Acoustic Detectors

We now come to a group of related detectors which are unlikely to find a use in the average home alarm system, being used principally for industrial installations. However, there could be some special situations for which they have an application.

The first of these is the acoustic detector. Also called sonic detectors, these are not to be confused with the ultrasonic devices. The term embraces a range of sensors that operate mostly by the sound generated by breaking glass.

They consist of a microphone, an amplifier and an output relay. A special filter circuit is included which passes only those sound frequencies generated by breaking glass, and thus is immune from false alarms caused by other sounds. One type, also termed piezoelectric because the microphone is of the crystal contact variety, is mounted on the glass by means of an adhesive. The complete unit can be very small, little larger than one inch in diameter. The filter eliminates low frequencies, passing those in the high 6000 - 8000 Hz range.

Others are mounted on an adjacent wall or ceiling and have a normal air pressure microphone. Some of these have a more complex filter that responds first to certain low frequencies which are produced on impact, then to the high frequencies generated as the glass shatters. They will not trigger unless both the required frequencies occur in the correct sequence, thereby giving an even greater immunity against false alarms.

The glass-mounted type must be positioned at least 2-3 inches from any window frame because the intensity of vibrations in glass reduces from maximum at the center of the pane to zero at the frame. Both types protect an approximately 10 foot radius of glass. Some have an indicator light which latches on to show if it has been activated, or which one if there are several. If glass has been broken this may seem to be superfluous, but it does help to identify the source of a false alarm if one should occur.

They offer an advantage over window foil for large areas where a lot of foil may be considered to be visually detracting, and especially for multi-pane windows where there may be practical problems in foil laying. The disadvantage is that the visual deterrent effect of foil is lost. For multi-pane windows the space rather than the contact sensor should be used, as each pane would require its own contact detector.

A similar principle is used in wall sound detectors. With these, either structure or airborne sounds are picked up by the microphone and filtered so that only those produced by hammering, drilling, and cutting activate the unit. Again it is mainly the high frequencies that are detected, so those produced by bumps, traffic noise, and other normal happenings are ignored.