A few weeks ago I had an opportunity to work on a theater designed by a well known acoustical design firm. The RT60 for this theater measured at 360 ms. It had a strong slap echo and was way too reverberant for quality theater sound.
The installation had too many diffusers compared to absorbers which was both expensive and ineffective. When you hire someone to do this you should ask for an RT60 measurement be taken after the job is complete at the prime seating position. I would accept an RT60 of between 200 ms and 250 ms extrapolated from the linear sound drop from a wide-band impulse signal like a hand clap. This can be done with a Sencore SP495 using the energy time graph.
This magnitude of error can be avoided by computer modeling the acoustical behaviour of your theater based on your room materials of construction, furnishings and geometry. One method of doing this is to use the CARA design software.
…and exactly where was this measured? At the listening position? It should also be noted that the RT60 in a room can be vastly different from various points in the room, and is also one of the many reasons that RT60 measurment is argued to not be relevant measurment in a small room. A measurment can be taken 5 feet away from a previous measurement and result in a completely different value which could be 100ms or more!
True to some extent, but the room was horribly live at all listening positions due to a great lack of absorption. Some type of metric should be used to specify the final performance.