Intensity Probe

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.


The microphones have a free field response optimised for use in face-to-face configuration intensity probes and a high sensitivity to enable low level measurements. As a pair, they fulfil the phase requirements for Class 1 intensity microphones in accordance with IEC International Standard 61043.

The solid spacers are for intensity probe configurations using microphone separations of 12 mm, 25mm, 50 mm and 100 mm to cover a full frequency range from 50Hz to 10kHz, and improve microphone protection even under the most adverse of measurement conditions. There are two right-angled adapters and one straight adapter for use with the 1/4” Preamplifier Type 26AASet.

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Complete set of a pair of phase-matched 1/2” intensity microphones, solid spacers and adapters for a 1/4” Preamplifier Type 26AA Set.

1-4 inch Intensity-microphone Set

Complete set of a pair of phase-matched 1/4” intensity microphones, solid spacers and adapters for a 1/4” Preamplifier Type 26AA Set.

Intensity Calibrator

For calibrating the phase response of a pair of microphones used for measuring sound intensity, e.g. Type 40AI and Type 40BI. It includes a calibrated acoustic resistance to enable the computation of particle velocity and intensity levels.

Complies with IEC International Standard 61043.

The remote-control functions make it possible to control the process of data acquisition entirely from the handle of the probe. Various versions of the Type 50AI

are available with different remote-control handles for direct use with a wide range of general-purpose frequency analysers as well as specialised sound intensity analysers. These are described in the following.

Intensity Probe 2

Has a remote-control function for direct connection to, and control of, sound-intensity measuring systems from a wide range of suppliers such as 01dB and Neutrix-Cortex.

Can be used directly with the G.R.A.S. Intensity Module Type 12AB, which provides all necessary voltages for the remote-control functions and powering the preamplifiers; also enables direct use with 01dB Sound Intensity systems.

Intensity Probe

Can be connected directly to any analyser with two standard 7-pin LEMO microphone-preamplifier inputs.
Can be used with the following G.R.A.S. Power Modules, which provide all necessary voltages for powering the preamplifiers:
• Intensity Module Type 12AB.
• Power Module Type 12AA (via the included adapter cable AC0003, which splits the output from a 12pin LEMO socket into two 7-pin LEMO plugs).
• The Power Module Type 12AA has two standard BNC output sockets for connecting to analysers that don’t have special microphone-preamplifier inputs.

Wide Band Intensity Probe

Flexible and adjustable four-channel sound-intensity probe. It enables sound intensity measurements using two different microphone spacers simultaneously to cover an extended frequency range. The spacers used with the Type 50VI-3 have lengths of 12 mm and 60 mm to cover a frequency range from 80Hz to 10kHz measured at the same point.

The remote control output can, for example, be connected to an RS-232 interface for computer control of the process of data acquisition.

The Wide-band Intensity Probe comprises two pairs of phase matched intensity microphones Type 40AI, two pairs of preamplifiers Type 26AA, 12 mm and 60 mm spacers, a 6-channel probe handle and a 10 m cable with LEMO connectors.

XYZ Intensity Probe

Two-channel intensity probe for serially measuring a three-dimensional sound intensity vector in a stationary sound field. It includes a pair of phase-matched
Intensity microphones Type 40AI mounted on a pair of Preamplifiers Type 26AA with adapters and spacers. The whole assembly is mounted on a swivel head that can be swiveled in three mutually-perpendicular directions by means of a pair of slider buttons on the probe handle.

The probe handle has inputs for 2 preamplifiers and a pair of LEDs for indicating two states as well as a pair of push buttons for interactive use with a remote control system. For example, the output socket for remote control can be connected to the RS-232 interface of a computer which controls the process of data acquisition.