INAUDIBLE SOUND

Sound is formed by propagating waves of pressure in a medium such as air. Its pitch is given by the number of cycles per second (at a stationary point through which the wave passes); the higher the frequency, measured in hertz (Hz), the higher the pitch. Up to about 18,000 Hz (18 kilohertz, kHz), we can hear it; the upper limit is individual, usually higher for women, and still higher for children. Beyond that limit sound still exists, even though we can't hear it, and it is called ultrasound. Dogs can hear still higher, and bats have an ultrasonic radar that works to some 100 kHz. And man, even though he cannot hear it, can produce it to some million Hz (MHz), mostly for some purpose, but also inadvertently, in most cases by electricity of the same frequency and some kind of electroacoustic coupling.

"Electroacoustic coupling" is an exalted expression for a gadget that converts electricity to sound. This can be done on several prin-ciples, but the most common is magnetic, and probably the simplest is a headphone as in a telephone set. Its essential in-gredients are a flexible, metallic membrane attracted by an electromagnet formed by a coil carrying the "signal" current. That is an alternating current varying in the rhythm of the sound which had originally impinged on some microphone. The membrane thus vibrates in the rhythm of the current and thus becomes a source of sound by compressing and rarefying the air in its immediate neigh-borhood. The pressure variations then propagate outward as sound waves.

The principle here is an oscillating magnetic field attracting a metallic component whose mechanical vibrations become a source of sound. This is, of course, done on purpose. But the same thing can happen inadvertently. A transformer, for example, also has current-carrying coils wound on an iron core, though not for producing sound. The core consists of thin laminations, which are tightly bolted together to form a solid frame. But not tightly enough. The bolts cannot be everywhere, and the segments between them will vibrate just a little in the rhythm of the 60 Hz current in the windings, thus giving rise to the well known "60 cycle hum" that one can hear near a large transformer and near some electric household appliances.

Ultrasound is no different. It is used for many applications, such as burglar alarms and probing inaccessible material for faults by using it in a kind of mini-radar. That includes testing welds and other structures for imperfections, and imaging internal parts of the body for signs of disease. (Before my operation a year ago it was used on me, and the doctor tried to put me at ease by assuring me that this had nothing to do with radiation. She was pleased when I told her that radiation did not frighten me, but less so when I maintained that radiation includes sound no less than electro-magnetic waves, whether ionizing or not. She evidently holds that the obscene R-word should be used only for obscene things of n***ear origin.)