system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do in-car audio systems avoid feedback when using speakerphones?
gpt
Fun fact, car audio is actually really complicated when you start adding features like that. 

First off, feedback only happens when the speaker is playing back the same signal that the microphone picks up. So if you're talking on speaker phone with someone who is on handset, no feedback will happen. Your voice will not come through the speakers and be picked up. 

So the only case where feedback can occur is when you're on speakerphone and the other end is also on speakerphone. And luckily, feedback only happens when there is little to no delay at all between the systems... in this case the shitty-ness of cellular networks is an advantage, there is always a non negligible delay between transmitter and receiver. So the feedback you experience is an echo. 

So the way you get rid of feedback has to do with getting rid of echoes. The logical and most significant way to reduce these echoes is by controlling the direction of the microphone and speakers; so long as the mic and speakers do not overlap in their radiation fields, there will be no feedback. To do this, there are simple designs (directional mics are easy to make). There are also complex designs where the speakers and mics are actually arrays of mics/speakers, controlling the delay and mixing between them will allow you to form "beams" where audio is picked up or transmitted. 

Another nice thing about echoes is that you can make some assumptions about them. An echo by definition is a delayed copy of something that has already happened... so if you record what comes through the mic, and listen to what comes through the speakers and check against recent things coming through the mic, you can remove those copies. Boom, no echoes or feedback. Again, the inherent delay between systems is your friend here. But actually getting that to work is not trivial.