Lasers have been used for loudspeaker measurements since the British manufacturer Celestion first set them up pointing at a cone in the early eighties.

Now the U.K national Physical laboratory has taken up their light sabres to produce a very neat visualisation of the pressure wave differences as the result of the acoustic output of a loudspeaker operating on a room.



So now we Audiophiles who spend hours tweaking speakers have a new laser-based tool to help perfect the setup. One of the fundamentals to an excellent sounding hi-fi is avoiding dead spots, the points at which sound waves overlap and cancel each other out through destructive waveform interference. These can be caused by incorrect speaker placement, but they also occur when a single speaker plays mid-range frequencies and sound from the woofer and tweeter interact.

Speaker manufacturers as in the aforementioned Celestion have been detecting these dead spots in their designs by performing microphone tests in the space around the speaker, which is a time-consuming process, or computer simulations, which can prove inaccurate. Now, the National Physical Laboratory (NPL) in London, UK has come up with this rather neat alternative that takes advantage of the way light changes as it passes through sound waves.

Researchers at NPL shone a laser vibrometer, which measures tiny vibrations, past the front of a speaker and then bounced the light back with a reflecting surface. Light waves change their phase when reflected and interacting with sound waves can affect this change because light travels faster in lower-pressure air. The laser vibrometer can pick up the differences, letting the researchers map out the sound waves and track down the dead spots. In the above YouTube clip you can see the process being enacted on a Genelec monitor in an anechoic chamber in the U.K.