If it's too loud, you're too old--or just not into heavy metal music. Power ballads aside, heavy metal is about volume. From amplified guitars--usually more than one of them--to pounding drums and thick, churning bass lines, metal is loud. Spiritus-Temporis explains some of the history and theory behind the music: "Early proto-heavy metal bands (along with The Who, for example)...set new benchmarks for sound volume during shows. The volume of the music was seen as a factor equal in importance to its other qualities. Though this influence is often denigrated as pointless extravagance, it has proven enormously influential, and still dominates many people's perceptions of the genre."
Forefront in heavy metal music is always the guitar, or several guitars. AllMusic Guide points out that heavy metal music displays "a reliance on loud, distorted guitars (usually playing repeated riffs)," and traces this unique characteristic of metal to the guitar-centric British music scene of the late 1960s and early 1970s, which centered around guitar gods like Eric Clapton and Jeff Beck. Both were veterans of the Yardbirds, as was guitarist Jimmy Page, whose subsequent band Led Zeppelin is cited by AllMusic Guide as the first example of a heavy metal band. "Guitar playing is very important in heavy metal," Spiritus-Temporis points out. "Distorted amplification of the guitars, with effects and electronic processing, is used to thicken the sound as well as downtuning the guitars."
A preoccupation with dark themes, such as death, the occult, violence and sexual sadism, is a characteristic of heavy metal lyrics, although not all metal bands prescribe to what Spiritus-Temporis identifies as "themes of darkness, evil, power, and apocalypse." The predilection for dark or disturbing lyrical content in metal music has created persistence of urban legends about how far bands will go to get their "evil" messages out. "One popular contention," says Spiritus-Temporus "[is] that heavy metal albums [feature] hidden messages urging listeners to worship the Devil or to commit suicide and murder."
Some bands are heavy but not metal, and some artists play metal-type music that's not heavy. Heavy metal bands who display the most common characteristics of heavy metal music include Black Sabbath, Led Zeppelin, Judas Priest, Metallica, Megadeth, Pantera, Corrosion of Conformity and Iron Maiden.