Ultrasonic Microphone

This interesting electronic project enables you to clearly hear a world of sounds beyond that of human ability. The possible applications of the ultrasonic microphone you will create (see Figure 26-1) range from the detection of leaking gases, liquids, the mechanical wear of bearings, rotational and recipro­cating devices, and electrical leakage on power-line insulators. A whole world of sounds coming from liv­ing creatures is also audible. Simple events like a cat walking across wet grass, the rattling of key chains, and even a collapsing plastic bag all are clearly heard. On a warm summer night, the sounds that can be heard are remarkable, as bats to small insects all perform a cacophony of nature’s own orchestra at its best.

Ultrasonic Microphone

Figure 26-1 Ultrasonic microphone with parabolic reflector

This handheld and directional microphone easily dclecls and locates these high-frequency sounds. The addition of a parabolic reflector further enhances the performance of this project. Expect to spend $30 lo $50 for this rewarding effort.

This project enables you to listen to a world of sound that few people even know exists. The unit is built in a gun configuration with the barrel housing the elec­tronics. A rear panel contains the on/off volume, the tuning control pot. and the headphone jack. The front of the unit contains the directional receiving trans­ducer. The handle houses the batteries.

The addition of an optional parabolic reflector greatly enhances the device’s performance, providing super-high gain and directivity.


One of the most interesting sources of high-fre – quency sound is the many species of insects emitting their mating and warning calls. On a typical summer night, one can spend hours listening to bats and other strange insect noises. A whole new world of natural sound awaits the user. Many man-made sounds also generate high-frequency sounds easily detected by the devicc. Several examples are as follows, yet these only represent a small fraction of the potential sources of high-frcquency sounds:

• Leaking gases and rushing air.

■ Water from sprinklers or leaks.

• Corona leakage, sparking devices, or lightning.

■ Fires and chemical reactions.

■ Animals walking in wet grass or in the brush can be heard over a considerable distance. This is an excellent aid for hunters or trackers, or for iust finding pets ai night.

■ Computer monitors. TV sets, high-frequency oscillators, mechanical wear bearings, rattle detection often developing in automobiles, plastic bags, and rattling change.

An excellent demonstration of this ultrasonic microphonc would involve Doppler shifts where motion toward the source produces a rise in fre­quency and motion away from it does the opposite.

Doppler shift is when an observer moving toward a source of sound experiences an increasing fre­quency. This is easy to visualize when one realizes that sound propagates as a longitudinal wave at a rel­atively constant velocity. As the observer moves toward the direction of the sound source, he inter­cepts more waves in a shorter period of time, thus hearing a sound that seems to be shorter in wave­length or higher in frequency.

A fun game both for children and adults is to hide a small test oscillator and attempt to have your oppo­nent locate it in a minimal amount of time.

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