Hi Peter and list,I have written a tool to create synthetic fricatives using Praat. This script can be used to modify center frequency, bandwidth and relative amplitude of three separate spectral peaks, as well as overall duration, rise time and fall time. It currently doesn't have a looping function to create a continuum, but you can set the parameters offline and then just type them them into the pop-up form as the script begins.
The script can be found here:
A short readme file with some suggestions on what parameters are useful for voiceless English fricatives is found here:
Let me know if you have success!
Matt