Hey Mike,
I usually assume that mathematically Q = f / Δf and this hasn't gotten me in trouble yet, but I am not a real audio engineering guy so maybe this could be false in some cases.
In ChucK's case, for brf, the code looks like this:
[...]
// set_brf
inline void set_brf( t_CKFLOAT freq, t_CKFLOAT Q )
{
t_CKFLOAT pfreq = freq * g_radians_per_sample;
t_CKFLOAT pbw = 1.0 / Q * pfreq * .5;
[...]
so in that last line, Δω = ω/(2Q), so Q = ω/(2Δω) = f/(2Δf). I don't know for sure if the extra 2 in the denominator is a scaling factor or actually part of the Q-bandwidth relationship for this filter. I would be surprised if it was the latter though given the "standard" definition of Q one sees, given above.
I can see how it would be confusing because the term Q is also used for the LPF and HPF filters as resonance, which is the gain of the peak at the cutoff frequency, in which case the f/Δf relation is not correct or even applicable.
spencer