Research on a future optically-based free-field calibration standard.

Research work is currently being undertaken towards the development of a new optically-based primary standard for the free-field calibration of airborne measuring instruments based on photon correlation spectroscopy.

The existing primary standard for the calibration of microphones and hydrophones is based on the reciprocity method. For sound in air, the calibration method that has become internationally recognised is coupler reciprocity, where calibration is undertaken essentially in a closed coupler. This is a calibration technique that has been developed and refined over many years to realise the pascal (the unit of acoustic sound pressure) 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 spectroscopy has been developed to the point at which a prototype system has been 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. In addition, and contrary to the existing calibration, it can be applied to any standard and non-standard dimension microphone.

NPL has developed an integrated delivery and collection optical system that is placed outside a fully anechoic chamber. Two laser beams cross inside the room and form a small ellipsoid volume with interference fringes. As airborne particles oscillate due to the propagating sound, scattered light (photons) is collected; further gated time domain analysis yields the particle velocity and thus free-field pressure at the crossover point. This operation is performed at distinct frequency steps and thus an acoustic measurement device can be calibrated directly against the acoustic unit of sound pressure. Research work is also concentrating on the investigation of a working frequency and pressure range, including associated uncertainties.

Publications

• Gated photon correlation spectroscopy for acoustical particle velocity measurements in free-field conditions
  Koukoulas, T., Piper, B., and Theobald, P.
  JASA Express Letters, 133 (3), pp. 156-161, 2013
• Towards a future primary method for microphone calibration: optical measurement of acoustic velocity in low seeding conditions
  Koukoulas, T., Theobald, P., Schlicke, T., and Barham, R.
  Optics and Lasers in Engineering Journal, 46 (11), pp. 791-796, 2008

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

For more information, please contact Triantafillos Koukoulas