Laser visualisation to lead to better speakers?

AUTHOR: Inavate

A key issue that plagues loudspeaker designers is the phenomenon known as deconstructive interference. This occurs when audio signals overlap and cancel one another out, creating dead spots in the wave field which, until recently, have been very difficult to track. Now, a team from Britain's National Physical Laboratory (NPL) has developed clever way to make speaker sound "visible" via the use of laser light.

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," said NPL team leader Ian Butterworth.

"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.