Drone doom (also known as drone metal) is a style of doom metal that is defined by the repetition of drones, that is, notes that last for long periods of time. It is generally influenced by noise and ambient music. Typically, the electric guitar is performed with a large amount of reverb or audio feedback while vocals, if present, are varied, and often growled or screamed. Songs often lack beat or rhythm in the traditional sense and are often between ten and thirty minutes long. Both Earth and Stephen O'Malley can be largely credited for the rise of drone doom as a recognised subgenre.