Eric Peterson: Have you ever been to a concert and heard that ear piercing squeal every once in awhile? That’s called feedback, and it happens when the sound being captured by an open microphone gets sent through an amplifier, projected out by speakers, then picked up again by the microphone, creating an exponentially increasing feedback loop. There are a lot of ways it can happen and one of the main responsibilities of a live sound engineer is to mitigate it. Guitars can feedback too, but instead of the diaphragm of a microphone vibrating, the body of the guitar vibrates at self-reinforcing frequencies, causing the strings to vibrate with ever increasing intensity. Sometimes, they squeal, but more often, they hum, rumble, or whine; it’s almost always a more pleasant and gradual sound than microphone-related feedback. This week’s piece is built on top of three guitars feeding back, to show that even mellow sounds can be built from that ear piercing squeal.