Let us suppose that Bob and Alice both have walkie-talkies. They are both 3m apart from one another. Alice pushes her walkie-talkie to speak but instead of speaking, she starts walking toward Bob. Both Alice's and Bob's walkie talkies are facing each other in the same general direction (speakers are in the same direction). Alice suddenly reaches a point along her path to which Bob's Walkie talkie creates a loud pitch screeching sound, Alice moves, even more, closer and the sound escalates in pitch, it becomes unbearable and she unclicks the speak button.
What is the cause of this high pitched sound coming from Bob's walkie talkie? Why only at a certain distance from Bob does the sound start?
Summary:
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Alice is 3 m from Bob when she presses the speak button and walks towards Bob, at a certain point a loud screeching sound can be heard from Bob's walkie talkie.
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She moves closer and the sound gets louder and higher in pitch.
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The walkies talkie speakers are facing each other the whole time.
This was a problem I came up with just out of interest and something I experienced myself.
Best Answer
It's called feedback . Here is what happens:
When Alice presses TRANSMIT, it turns on the microphone in her radio and hence begins to transmit any noise that hits the mic. With Bob's radio on RECEIVE, its speaker is turned on and it plays out anything that it receives at that moment- which in this case is the audio signal transmitted from the mic in Alice's radio, which responds to any sound source near Alice.
As Alice approaches Bob, her mic begins to detect noises coming from Bob's speaker, which her radio then transmits to Bob's radio, which plays it through Bob's speaker, which is picked up by the mic in Alice's radio, etc., etc. and the signal gets looped around and around and around, getting stronger all the while as Alice gets closer to Bob.
The critical case for best looping happens when a sound in the loop has a wavelength in air approximately equal to the distance between the two radios, which at 10 feet is about 100Hz, at 1 foot it is 1000Hz and at 1 inch it is about 12,000Hz, so the pitch of the feedback will go higher and higher as the radios approach one another.