Loudspeakers have improved a lot in the last 50 years but one pesky issue has remained; dead spots.  
 

Modern oudspeakers 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 radiating sound waves overlapping and cancellng each other out. This often happens when the sound is radiating from two or more sources, like in the mid-frequency ranges where both the 'woofer' and 'tweeter' loudspeaker cones are both active. This creates areas where the frequency response of the loudspeaker is less smooth, and sound quality is diminished.

Determining the nature of these dead spots is difficult. Acoustic measurements are 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 numerical simulations but those are only as accurate as the input for materials; the actual loudspeaker may differ dramatically due to the real-world variability of the materials and manufacturing process.

The National Physical Laboratory (NPL) has developed a laser-driven technique which allows remote, non-invasive and rapid mapping of sound fields. They say it will provide loudspeaker manufacturers with reliable 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 on research for its application to the 3D characterization of underwater sonar arrays. This NPL work has shown that in air, the acousto-optic effect, the resulting optical phase change of light as it passes through an acoustic field, is significant enough to be detected.

To measure the acoustic output from the loudspeaker, the laser is positioned to the side of the loudspeaker and is rapidly scanned through a series of points in front of the loudspeaker, being reflected back to the laser vibrometer by virtue of a retro-reflective mirror on the other side. By measuring the laser as it returns to its source, the technology can rapidly provide spatially distributed phase shift data, enabling an image, or video, of sound propagation around the source to be constructed. 

Ian Butterworth, project lead at NPL, said, "We're now looking to conduct further studies, scanning larger areas with higher definition, to get a better picture of how sound is propagating away from these loudspeakers."

The measurement technique is ideally performed in conditions that minimize sound reflection, such as NPL's hemi-anechoic chamber, but measurements can also be carried out outdoors given the natural hemi-anechoic nature of fields.