The audio frequencies between 15 kHz and 18 kHz is normally not audible to humans. This audio range is also called ultrasound. In order to hear it, it must first be converted to a
frequency range which is audible to humans.
The circuit featured here mixes the ultrasound with a frequency generated by a BFO circuit. The result
is a mixture of several frequencies from which one (the difference) can be clearly heard. The undesirable signals are filtered out by a 4 kHz low pass filter.
The transducer is a special ultrasound sensor. Portable batteries can be used to power this circuit.
Electronic Circuits volume 1.0 - Circuit Nr. 22
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.