Month: April 2015

So we’ve established our target: shortish decay time (~0.75 seconds) that is flat across all frequencies, and that is the same no matter where in the room an audience member is.  As few resonant frequencies as possible, and those resonances should be “weak”.

Weak?  Huh?  The short explanation of that: a simple resonance is one where the sound bounces back and forth between two walls that are exactly as far apart as the sound wave is long.  Since sound moves at 340m/s, a 5m long room would resonate at 68Hz, which is approximately C#2.  (It would also resonate at multiples of that, but we’re going to stick with the simplest option here.)  Now if the room is also 5m WIDE, we have a second 68Hz resonance on top of the first, making it stronger.  If the room is 5m TALL, we get a third.  In the other direction, if we were to add material to the walls to damp the reflections, it would be weaker.  Yes, this is oversimplified, but you get the point.

We’ve all been in theaters that are more or less shaped like a scallop shell.  Angled sides, curved rear wall?  Those angled sides don’t reflect sound at each other, so no resonances can form between them.  If that back wall is constructed correctly, sounds go there to die – no reflecting back into the room.  It’s easier to build one very absorptive wall than 4.  I’ll bet the floor and ceiling aren’t parallel, either.  Sound will still bounce around a bit, but we want a little so that’s OK.  Yes, there are very good shape options other than this one, but it’s the one I understand best.  The Troy Savings Bank Music Hall has parallel walls, and I have no idea how that works.

There’s more to it than just shape, though.  This is also where my expertise gets very thin, so here we start to gloss a bit.

What if the walls resonate?  Or the floor?  Or a space on the other side of the wall?  That can pass energy back across the wall the same as it got there in the first place.  There’s a great story about the acoustics at Carnegie Hall in New York where a renovation in 1986 was said to have “diminished the famed acoustics”.  Turned out that there was a slab of concrete poured in under the stage causing the problem, and when it was removed in 1995 things were much better. (See: this New York Times article from 1995) So it’s possible that such resonances are bad, but equally possible that they are good or even necessary.

Then we can get in to the use of resonators to tune the room.  The Troy Savings Bank Music Hall has small cavities built in to the ceiling for exactly this purpose.  In modern design, this sort of thing is generally used as “bass traps”, to damp down low frequency resonance, but I have no doubt there are other uses.

All of this, from the shape of the room to the wall materials to building in resonant cavities must be planned before construction starts.  It can be retrofitted later, but it’s cheaper to do it right the first time from the beginning.  The lesson to be had here is: When you start a construction project on a theater, whether you are building new or repurposing something existing, bring in your architect and an acoustics expert at the beginning, and design it right.  Then build to that.  You’ll be glad you did.

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.

Possibly the one specialty field that is less understood than Sound Design in theater is Acoustics.  I know of no other aspect of the world around us for which nearly everything that lay people say is wrong.  Usually people have at least some small grasp on reality, but you can’t see or touch acoustics, and when you hear them it’s almost more what you don’t hear than what you do. Or maybe it’s what you can’t hear well.  Or possibly what you… hmmm.

So let’s start with the concept of good vs. bad acoustics.  Two problems with this seemingly basic concept: it’s subjective, and it depends on what the space is going to be used for.  Think for a moment about a large stone cathedral.  One where you can hear your own voice for several seconds as it bounces around.  These sorts of spaces led to the creation of musical styles such as Gregorian Chant, as the long, slow tones combined with the long, slow decay time (we’ll talk about decay time later) make for an amazingly powerful instrument that can lend a physically moving component to what is supposed to be a spiritually moving circumstance.  Now try to imagine singing Gilbert & Sullivan in that same space, preferably one of their patter songs.  After the first line the entire room is filled with audio mud and the cast can’t hear themselves well enough to keep singing.

Another detail that is often missed is that there is a sweet spot designed in to good churches such that someone standing at the altar can be clearly heard in every seat.  I know that modern technology can be used to make the preacher loud enough to be heard everywhere, but in the 14th Century that simply wasn’t an option.  It’s really quite amazing walking around nearly that sweet spot talking and hearing how the whole room changes when you step on it.

So does that cathedral have bad acoustics? No. It has precisely the acoustics it needs for what it is meant for. It wasn’t meant for Operetta, so if you try and repurpose the space for your new G&S Company, you are going to be very sad with the result unless you put a lot of effort and money into changing it. I’ll guarantee that keeping the look will be nearly impossible if you fix the sound, too.

Over the last 5 years a company I work with on a moderately regular basis purchased a “new” building, in the form of an 1850s timber framed church. People involved with the purchase touted the “wonderful” acoustics and told me I’d love working in the new space. Since moving in there have been nothing but complaints about how bad the sound is.  Guess how much time and money went in to changing the acoustics from the old purpose to the new?