Acousto-optic system enables speaker output to be analysed

A new laser system that can visualise sound waves could help speaker manufacturers improve their products’ audio quality.

Measurement institute the National Physical Laboratory (NPL) has developed a technique to create a computer image of sound waves emerging from a speaker, by measuring how the audio vibrations affect a laser beam.

This provides a way of identifying ‘dead spots’ in a speaker’s output — small areas where there is audio interference from two similar sound waves from different speaker cones that cancel each other out.

The technique combines specially designed software with technology developed to analyse underwater sonar arrays.

A laser vibrometer fires beams of light across the speaker from many slightly different positions. These are then reflected back by a type of mirror known as a retroreflector, and captured by a receiver.

Afterwards, the equipment measures the vibrations from the speaker by calculating how much the phase pattern of the light waves has changed as they interacted with the sound waves — this is known as the acousto-optic effect.

‘The acousto-optic effect is stronger underwater, so it’s slightly easier to map than in air,’ NPL research scientist Ian Butterworth told The Engineer.

‘We wondered whether it was possible to do in air and, due to the sensitivity of the equipment we’ve got, we can actually do it.’

The retroreflector ensured the vibrometer was able to capture enough light to calculate the vibrations, he added.

‘If you have a regular mirror, many of the reflections of the mirror won’t go back into the receiver when the angle changes.

‘With a retroreflector, the light goes back to the receiver no matter what angle you hit it at. It’s exactly the same material that road signs are made from.’

NPL is now developing a larger version of the equipment to study bigger speakers and other objects that affect sound, such as acoustic diffusers, which are designed to spread sound out across a space evenly or even to partition walls that aim to block sound.

The company also hopes to work directly with speaker manufacturers, which already use similar equipment to analyse their products.

‘The laser vibrometer is quite a common bit of kit nowadays for scanning vibrations on surfaces,’ said Butterworth. ‘It’s quite common to see speaker manufacturers doing things such as scanning speaker cones, to see how they move.’