That RT60 Metric

By Peter Patrick

There is a lot of confusion about the Metric RT60. Peter Patrick answers this question, “What on earth does 2 seconds RT60 mean??”

Why do we all seem to want to reduce measurement outcomes to single numbers? T60, RT60 or room reverberation time is a measurement that can be seen as one of the best, or worst depending on you perspective, examples.

What on earth does 2 seconds RT60 mean??

Let’s suppose for a minute that quoting the RT60 at just one frequency means something …..

In a large board-room it means a nasty space where a conversation between two people at one end of the room is irritating, loud and difficult to understand to a couple trying to either listen or hold their own conversation at the other end of the room. Music is probably boomy at one end of the scale or other and not nicely balanced at all.

In a 2,000 seat well designed concert hall it’s possible to hold a conversation with another person 50 ft away, other conversations are not intrusive on yours and music sounds as it should.

2 s RT60 sounds very different in different spaces – especially different size spaces.

Then there’s the variation in RT60 across the spectrum.

I actually use the RT60 calculation in the control room design process – it just helps make sure there’s a balanced amount of absorption in the room across the spectrum – that’s all – the metric is otherwise useless in control room design.

In rooms predominantly constructed of Gypsum board surfaces there will usually be a large peak in RT60 in the male voice register – bass disappears, consonants are swamped by lingering vowel sounds and so forth.

2s RT60 in the mid band can sound very different in different spaces constructed of different materials

Then there’s the dear old stadium.

RT60 is often defined by the mean free path length – that is – the mean of all unobstructed flight paths for sound in a space. Three dimensions – length, breadth and height – what’s the mean – that will give you a clue about the volume and the amount of time sound will traverse the space.

In a stadium or outdoors, one dimension, height, is infinity. Factoring that number into an equation gets pretty interesting.

Yes sound does linger in a stadium and yes an impulse can be heard to linger outdoors if it’s loud enough but is it reverberation we hear ?

I think not!

2 s RT60 means very little in a stadium