The acousto-optical effect that uses lasers to see sound

If you've ever sat in a sound "dead zone" in a movie theater or concert, you know what it's like to be at a loud event where everything sounds muffled and distorted. Luckily, lasers may soon put an end to these dead zones. The UK's National Physical Laboratory has figured out how to use lasers to see sound waves in a room - as well as acoustic dead zones.

Sound travels in waves. This generally works very well, since it allows massively varied information to travel long distances by causing minor, rhythmic changes in air pressure. The problem is, these waves of high and low pressure can be emitted by different speakers at once, and when they cross, things get weird. When a high pressure area meets a low pressure area, the sounds can cancel out, resulting in no pressure change at all. A person standing there will hear nothing. These dead zones are anathema to theaters and stadiums, where sound needs to carry well. Finding them is a challenge, since getting people to stand at attention in every area of a room is expensive, inexact, and time-consuming.

A new piece of equipment may be able to get the job done with some computers and a thorough knowledge of the acousto-optic effect. We all know that light travels through a vacuum at a smidge under 3 million meters per second, but it can't get anywhere near that fast on Earth. Earth is covered with a material - the atmosphere. When light travels through matter, it has to deal with that matter's refractive index. This is a measure of how fast the medium slows down light. The slowing of light depends both on the medium and the wavelength of light.

As pressure waves move through the air, they change the air density. That shift in density changes the air's refractive index. When light hits a high-density section, it will be slowed down. When it hits a low-density section, it will shoot ahead. These regular high-density and low-density sections can also act as a diffraction grating, causing the light to splay out and cause interference. By bouncing laser light off a reflective sheet on the far side of the speaker, light makes two trips through the acoustic waves. It's then taken up by a detector. The acoustic waves will have caused the light to move at different angles and different speeds, causing phase shift.

Laser light is light that is emitted as a single wave. No matter how much light, how many photons, is emitted, all the peaks of the waves line up, and all the troughs line up. It should come back as a unified wave, too. But because some of the waves are sped up, or slowed down, or scattered around, the peak of one light wave will have shifted to be out of phase with the peak of another. A computer takes these shifts, and the physical data about the room, and analyzes exactly what kind of sound waves caused these phase shifts. This allows the device to build up a picture of what sound waves are light in every part of the room. People could create maps of dead acoustic spots and either alter the room or the seating pattern accordingly.

Take a look at a depiction of one such map.

Via GA Tech and National Physics Laboratory.

Image of Laser Interference: Falcorian