GROWL is an approximate implementation of the formant filter used in the iOS vocal synthesizer, HOWL.
The GROWL algorithm itself depends on Soundpipe. Sporth is required to use the Sporth plugin.
By default, GROWL compiles to a Sporth plugin. This can be compiled by simply running "make".
To hear what the filter sounds like, run the Sporth patch included with "sporth test.sp". This will generated an audio file called test.wav that you can then listen to.
The code is also ready to be used just like any other soundpipe module. Just drop the header and c file into your project, and you should be good to go!