Freeverb (stereo) for two-channel audio function. Based on Faust implementation.

afn - input stereo audio function
room-size - size of room [0,1.0] 
hf-damping - high-frequency damping according to frequency [0,1.0] 
stereo-spread - adjusts differences of lengths of delay lines [0,1.0]

For more information, see:

https://ccrma.stanford.edu/~jos/pasp/Freeverb.html

Freeverb (mono) for single channel audio function.