A laser-driven technique that allows remote, noninvasive and rapid mapping of sound fields has been developed — a solution expected to help high-performance loudspeaker manufacturers overcome the issue of dead spots. Dead spots are caused by deconstructive interference from radiating sound waves overlapping and canceling each other out. The biggest issue is when sound radiates from two or more sources, common in the midfrequency 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. The laser-driven technique developed by researchers at the National Physical Laboratory (NPL), the UK's national measurement institute, builds on technology developed for the study of mechanical vibration — the laser vibrometer — and on research for its application to the 3-D characterization of underwater sonar arrays. 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 rapidly scanned through a series of points in front of it, being reflected back to the laser vibrometer by a retroreflective 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. “This is a significant breakthrough for loudspeaker manufacturers. By having actual data to rely on, they 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,” said Ian Butterworth, NPL project leader. The team said applications will be for high-end, in-home surround-sound systems as well as for outdoor systems typically used for concerts and live events. “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,” Butterworth said. For more information, visit: www.npl.co.uk/acoustics