good audio filters

[Heiko van der Linden]

Hi there,
a couple of friends of mine wondered if I could built an audio filter for
them. I was wondering if anyone on the list knows schems for good filters
and I was also wondering what would be the best filters to build. 
Should I use low-pass, high-pass, band pass. Do the filters have to be
24dB/Oct. or are 12 dB/Oct. filters good enough.
If anyone has some good suggestions. Please mail me.
thanks
Heiko