Acid techno is built around the Roland TB-303 bass synthesizer's squelching, resonant filter sweeps, layered over driving techno drums. The TB-303 became central to dance music after Phuture's "Acid Tracks" in 1987, and acid techno consolidated in the early 1990s in London (the Stay Up Forever crew) and Chicago. Tempos run 130–145 BPM and arrangements are typically loop-based and high-energy. The genre is closely associated with free parties and warehouse rave culture, and remains a fixture of underground club programming. Modern acid producers like Tin Man, DJ Bone, and Charlotte de Witte's harder edits keep the style in active rotation. Acid techno also spills over into acid house and harder rave subgenres.