Drone metal, also known as drone doom, began as a derivative of doom metal and it is largely defined by drones; notes or chords that are sustained and repeated throughout a piece of music. Typically, the electric guitar is performed with large amounts of reverb and feedback, while vocals, if present, are usually growled or screamed. Songs are often very long and lack beat or rhythm in the traditional sense. Drone doom is generally influenced by drone music, noise music and minimalist music. The style emerged in the early 1990s and was pioneered by Earth, Boris and Sunn O))).