In my experience the little “whorls” as Richard I think, or maybe Don Davis called them are typically resonances in loudspeakers.
I think that's right. I ran the transfer functions of a number of simple filters to get a feel for how they are represented. A bi-quad makes a perfect (looking) circle irrespective of its Q. Changing the Q simply changes the rate at which the locus moves along the (normal-to-our-view) frequency axis. Changing the gain changes the radius of the circle, as one would expect. Example below is of a 1 kHz bi-quad with +6dB and Q=1 (by my definition) and the same with 1 cycle of delay at the max freq of 8 kHz.
Hi- and low-pass filters make spirals. Not surprisingly, all-pass filters make circles around the origin, and a straight wire makes a point at 0 or 180 deg depending on the polarity. Delay winds up whatever is there like a clock spring.
So are the whorls useful in identifying simple (2nd order) resonances that can be corrected with bi-quads?
--Frank