# What do these values in the freeverb diagram mean?

I am implementing a reverb based on the Freeverb implementation detailed here. There is also a more in-depth description of the LBCF in question here, but I am not able to find the information I want at that link.

In the diagram given, they use a lowpass-feedback comb-filter, denoted by LBCF(f,d,N). N is clearly the number of samples. I suspect d means the feedback gain, though I'm not sure if a value of .2 makes sense for that. I have no idea what f means.

So here are my questions:

• What does d mean?
• What does f mean?
• What cutoff frequency is used for the lowpass filters?

$$d$$ is the pole location of the lowpass filter. That determines the cutoff frequency of your lowpass filter
$$f$$ is the feedback gain and determines how quickly the repeating echoes decay. That should be a function of the reverb time (at low frequencies) and the delay size
• Depends on how you define it. This is not a "true" lowpass filter since it doesn't have a zero at Nyquist. In general, amplitude goes down with frequency but you won't hit -3dB if $d$ gets too small. A $d$ of 0.2 you'll get a -3dB point of about 15.5kHz @44.1kHz sample rate. For a $d$ of 0.1 there is no -3dB point at all – Hilmar Jul 2 '19 at 11:13