Eli5: Initial Microphone Noise

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Eli5: why does your microphone seem to be picking up noise when you first start an audio recording with it, even if you have no audio going to the microphone? After the first few seconds it drops off to nothing again.

In: Technology
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You’re most likely seeing the effect of something called “Automatic Gain Control” (AGC).

It’s not the microphone that’s doing it, but the recording system. It doesn’t know what level to expect from the microphone, so it starts at a specific level. Once it has a stronger signal than the background noise it was just listening to, it can reduce the gain so that the signal doesn’t blow out the recording. This has the result of capturing the audio at the correct level, as well as reducing the volume of that initial background noise.