Humans can only perceive sound in a frequency range of 16 Hz to 20,000 Hz with pressure fluctuations of 0.00002 Pa to 20 Pa. This range is referred to as the human hearing range. The following circuit can be used to make ultrasound audible to humans. The following circuit can be used to make ultrasound audible to humans. The circuit was developed by Burkhard Kainka. I have only modified it a little bit. So I use only one supply voltage and added a MOSFET switch so that you can enable and disable the circuit. It is basically a bat detector with an AM/FM radio chip (CD2003) in its core.

The PCB is completely designed with SMD components.

The speaker will later sit directly on the ear.

Of course, you can also use it to make the emitted ultrasonic waves of the HC-SR04 ultrasonic sensor audible. They are heard as a loud, knocking noise.