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Laser "sound visualizer" may lead to better loudspeakers

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November 9, 2011

Visualization of the acousto-optic effect (Image: NPL)

Visualization of the acousto-optic effect (Image: NPL)

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We've been following the quest for the world's best speakers for some time but remarkably, there's still room for improvement. A key issue that plagues proper sound reproduction (and thus its perceived quality) is a phenomenon known as deconstructive interference. This occurs when audio signals overlap and cancel one another out, creating dead spots which, until recently, have been very difficult to track. Now, a team from Britain's National Physical Laboratory (NPL) has figured out a clever way to make speaker sound "visible" - and they do it with laser light.

"Speakers are very common in modern life, whether it's for entertainment, communication or safety," observed NPL team leader Ian Butterworth. "And you've probably noticed the loudness of sound you hear coming from a loudspeaker is likely to vary depending on position. At low frequencies, most speakers are pretty even, which is termed omnidirectional. But at high frequencies they become much more directional- we've much stronger radiation in certain directions."

Listeners sitting behind or to the speaker's side often detect a fall-off in the higher frequencies. In previous efforts to reduce or eliminate this directionality, speaker designers resorted to the time-consuming task of placing a microphone in several positions to map out the dead spots. The NPL team saw this as an opportunity to make a big difference in the way new speakers are designed.

Schematic of the sound visualization approach (Image: NPL)

"So we've been developing a new measurement technique that uses a laser vibrometer, which is a device that's usually used to look at mechanical vibrations of surfaces," Butterworth continued. "But instead, we're reflecting the laser off a stiff board that's covered in retro-reflective material. By passing the laser light past the speaker and reflecting off the board behind the speaker, the light is subject to the acousto-optic (AO) effect."

In the AO effect, light passing through an acoustic field undergoes an optical phase change that is significant enough to be detected visually. Butterworth's team is currently conducting additional studies, performing higher definition scans of even larger areas to get much better images of how sound emanates from high-end loudspeakers.

"Basically, light travels faster in lower pressure air, therefore its speed is very slightly affected by the presence of sound," he continued. "By monitoring these subtle variations and scanning the laser through the air in front of the speaker, we can build up a picture of how the sound radiates from the speaker. And we're able to do this at roughly 100,000 frames per second."

The new technique involves setting the laser to the side of the speaker to be tested. Its beam is then rapidly scanned through numerous points in front of the speaker. The beams are then reflected from a mirrored surface back to the vibrometer's receiver where, borrowing on its ability to characterize underwater sonar arrays, the device yields spatially-distributed phase shift data. From this, useful images (or videos) of sound waves around the source can be assembled.

"This is a significant breakthrough for loudspeaker manufacturers. By having actual data to rely on, they will be able to better understand how different designs impact the loudspeaker's directionality, and design out the dead spots which could limit the quality of the loudspeaker," Butterworth added, and that sounds mighty good to us.

Source: NPL

See the video below for an explanation and demo of the technique:



About the Author
Randolph Jonsson A native San Franciscan, Randolph attended the U.S. Naval Academy at Annapolis, Maryland before finding his way to the film business. Eventually, he landed a job at George Lucas' Industrial Light + Magic, where he worked on many top-grossing films in both the camera and computer graphics departments. A proud member of MENSA, he's passionate about technology, optimal health, photography, marine biology, writing, world travel and the occasional, well-crafted gin and tonic!   All articles by Randolph Jonsson
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