National Physical Laboratory

Seeing sound

NPL has developed a non-invasive method to visualise sound propagation and help sound engineers design out dead spots in loudspeakers.

HIFI loudspeakers can be designed to deliver the full frequency range of audible sound, but it is difficult to achieve a smooth frequency output in all directions. Dead spots are caused by deconstructive interference, as a result of sound waves overlapping and cancelling each other out. This creates areas where the frequency response of the loudspeaker is less smooth, and sound quality is diminished.

NPL's hemi-anechoic chamber
The NPL hemi-anechoic chamber provides ideal
conditions for sound field measurements

Determining the nature of these dead spots has proven difficult in the past. High accuracy acoustic measurements can be made using a microphone, but to build up a picture of the spatial distribution of the sound many point measurements are required within the 3D space. Manufacturers can conduct computer-aided simulations, but these can prove inaccurate to the actual loudspeaker performance through the variability of the manufacturing process.

NPL has developed a solution to this problem. A new laser-driven technique allows remote, non-invasive and rapid mapping of sound fields, which will provide loudspeaker manufacturers with detailed data on which to design their technology.

The technique builds on a piece of technology developed for the study of mechanical vibration; the laser vibrometer, and relies on a phenomenon called the acousto-optic effect. This describes the slight change in the speed of light in air when it passes through an acoustic field, causing a phase shift in the light that can be detected using the vibrometer.

To measure the acoustic output, the laser is positioned to the side of the loudspeaker and rapidly scanned through a series of points in front of the loudspeaker, before being reflected back to the instrument by a retro-reflective mirror (see diagram below) .

By measuring the laser as it returns to its source, the technology can rapidly provide spatially distributed phase shift data. This data can then be used to construct an image, or video, of sound propagation around the source, i.e. the loudspeaker.

An overview of the experimental set-up   The data is used to construct an image of sound propagation around a source such as a speaker
An overview of the experimental set-up     The data is used to construct an image of sound
propagation around a source such as a speaker


Ian Butterworth, the leader of the project at NPL, said:

"Being a rapid technique that provides detailed spatial information, manufacturers will be able to better understand how different designs impact the loudspeaker's directionality, and design out the dead spots which could limit the quality of the loudspeaker."

View science poster entitled 'Seeing Sound: Exploiting the Acousto-Optic Effect'

Find out more about NPL's work in Acoustics  |  NPL's Acoustic facilities are also available for hire.

Last Updated: 29 Aug 2013
Created: 9 Nov 2011


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