What is Acid bassline?
An acid bassline is the squelchy, resonant bass sound created by the Roland TB-303 synthesizer, defined by a sawtooth or square wave run through a resonant low-pass filter whose cutoff is modulated while notes slide into each other. It defined acid house and remains a staple flavour in techno.
The classic acid sound comes from three ingredients: slides between notes (glide/portamento), accents that push the filter harder on chosen steps, and constant automation of the filter cutoff and resonance. Any 303 emulation — or a simple mono synth with a resonant low-pass filter — can produce it. In techno, acid lines are usually a single repeating pattern whose character evolves entirely through filter movement and distortion rather than new notes.