Forgot the name of this circuit (description inside)

[DeeJayCruiser]

A circuit that measures two resonant frequencies and compares them with an LM741. One 555 is a metal detector with a coil of wire, and the other 555 is to be matched in resonance.

Any idea what the name of this circuit is? I thought it was an OP Amp differential amplifier, for metal detection, but as you can imagine double checking that on google is a little difficult

 

[vk6kro]

That would be a mixer.

The two oscillators produce an output above the limit of human hearing, but when the two mix, the difference frequency is audible.