Where does that ugly metallic sound originate from?
As is well known, a room can only develop reverberation on frequencies where room resonances (»modes«) are available. The less of the useful signal “slips through the cracks”, the lower the coloration of the reverb sound. A feedback delay line of 1 sec has a natural frequency density of 1 Hz, so it can form reverberation at e.g. 98, 99, 100, 101, 102 Hz. For a delay loop with just 0.1 sec, i.e. one tenth of the delay,, the natural frequency density would be 10 times more patchy, e.g. 80, 90, 100, 110, 120 Hz. Imagine now operating 10 loops in parallel with an average (but not exact) of 0.1 sec, specified in a clever way so that the natural frequencies of each loop fill the gaps of the others, then again an average density of 1 Hz will result.
What I’m aiming for: to estimate the natural resonance density of a reverberation device in a first approximation, one could simply add up all delay lines built into the algorithm, and then form the inverse.
What exactly needs to be fixed in a bathroom to get rid of its coloration?
Increase the natural resonant density or, in other words, add more delay lines. Based on the loop calculations above: a bathroom of 3 m cannot deliver delay times of more than 10 ms, but you’re still free to increase the number of paralleled delay lines, and try to bend them away from the uniform 10 ms whenever you can.
Ultimately, this leads to the well-known reverberation chambers that attempt, with crooked walls and ceilings, to stuff as many independent delay lines into a given space as possible.
I should perhaps mention that such a strategy does reduce coloration indeed, but, even with pre or post delay, cannot pull a church from a small echo chamber. Delays of a mere 10 ms simply don’t “reach” far enough for a convincing impression of auditary spaciousness. In addition, such agile small echo chambers lack the sedate complacency associated with large churches.
- Michael Ottersbach / pixelio.de (header image)
- Wimox (wikimedia)