1. Pickups and Amplification: Electric guitars have pickups, which are essentially electromagnets near the strings. When the strings vibrate, they induce a corresponding electrical signal in the pickups. This signal is then amplified by the guitar amplifier.
2. Resonance: When the amplified signal is played back through the guitar's speaker, it creates sound waves that can interact with the strings. If the frequency of the sound waves matches the natural resonant frequency of a string, that string will start to vibrate sympathetically.
3. Positive Feedback Loop: As the vibrating string moves past the pickup, it induces another electrical signal, which is again amplified and played back through the speaker. This creates a positive feedback loop, causing the amplitude of the vibrations and the loudness of the sound to increase rapidly.
4. Sustained Squeal: As long as the conditions for sympathetic vibration are maintained, the string will continue to vibrate and produce a high-pitched, sustained squeal. This squeeky noise can be controlled by adjusting the guitar's volume, tone, and proximity to the amplifier.
In essence, feedback occurs when the amplified sound from the speaker is picked up by the guitar's pickups, creating a self-reinforcing loop of sound that resonates with the strings. While feedback can be an unwanted occurrence in some situations, guitarists can also intentionally use it to create a variety of musical effects.