Sight as well as sound

Researchers at the National Physical Laboratory (NPL) have developed a technique for imaging acoustic fields that could lead to huge improvements in microphone and loudspeaker design.

Peter Theobold is a senior research scientist in NPL’s acoustics department, where researchers have been using laser vibrometry to measure the acoustic waves as they pass through air and water.

He explained that when an acoustic wave passes through a medium – either air or water – the pressure oscillates between high and low, which produces a corresponding change in density. As a result of these density changes the refractive index is also altered, a shift that can be detected using scanning Doppler laser vibrometry.

‘Using interferometry or vibrometry you can detect the change in optical path length,’ said Theobold. ‘As you change the refractive index the laser travels further and this is something that can be detected. This gives us an image of how the waves are moving through the air or water.’

The team’s work on detecting the acoustic waves in water also used innovative tomographic techniques — similar to those used in CT scans — which can be used to build up a 3D image of the waves.

This technique has a number of implications across a variety of fields, according to Theobold. The team hopes that optical imaging of acoustic waves could be used in the medical field by imaging waves used in High-Intensity Frequency Ultrasound (HIFU) — a technique that uses ultrasonic waves to heat and destroy tumours. By monitoring these ultrasound waves, Theobold believes the safety of HIFU could be considerably improved as they gain better understanding of how the waves propagate.

Imaging the acoustic scattering produced by commercial loudspeakers could also lead to a huge improvement in loudspeaker design. While speaker manufacturers now use modelling techniques to design their products, Theobold believes that imaging the waves the speakers produce could result in far more efficient designs.

He said: ‘We are looking to talk to loudspeaker manufacturers because we think it would be good for their design if you could see how the waves propagate and how the speaker itself interferes with the waves. It could help them design a product which has a reduced signature when it is in an acoustic field.’