The rq (reciprocal-Q) parameter of a resonant filter controls the sharpness of the resonance peak at the cutoff

A resonant filter (RLPF, Resonz) adds a peak in frequency response at the cutoff frequency, boosting that frequency before the rolloff begins. The sharpness of this peak is controlled by the rq parameter — ‘reciprocal of Q’ — where Q is the quality factor. Lower rq = narrower, higher peak (more resonant); higher rq = broader, flatter peak (less resonant). With rq near 0, the filter can self-oscillate, producing a sine-like tone at the cutoff frequency. In SuperCollider, RLPF and Resonz both accept the rq argument (Resonz calls it bwr). Resonz is described as a ‘stronger’ resonant filter.

Examples

// Resonant LPF — rq near 0 = strong resonance:
RLPF.ar(in: WhiteNoise.ar(0.4), freq: 4000, rq: 0.05)
// Mouse-controlled sweep with high resonance:
RLPF.ar(in: WhiteNoise.ar(0.1), freq: MouseX.kr(0,1600), rq: 0.005)

Assessment

Set up a resonant LPF on white noise with rq=0.005 and sweep the cutoff frequency across the spectrum. Describe what you hear and explain which sonic phenomenon is occurring at extreme low rq values.