A new generation of acoustical measurement standards.

Work is being undertaken to develop optical techniques as the basis of new measurement standards for the calibration of airborne measuring instruments, and to establish proof of feasibility for photon correlation as a primary standard for airborne sound.

The reciprocity method has become the standard method for calibration of microphones or hydrophones. For sound in air, the calibration method that has become recognised internationally is coupler reciprocity, where calibration is undertaken essentially in a closed coupler. This is a simple calibration technique that has been developed and refined over many years to realise the Pascal to an accuracy unsurpassed by any free-field technique, and it is the basis of UK NMS primary standards research. Traceable calibrations of devices such as microphones, sound calibrators and sound level meters are mainly achieved by pressure-based techniques using a calibrated reference microphone. However, the calibration of non-standard microphones is not easily accommodated by these methods because of coupling considerations, and this is quite a restriction and an inhibitor to the introduction of new technology. Furthermore, calibration of miniature array microphones or MEMS sensors is also not possible. These limitations would be overcome if free-field calibration techniques could be implemented more easily, and this can be achieved using optical techniques as they offer the ability to realise an accurate determination of the Pascal at a point in a sound field.

Photon correlation methods have been developed to the point at which a prototype system can now be developed as a potential method of improving the accuracy of primary acoustical standards, and thereby enabling them to be more accurate and directly traceable to SI base units. Scattering of light from natural particles in the air as they pass through a fringe pattern established by two intersecting laser beams will be used. There will be a need to undertake research studies on particle size and concentration, signal-to-noise and photon correlation signal analysis techniques. Implementation and testing of a pilot facility in the small anechoic chamber at NPL will be the ultimate objective.

A parallel research project is being undertaken in the underwater acoustics area.

For more information, please contact Pete Theobald