Let's cover ultrasonic detectors. The term ultrasonic denotes sound frequencies that are above the range of human hearing, the upper limit of which is 16 kHz in healthy people in their twenties. The frequencies used in ultrasonic detectors vary between different models, ranging from 23 to 40 kHz. The frequency is generated by an electronic oscillator and is fed to one or more loudspeakers. These need to be very small, smaller in fact than the tweeter in a hi-fi loudspeaker, because the moving parts must move very rapidly and so their mass must be kept to a minimum. This is an advantage of an ultrasonic detector alarm system as the units can then be made small and unobtrusive. The same unit contains a microphone with amplifying and processing circuits.

Ultra-high frequency sound is thus projected into the protected area of the ultrasonic detector. Some of it is received directly by the microphone from the loudspeaker, while some is picked up after being reflected from walls and objects in the room. When there is no movement, both direct and reflected sounds are of the same frequency. Should movement of an object occur, the sound reflected from it will undergo a change of frequency owing to the Doppler Effect. Thus the ultrasonic detector microphone now picks up two frequencies: the original received directly from the loudspeaker, and the shifted frequency reflected from the moving object.

If two different frequencies are mixed, the result is that a third one appears that is the difference between the two. When the two frequencies are close, the third is often termed a beat note. The effect can sometimes be heard when two internal combustion engines are idling at slightly different speeds: the beat note is heard by the ultrasonic detector as a throbbing sound that varies in frequency as the speed of one or other of the engines changes.

The processing circuits in the ultrasonic detector detect the presence of the beat note and actuate the output relay which is of normally closed type. The ultrasonic detector can thus be connected into a normal loop, but it is preferable for space detectors to be connected to a separate zone from the perimeter sensors so that rapid identification of the alarm source can be made. A power supply of 25 - 50 mA is required from the control box.

Ultrasonic detectors usually have a range up to 30ft, but they have a sensitivity control which can reduce the range down to about 10 ft. The polar pattern is in the form of a narrow lobe and so the device affords little protection at the sides. It should therefore be pointed towards the expected point of entry. As the Doppler Effect is greatest for forward or backward motion, the sensor is more sensitive to these than to side-to-side movements. This is another reason to mount the ultrasonic detector facing the possible point of entry.