From Wikipedia, regarding "power chords" as played on the electric guitar:
"... in a power chord, the ratio between the frequencies of the root and fifth are very close to the just interval 3:2. When played through distortion, the intermodulation leads to the production of partials closely related in frequency to the harmonics of the original two notes, producing a more coherent sound. The intermodulation causes the spectrum of the sound to be expanded in both directions, and with enough distortion, a new fundamental frequency component appears at an octave lower than the root note of the chord played without distortion, giving a richer, more bassy and more subjectively 'powerful' sound than the undistorted signal.[5]
Even when played without distortion, the simple ratios between the harmonics in the notes of a power chord can give a stark and powerful sound, owing to the resultant tone effect.
Power chords also have the added advantage of being relatively easy to play
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