It’s easy to think that if too much reverberant echo is bad for theater, then less would be better and none would be best. Well, none would be awful, actually. Our brains aren’t designed to work in an environment with no reverb. The closest that exists in nature is the outdoors, but even there sound does bounce off of everything it encounters and we do use those bounces as part of our understanding of our environment. An actual absence of reverb is quite unsettling, and spending too much time in an anechoic chamber will quickly lead to hearing the blood flow inside your own ears.
So what does make for good acoustics for theater?
Clearly, we need reverb that doesn’t last too long, but also isn’t too short or nonexistent. The usual numbers I’ve seen quoted are on the order of 0.75 seconds RT60.
RT60, you ask? So last post I mentioned decay time and said we’d get back to it. Then is no longer soon, it is now. RT60 is the time required for a sound to decay by 60dB. Originally this was measured using a stopwatch and Wallace Clement Sabine’s ears, and it is roughly the amount of decay required for a sound to become inaudible.
But, of course, there’s more to it than that. Go back to the big stone church again and think about what you hear as a sound decays. It doesn’t just sound like the original noise is stretched out as it slowly fades away. No, it tends to be the low notes that last the longest and the higher notes tend to fade away faster. Or in a smaller room with hard parallel walls, the higher notes may last longer than the low ones, and may even sound oddly fluttery (a “flutter echo”). Worse yet, you could have a single note that jumps out from all the rest – that one will reverberate much longer than others around it, and will be louder to begin with. That’s a resonance, where the room itself is helping the note along. All of these things are bad.
So for theatrical use, we want a room with as few resonances as possible, and any that are there are as small as possible. We also want a room that decays as the same rate no matter what frequency (pitch) a sound is at.
Lastly, we want a room where the sound is the same, no matter which seat you are sitting in. As it turns out, while those other things are hard to do, this is the hardest of all.