Not-so-super sonics

The Holosonics system (Feature, 18 June) is already in use at Heathrow Airport and by major retailers, allowing sound to be directed selectively.

Your report of this 'striking development' was enthusiastic and uncritical — but there is more to the story.

The idea may be new for directing sound in public spaces, but it has long been used in underwater sonar systems.

For instance, a submarine may project a low-frequency sound by emitting a pair of intense, high-frequency beams. The non-linear properties of sea water 'mix' the high frequencies to make the low frequency, which achieves greater range for the sonar, and the transducers can be small enough to fit within the submarine's hull.

Great caution is needed before Holosonics' ultrasonic systems are turned on an unsuspecting public. Non-linear mixing in air can only generate audible sound when the actual sound levels are far greater; yet the ultrasound may not be perceived at all, or just as a vague feeling of distress.

Insidious hearing damage could be caused to large numbers of susceptible individuals before the cause was realised. Conventional sound level meters are 'weighted' (read filtered) to indicate the audible sound, so will register nothing for the intense ultrasound.

I hope the Health and Safety Executive is aware of this potential danger, which is far more plausible than the much-publicised hazards of low-level radiation and electromagnetic fields.

John Cole

Yelverton, Devon