PMC & NPL Collaborate On Speaker Measurement

The UK's National Physical Laboratory (NPL), based in Teddington, south-west London, has developed a new laser-based acoustic measurement technique with the help of PMC loudspeakers — a cutting-edge process which will be used in the development of future PMC products.

NPL is the UK's National Measurement Institute, and one of its responsibilities is to develop and apply the most 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 how sound propagates around the device, and the microphone has to be moved many times. Moreover, the presence of the microphone can itself distort the dispersion pattern being measured.

NPL's new laser-based measurement technique, known as Rapid Acousto-Optic Scanning (RAOS) removes the need for a physical measuring device such as a microphone, and instead relies on 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 with ease. 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, turning the theory into practice so that it could be used reliably in a test and measurement context was another matter. Acoustically neutral PMC speakers were ideal test products to help NPL develop their experimental technique, honing it into an industrially relevant and valuable process for the design and testing of acoustic products. RAOS can now provide the most detailed 3D picture of how sound radiates from a loudspeaker.

Ian Butterworth, Higher Research Scientist at NPL, comments: "Working with PMC design engineers gave us a fantastic relationship, helping to home in on some pertinent acoustic characteristics of various PMC speakers, and 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," agrees PMC co-founder and Chief Designer Peter Thomas, "and allows us to understand, with much improved accuracy, the effect of cabinet, driver and crossover design on what we hear. 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."

For more on RAOS, see www.npl.co.uk.