How speakers make sound
On the other end of the audio stack, I highly recommend this multipart video lecture about microphone design. The guy is a legit microphone engineer who knows what he's talking about and explains it really well. Some of the principles, like how directional vs. omnidirectional microphones work, are actually quite intuitive and much simpler than I would have guessed.https://youtube.com/watch?v=ihAG6cMpUlY