Drone metal 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))). Epic doom has a heavy classical influence. One of the main characteristics are the vocals; vocalists typically employ clean, operatic and choral singing, accompanied by keyboarding and drumming performed in a bombastic fashion in order to evoke an "epic" sensation. Lyrics and imagery are typically inspired by fantasy or mythology. Examples of prominent epic doom bands include Candlemass, Solitude Aeturnus, Solstice, While Heaven Wept, and Doomsword. Gothic-doom, also known as doom-gothic, is a style that combines more traditional elements of doom metal with gothic rock. Gothic-doom bands usually play at slow and mid-tempos and employ the usage of instruments that are more related to classical music, alongside traditional doom metal instruments, in order to create darker and meditative atmospheres. Doom-gothic lyrics combines the dramatic and romantic elements of gothic rock with the sorrowness and melancholy present in doom metal, while being more introspective and focused on personal experiences such as love, grief, irreparable loss, loss of faith, etc. Unlike in gothic metal and death-doom, gothic-doom bands prefer the use of cleaner vocals instead of employing death growls, although some of them employ harsher vocals occasionally, and avoid the usage of death metal-like riffage. Bands labelled as gothic-doom include Weeping Silence, The Foreshadowing, Grave Lines, Artrosis, Ava Inferi, Draconian, and Type O Negative. Progressive doom is a fusion genre that combines elements of progressive metal and doom metal. Notable bands include King Goat, Below the Sun, Sierra, and Oceans of Slumber. Sludge metal (also known as sludge doom or sludgecore) is a style that combines doom metal and hardcore punk, and sometimes Southern rock. Many sludge bands compose slow and heavy songs that contain brief hardcore passages. However, some bands emphasise fast tempos throughout their music. The string instruments are heavily distorted and are often played with large amounts of feedback to produce an abrasive, sludgy sound. Drumming is often performed in typical doom metal fashion, but drummers may employ hardcore d-beat or double-kick drumming during faster passages.