The UK’s National Physical Laboratory (NPL) has developed a new laser-based acoustic measurement technique in collaboration with PMC loudspeakers.

As the UK’s National Measurement Institute, one of the NPL’s responsibilities is to develop accurate measurement standards. Existing acoustic measurement techniques, used in the test and measurement of acoustic products such as transducers and loudspeakers, make use of microphones to map the soundfield around the device being measured, its directivity and dispersion pattern. However, measurements have to be made at many points to create an accurate picture of sound propagation. Moreover, the presence of the microphone can itself distort the dispersion pattern being measured.

NPL’s new laser-based measurement technique – Rapid Acousto-Optic Scanning (RAOS) – removes the need for a physical measuring device such as a microphone, using the fact that sound waves affect the refractive index of the air they travel in. A laser passed through the air is affected to different degrees depending on the nature of the sound passing through the air at the same time as the laser. By accurately measuring the delay of a laser beam passing the device under test as it generates audio, a complete picture of the dispersion pattern of the device can be built up. Because RAOS uses light, and requires no physical measurement device, and because the measurements with the laser can be carried out extremely quickly from a single position, the acoustic field around the device under test can be sampled and mapped rapidly.

Although the principles behind RAOS have been understood for many years, applying it in a test and measurement context was another matter. Now a viable T&M procedure, RAOS can provide a detailed 3D picture of how sound radiates from a loudspeaker.

‘Working with PMC design engineers gave us a fantastic relationship, helping to home in on some pertinent acoustic characteristics of various PMC speakers,’ says NPL Higher Research Scientist, Ian Butterworth . ‘The process has allowed us to develop our technique into a valuable industrial tool.’

‘NPL’s laser-based acousto-optic measurement technique provides a rapid, reliable method of viewing every aspect of loudspeaker dispersion and allows us to understand, with much improved accuracy, the effect of cabinet, driver and crossover design on what we hear,’ agrees PMC co-founder and Chief Designer, Peter Thomas, ‘We are delighted to have collaborated with NPL to pioneer this highly innovative technique and we will apply this groundbreaking knowledge to further the development of all our products.’

The process will be used in the development of future PMC products.