Resonance
Resonance (or Q) is a filter parameter that boosts frequencies around the cutoff point, creating a peak that emphasizes those frequencies. High resonance on a low-pass filter produces the iconic analog squelch and 'wah' effect of acid house basslines—and can cause self-oscillation at extreme settings.
Resonance interacts with the cutoff frequency: wherever the cutoff is set, resonance emphasizes that frequency band. Sweeping the cutoff while resonance is engaged creates the characteristic 'wah' sound because different frequencies are emphasized as the cutoff moves through the spectrum.
At extreme resonance settings (close to maximum), many analog-modeled filters will self-oscillate—the filter itself generates a sine wave tone at the cutoff frequency. This is a feature, not a bug: self-oscillating filters are used as a synthesis tool to create tonal bass sounds and effects without any oscillator input.
For acid basslines (Roland TB-303 style): use a sawtooth oscillator, set cutoff to around 800–1200 Hz, resonance at 60–75%, and automate the cutoff with an envelope set to a short decay. Accent certain notes (higher velocity) to trigger more dramatic envelope movement, creating the classic acid squelch pattern.