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I thought that "beamforming" only applied to actively emitting signals, so that the waveforms would cancel/reinforce each other, to get the desired "direction". I have no idea how that works for microphones. Google is not very helpful, I get lots of hits for products.

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A much more intuitive name for beamforming is spatial filtering [1]. It just means using mutiple receivers along with knowledge about their location to filter out noise and other signals you don't want. The term also applies to emitters like phased radar arrays or MIMO cell towers which can use spatial filtering for beamforming but it's a general technique.

[1] https://en.m.wikipedia.org/wiki/Beamforming

Thanks, I've always seen a Beamforming option in my DD-WRT Router but didn't know the exactly technical implementation.

Thank you!

It's a reciprocal process, meaning that it works for transmit as well as receive. In the receive direction, the antennas/microphones sample the incoming waves at various spatial points, and then the cancelling/reinforcing occurs when the received signals are phase shifted or delayed and summed.

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