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.'