TU Dresden - reverberation chamber

Dismantling the myths of reverberation chambers

A real reverberation chamber with driving speakers and capturing microphones – that’s it, right?

Not necessarily! – Reverberation chambers are usually much too small, and Mother Nature cannot be cheated with regard to room size. There are a number of shortcomings with reverberation chambers, that cannot be compensated with ugly tricks like pre or post delay.

Which is?

With the usual two speaker feeds for the reverberation chamber, you’ll only get two room mode spectra, nowadays often referred as »IRs« - a spectrum originating from the left speaker, and a spectrum originating from the right speaker.

On the contrary, both orchestra and church organ consist of a large number of sound sources distributed throughout the room; each of them stimulates the room in a highly individual way. So to speak, every instrument and every organ pipe comes with an own, very personal IR. Such behavior cannot be mapped easily with just 2 feeds and 2 IRs, and not even with a few more. For my part, a few hundreds of IRs may do it, but don’t overlook the need for a corresponding number of echo send paths …

What’s next?

Reverberation chambers usually have a relatively small volume. Anyway, they’re incomparably smaller than e.g. churches. In comparison to large rooms, small rooms show a much lower mode density (=density of room modes on the frequency axis).

TU Dresden - Reverberation chamber
TU Dresden – Reverberation chamber

Since a room can only form reverberation at points of resonance, smaller rooms show a lot more “holes” in the frequency spectrum of the reverb. One can clearly hear that, as a more or less metallic coloration, especially with narrow-band sounds like choral music. Reverb with insufficient self-resonant lines on a soprano aria may even cause physical pain in your ear …

That’s it then, right?

No, one more step. Picture yourself on a church gallery playing a trill on the organ. What does a trill? It spreads through the room. Let’s now trace one of those signal paths that hit the front of the sanctuary, bounce back, and eventually return to your ear after several 100 milliseconds. You’ve guessed it, still warbling, why not? It’s the interaction of a variety of return paths with different lengths that blurs and neutralizes time factor, and turns the narrow-band trill elements into a continuous, wide-band reverb tail. But the molecules inside the wide-band reverb tail continue warbling. When listening carefully to the reverb tail of a large church, you may occasionally hear distinct audible trills rising up from the crowd; that’s part of the game.

For small rooms like a reverberation chamber, due to much too short signal paths, the individual trill elements can no longer be sequencially aligned, because some reasonable size of “sampling buffer” is missing for capturing the trill. Inside small rooms, whether real or digital, a spreading of musical sequences cannot happen, so spatial depth is not possible. Result: in contrast to a large real room, the spacious impression will be drastically reduced, or should I say compressed, for conventional reverberation chamber sizes.

Picture Credits
  1. Wimox (wikimedia) (header image)
  2. Wimox (wikimedia)

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