This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This circuit suggested by Siemens can provide time intervals for triggering values that are given...
This circuit was found in a 90s American documentation from the 90s. The components are from that...
In the figure we have an inductance bridge for values between 1 and 30 uH at a frequency of 5 MHz....