Been thinking about this for a while....

Say I wanted a high pass cut off at 100Hz, say 3dB per octave. If I set -3dB at 50Hz, -6dB at 25Hz, -9dB at 12Hz, would this approximate to something like a high pass crossover curve? Am I right in thinking that a Q of 0.5 in each case would give roughly the right shape curve? It would be very lumpy but would have at least a downward trend and cut more bass with lower frequencies.

If I wanted a steeper curve (say 12dB per octave) would that work with cutoffs of -12dB, -24dB, -36dB, but with a Q of 2.0? Is there a mathematical formula that defines the shape of the parametric curve? I'd like to plot these curves graphically, then compute the sum effect of each equalisation step to derive the overall effect. Anybody know of any good (free) software that would allow this kind of graphing?

Alternatively, maybe this could be handled in the digital domain instead? The Auto volume thingy in hijack analyses the outgoing sound data stream and tweaks it to apply a multiplier before it reaches the DSP. Could the same approach be applied to create a low/hi pass filter effect? Hmmm... maybe this would only be in stereo so I don't think it would solve my problem...

My brain hurts now, so I might just go and buy a variable high pass filter instead. Sigh.