Intermodulation distortion creates new tones with frequencies at the sum and difference of the frequencies (and integer multiple of the frequencies) of each pai

Intermodulation distortion creates new tones with frequencies at the sum and difference of the frequencies (and integer multiple of the frequencies) of each pair of tones you're distorting. Usually this produces tones that are harmonically unrelated, but in the case of a power chord it produces a harmonic series with a fundamental one octave below the root of the power chord (0.5 times its frequency). The fundamentals of the notes of the power chord have frequency ratio 1:1.5 Where i, j, k are all integers: 0.5 * i = 1 * j + 1.5 * k 0.5 * i = 1 * j - 1.5 * k No matter what combination of frequencies you select, you always end up with multiples of 0.5. Even if you extend the power chord with the octave (frequency ratios 1:1.5:2) you still only get multiples of 0.5. The same applies if you add the harmonic overtones of the fundamentals. This generation of a new bass note contributes to the "heavy" sound. But in reality guitar strings have non-zero thickness, which means they produce notes with overtones that aren't perfectly harmonic (they are non-integer multiples of the fundamental) . This means you actually won't get perfect multiples of 0.5 of the root of the chord, but it's close enough that the inharmonicity just adds even more "heavy" sound without sounding like noise.