Couple of things:
Low_Freq should be approx: Sample_Rate/4/1.3 to prevent noise from aliasing into the low frequency regions. In other words, the bandwidth plus roll-off of the filter should be less than half of the nyquist freq.
Rp controls the rippling in the low frequencies. It all depends on what you want to do, but I suggest [0.5-0.2]
Rs controls the distance (amplitude) of the filtered frequencies relative to the unfiltered frequencies. I suggest Rs > 40 db
To apply the filter I would use: "filter" (>> help filer)
