Researchers at the National Physical Laboratory (NPL) in the U.K. have developed a way to find “dead spots” in front of loudspeakers. Dead spots result from destructive interference from the output of two different speakers, whether from two separate speaker units or from the low-frequency woofer and high-frequency tweeter within the same enclosed unit.
In the video, NPL’s Ian Butterworth explains how shining a laser through the sound field in front of a speaker and bouncing the light off a reflective surface can pinpoint dead spots from tweeter–woofer interference. The idea is to help manufacturers root out design flaws in their loudspeakers.
The technique makes use of acousto-optics, the interaction of light and sound waves. “Basically, light travels faster in lower-pressure air, therefore its speed is very slightly affected by the presence of sound,” Butterworth says.
“By monitoring these subtle variations and scanning the laser through the area in front of the speaker, we can build up a picture of how the sound radiates from the speaker.” The video doesn’t discuss dead spots caused by interference between two different speaker units, which seems to be a fairly common problem, but perhaps the same principles could be adapted to that use.
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