Analysis
When two or more notes are played through a distortion process which non-linearly transforms the audio signal, additional partials are generated at the sums and differences of the frequencies of the harmonics of those notes (intermodulation distortion).
When a normal chord (for example, a major or minor chord) consisting of three or more different degrees of the scale is played through distortion, the number of different frequencies generated, and the complex ratios between them, can cause the resulting sound to be messy and indistinct (furthered by equal temperament and inharmonicity).
However, in a power chord, the ratio between the frequencies of the root and fifth is 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.
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, due to the resultant tone effect
Power chords also have the added advantage of being relatively easy to play (see "Fingering" below), allowing fast chord changes and easy incorporation into melodies and riffs.
Read more about this topic: Power Chord
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