Clapton used 100-watt Marshall heads and 4×12 cabinets (Whereseric.com). These large amps allowed a larger sound field along with sustain and feedback. Sustain extends the length that a note is audible and feedback is when the sound from a source is looped through a speaker or amplifier many times. Both sustain and feedback are strong characteristics of rock music. Below are three examples of guitar feedback through an amplifier.