Heavy metal music's scary sound comes from a 'forbidden' musical note combo once called the Devil in Music.

A tritone is a musical gap between two notes that are three whole steps apart, like C and F-sharp. It splits a musical octave perfectly in half, which creates a sound that feels 'unfinished' and messy to our ears. In the Middle Ages, people called it 'Diabolus in Musica' or the Devil in Music because it was so jarring.In 1970, a guitarist named Tony Iommi from the band Black Sabbath wanted to make music that sounded like a horror movie. He used this specific sound for the main part of their song 'Black Sabbath.' This one choice changed music forever and started the heavy metal genre.The science of why it sounds weird is all about math. Most pleasant sounds have simple ratios, like 2:1 for an octave. The tritone has a messy ratio of about 45:32. This complexity causes a physical reaction called 'beating' in your inner ear.Your brain struggles to process these clashing frequencies at the same time. A study in the journal 'Nature' shows that humans naturally prefer smooth sounds over clashing ones. This is why the tritone makes us feel uneasy or scared. Today, movie composers use it whenever they want to make an audience feel like something bad is about to happen.