Waveguide parameters

[NiemandHH]

I am not sure if I have understood the waveguide parameters. According to the online documentation the cutoff frequency is the limit between the waveguide low frequency simulation and the geometric simulation. What is the usable portion ? Is the portion of the IR which will be used to extract the results for the waveguide simulation ? By changing that parameter, I see that the 'effective sr' changes automatically. The smaller the usable portion becomes, the higher the effective sr becomes. Is this the sampling rate used for the extraction of the IR in wav form ? Could the developer or someone else help me ? I would highly appreciate it. Thanks