Exponential works better IMO.
BTW using “find” hp, “replace” with lp on the above code gives the same effect as Steve’s 2011 code, but now with a smoother envelope …
Low-pass variant of above code …
(setq start-freq 22000) ; cannot be greater than 1/2 the sample rate
(setq end-freq 20)
(setq passes 1) ; more passes for a steeper filter cut-off
(setq sweep-type 0) ; 1 for a linear sweep, 0 for exponential sweep
(let* ((nyq (/ *sound-srate* 2.0))
(f0 (max 0 (min nyq start-freq)))
(f1 (max 0 (min nyq end-freq))))
(if (= sweep-type 0)
(setf lpfreq (pwev f0 1 f1))
(setf lpfreq (pwlv f0 1 f1)))
(dotimes (i passes s)
(setf s (lp s lpfreq))))