Is it always meaningful to talk about T60?

by Ray Rayburn

In this article, Ray Rayburn helps us understand more about T60.  He addresses this question, Can T60 be applied to large rooms?

Another blogger mentioned a control room having a T60 of 0.3 seconds.  That inspired this post.

To talk about T60 in a room implies the room has a statistically reverberant field.  For this to be true there needs to be about 50 reflections of the sound before it dies down to less than -60 dB relative to the initial sound (which is where the 60 in T60 comes from).

T60 only has any meaning in mid-sized rooms.  In very large rooms such as the Georgia Dome (70,000 seats around a football field) the room is so large that air absorption dominates and you don’t get 50 reflections, and therefore can’t meaningfully talk about T60.  You can talk about reflections, and you can talk about STI, but T60 has no meaning.  If you move the acoustic source and the test location you will get radically different reflection patterns.  In a room with an actual statistically reverberant field, the same T60 will be measured in most locations in the room.  This is not true in these very large rooms.

In a similar way in small very dead rooms like the typical recording studio control room the absorption of the average surface is so great that once again you don’t have a statistically reverberant field.  Your handheld T60 meter might claim you have a T60 of 0.3 seconds, but if you looked at the actual data you would see a series of reflections that died out to under -60 dB without ever becoming reverberant.

This is the source of Al Fierstein’s big fight with Don Davis back in the early days of Syn-Aud-Con.  Al had made some very useful audio measurement devices which he sold under the Acoustilog name and Don promoted them.  When Al developed a T60 meter and started talking about fractional second reverb times in small control rooms, Don called him out.  Don was right, but Al would not back down and went on to oppose all that was learned about control rooms and listening in the early TDS days.

Ray A. Rayburn

K2 Audio, LLC