2-way analog intercom

[Audioguru]

A modern full duplex intercom or speakerphone does not use a delay. Instead it used an echo-cancelling IC.
The IC cancels the speaker sound that is in the mic circuit.

 

[BradtheRad]

Phase advance is also a possibility. It's done to a sinewave going through a series capacitor.

The values in this simulation cause a waveform to advance by 0.3 mSec (4 inch distance of sound travel). Since feedback is sine shaped conceivably you can cancel it by cascading 3 or 4 RC stages. Play with values to change time constants.

Theoretically (if it's not too paradoxical) you may be able to stop feedback before it begins.

Additional tactics:
* invert polarity
* amplify
* sum signals
* subtract signals

No guarantee it solves anything, nevertheless it's easy and could be worth experimenting with.

Furthermore you can create phase delay by making integrators instead... that is by reversing positions of the capacitors and 1k resistors. With sufficient RC stages you can mimic several inches worth of sound travel.

 

[Audioguru]

Phase shifting to cancel acoustical feedback works only for one frequency and its harmonics. But intercoms play many frequencies.
Some PA systems use frequency shifting but performers say it sounds wrong to them.