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Yes, we can now measure loudspeaker directivity with lasers!

by ct_acoustic_frontier July 06, 2022

It's been just over a year since the UK's National Physical Laboratory first released information on their innovative laser-based acoustical mapping technique. PMC testing Thanks to UK loudspeaker company PMC we now have a real life use case for this technology - using lasers to study the dispersion characteristics of loudspeakers. All speakers have off axis frequency responses that vary from their on axis. The off axis response is critical in determining the spectral balance of sound as heard at the listening position. In fact at typical listening distances in normal sized domestic rooms the spectrum of the early reflected sound dominates the frequency response heard by the listener above about 300Hz. So validating a speaker's dispersion performance, particularly around the crossover point between different drivers, should be an important step in all loudspeaker design process. Loudspeaker measurement with lasers To date however studying off axis response has required physically moving a measurement microphone around the loudspeaker to assemble vertical and horizontal off axis response data. This technique is invasive (since it inserts a mic in the soundfield) and time consuming. The new technique, called Rapid Acousto-Optic Scanning, enables a map of the soundfield to be built up by analyzing how the sound emitted by the speaker causes the refractive index of air to change. Here's more from the press release:
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.



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