones at the same time, then it must be travelling in a direction perpendicular to the pair of microphones.
The ability of a pair of microphones to determine accurately small differences in arrival times depends on how small the difference is between the phase responses of the two microphones. Therefore, phase-matching is an all-important factor for a pair of intensity microphones.
The Types 40AI and 40BI intensity microphone pairs have been carefully manufactured and selected to have minimum phase difference. To ensure maximum measurement accuracy, the spacing between the microphones should be optimised for the particular measurement conditions. At low frequencies and in highly reverberant conditions, spacing should be large, whereas at high frequencies, it should be small.
The G.R.A.S. Intensity Probe Type 50AI comes with a selection of solid spacers for microphone separations ranging from 12 mm to 100 mm. The design of the probe enables spacers to be swapped without dismantling the probe. The useful frequency range of a sound intensity probe depends on the phase response of the microphones and the distance between the microphones. The G.R.A.S. sound intensity probes have been designed to switch easily between different microphone spacers to cover different frequency ranges.