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Laser acoustic mapping for loudspeaker designers

06 March 2013

NPL's collaboration with the high-end loudspeaker specialist, PMC has provided vital industrial input on loudspeaker design and development, honing what has hitherto been an experimental technique into a valuable industrial tool. The PMC collaboration has seen NPL’s laser-based acoustic mapping technique adapted into a valuable design tool for all manufacturers of acoustic products, as Ian Butterworth explains.

Measuring how sound propagates is classically an arduous task as it is usually necessary to use manual microphone scans to build up a spatial map of the acoustic field. For instance, looking at how sound radiates from a loudspeaker (a property termed the 'directivity' of the loudspeaker) would require a large number of positions to be measured, each one requiring careful re-positioning of the microphone.

Computer-aided modelling can also be used to provide an insight into the various performance characteristics of the loudspeaker, but the validity of this technique is often limited. As a result, high-resolution directivity studies are rarely conducted, despite the fact that they are potentially valuable tools at the disposal of the loudspeaker designer.

A different approach
NPL's new technique uses lasers instead of microphones or computer modelling, and can rapidly map acoustic fields non-invasively. It provides a way of studying acoustic properties, such as a loudspeaker's directivity, in high-resolution and ultra-slow motion.

The technique, dubbed 'Rapid Acousto-Optic Scanning' (RAOS), uses the acousto-optic effect, which describes how light bends as it passes through an acoustic field.

When sound travels through air it causes the air's refractive index to change. This change can be detected by passing laser light through the air. The changes in refractive index bends the laser light slightly as it passes through, so by monitoring the light's speed it is possible to measure the bending effect.

NPL has shown that the subtle speed change due to typical sound pressure levels in air can be detected using a laser-interferometer, a device which monitors laser light phase changes. In this case, laser light is reflected off a stiff, optically retro-reflective board on the far side of the acoustic field, isolating the detectable effect on the speed of the laser light to the acoustic field.

Using a laser scanning vibrometer (a scanning version of the laser-interferometer), high resolution rapid scans of the sound field are possible. These provide a detailed insight into acoustic characteristics such as the directivity of loudspeakers and ultrasonic transducers, and the reflection characteristics of structural acoustic treatments such as diffusers and absorbers.

In this collaboration, PMC loudspeakers were evaluated using RAOS for various key acoustic characteristics, and a 3D tomographic reconstruction technique was explored for resolving greater detail in the acoustic field. PMC, in turn, provided guidance on the validity and efficacy of these studies.

The collaboration with PMC has enabled NPL to develop, trial, and improve the technique guided by the requirements of an industrial partner, developing a service for customers which provides valuable information and is cost-effective. PMC founder, Peter Thomas concludes:

"NPL's laser-based acousto-optic measurement technique provides a rapid, reliable method of viewing every aspect of loudspeaker dispersion. We are delighted to have collaborated with NPL to pioneer this highly innovative technique and we will apply this ground-breaking knowledge to further the development of all our products."

Ian Butterworth is a higher research scientist at NPL, specialising in the Sound-in-Air and Ultrasonic fields

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