Originally posted by HT-DJ
I appreciate all your posts, but would it be possible for you to explain the above paragraph in layman's terms? I'm trying to digest all of this information, and do not understand what you wrote.
That's a tall order, but I'll try.
A minimum phase filter is one which alters the phase delay of a signal through it down to its theoretical minimum. There are implications for stability of the filter (its response is guaranteed to be stable and not blow up over time) as well as causality (no sound through the filter can create an earlier effect in time, which is, believe it or not, not true for every possible filter).
Every linear filter (one for which 2X the input produces 2X the output) can be decomposed into two different filters: a minimum phase filter and what's termed an all-pass filter. The latter introduces a delay across all frequencies. One example of a process which cannot be modeled without an all-pass filter is slap-echo, or any other discrete echo.
But the cool thing about minimum phase filters is that any particular one always has an inverse filter, which is also minimum phase. This means that you can cascade a minimum phase filter with its inverse (in either order), and completely undo the filtering process.
So what has all this got to do with room modes? Simply that the resonance of a single room mode, the back and forth wall reflection and amplification at a particular frequency, is minimum phase. So that means it can be completely canceled by its inverse filter. You can readily make such an inverse filter using an off-the-shelf parametric equalizer. You just dial in the frequency, bandwidth, and gain which reverses the effect of the room mode, and you have zapped it. And it doesn't matter that the inverse filter of the EQ came BEFORE the room resonance. The minimum phase principle doesn't care about what comes before what.
Hope this helps,