I'm working on a building a parametric EQ in Pd, and was following the model in this post, which at first appears to work really well using para_bp2~ from iemlib.
However, what I've noticed is that whenever the center frequency of a filter is set relatively low and the q approaches an octave bandwidth or so, I'm getting a lot of extraneous high frequency noise even with a gain of 0 whenever a low frequency tone is sent in to the filter. So, everything sounds well until you send a sine sweep through it and you'll notice a lot of noise when the tone is below 40 Hz or so, but again only when the center frequency of the filter is set relatively low as well.
Here is an example with very common values found on the bottom filter of a 10-band graphic EQ (31Hz, Q of 1.41), and you can hear the issue. This same issue seems to persist when using coefficients for biquad~ as well although I haven't tried that exensively.
I thought it might be aliasing caused by some ultrasonics the filter is producing for some reason, but oversampling and filtering doesn't seem to change matters much.
Anyone have any thoughts on this? Thanks for your help!