Anyone can knock out the main riff to Deep Purple’s ‘Smoke on the Water’; four sets of perfect fourths played on two strings only, it’s the caricature of the easiest riff in metal.
And yes- power chords, the musical basis of metal, are easy to play. A power chord is a simplified chord; whereas standard chords would have a triad- three tones which when played together create a rich sound- power chords have only two tones, an interval of either a 5th (usually) or 4th. They often incorporate an octave as well.
They may be musically simple, but acoustically they are anything but. The power chord has a highly complex aural identity, and when you combine a power chord with distortion something very exciting happens, that gives metal its distinctive sound.
The explanation below will be obvious to seasoned metal musicians, but was a complete revelation to me when I started to play the guitar, and went a long way to explaining why heavy metal makes me tick.
Distortion is the other musical basis of metal. Distortion means to overload, to push beyond the norm, to manipulate out of shape, to put under strain. It implies something negative, unwanted. Distortion is the sonic effect created when amplifiers are played at too high volumes, and it was discovered by accident when the earliest amplifiers sustained minor damage or were played too loudly. The subsequent fuzzy or dirty sound was considered desirable by blues and rock musicians in the 1940s and 1950s and guitar distortion effects evolved from there. Distortion has a visceral effect on the human brain. Distortion generates higher frequency energy by clipping the signal and therefore the waveform, emphasising the higher frequencies and giving the impression that things are closer than they really are, enhancing the idea of something looming, a perception of heaviness. Distortion implies effort, and since too much distortion can tip a sound over into the unpleasant, a distorted guitar is always on the edge, on the brink of collapse.
When two or more notes are played simultaneously through an overdriven amp, something very exciting happens, something called intermodulation distortion, whereby additional sounds are generated at the sums and differences of the frequencies of these notes’ harmonics. With a normal chord, the resulting sound is messy and indistinct, but with a power chord, the ratios between the frequencies of the root and the fifth are so simple that a coherent sound is created. The spectrum of sound is expanded in both directions, and crucially, a new fundamental frequency appears an octave lower than the root note, making the chord sound lower than it really is. This phenomenon is called resultant tones, and with feedback we can achieve not only resultant tones in many registers, but infinite sustain- the thick chord just keeps going.
This powerful musical effect has a strong physical reaction in the body; hitting the nervous system on so many levels, it’s why we hear heavy metal with our whole bodies, from the inside out.