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NPL's new technique uses lasers instead of microphones or computer modeling, and can rapidly map acoustic fields non-invasively. It provides a way of studying acoustic properties, such as a speaker's directivity, in high-resolution and ultra-slow motion. The technique, which is called “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 you can 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 described above), 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.Very very cool stuff! FYI the NPL appear's to be offering this as a commercial service.
Nyal Mellor, Founder, Acoustic Frontiers