Let say we have a microphone and a speaker. The microphone will receive the surrounding sound, and the speaker will amplify it. Assume we have unlimited power for the microphone and speaker, so they can work forever. When we speak to the microphone, the sound should be repeated by the speaker. While the speaker generates sound, the sound should input to the microphone, and the microphone should repeat to the speaker. So, the sound should never stop, in a loop. But in reality, we don't have this issue. What's going on? Do my assumptions have something missing? Thanks.
Anyone who's ever set up a public address system knows that we do have this issue. It's generally called feedback, and tends to result in a high-pitched screaming sort of sound. It can be kept under control by careful use of EQ (graphic equaliser) and correct positioning of the microphone and speakers. (Pro audio people probably have lots of other tricks for dealing with it as well.) Basically you get feedback if the overall gain of any frequency is greater than 1, so you need make sure that doesn't happen. If you have a specific system such as a phone or computer where feedback doesn't happen, this is because it's been carefully designed to avoid it.