Though I try to avoid Wiki...here's one link about comb-filtering that may prove useful:

http://en.wikipedia.org/wiki/Comb_filter

And here's another:

http://www.google.com/imgres?imgurl=http://www.music.mcgill.ca/~gary/307/week2/images/backward-comb.jpg&imgrefurl=http://www.music.mcgill.ca/~gary/307/week2/delaylines.html&usg=__ZRNNlt70fyz_nsQ3glr8c3hczvw=&h=291&w=713&sz=49&hl=en&start=20&zoom=1&tbnid=4EJaP1DQZRuOtM:&tbnh=57&tbnw=140&ei=dXl7T83PLInftgfBh4G4CA&prev=/search%3Fq%3Dcomb%2Bfiltering%2Bdelay%26um%3D1%26hl%3Den%26sa%3DN%26gbv%3D2%26rlz%3D1R2ADSA_enUS355%26tbm%3Disch&um=1&itbs=1

I hope the links work...