Originally Posted by HopefulFred
Originally Posted by umr
The best sound I hear is in systems that do the following:
- room acoustics that conforms to the Dolby specification based on T10 measurements
I don't want to put you on the spot, but can you elaborate a little about this?
I was unfamiliar with Dolby's particular recommendation here, so I searched, and found "Dolby Stereo Technical Guidelines for Dolby Stereo Theatres" (1994) which seems to say (if I'm reading the guidelines right) that RT60 for small rooms (say 3000cuft) should be pretty low - like 0.2s. They allow for a little blooming at very low frequencies, and a little extra dryness at very high frequencies, which I understand.
I've been shooting or a pretty short reverb time myself in my own (fully DIY) design, but I was a little concerned about going below 0.3s, so I just want to understand if I'm reading right, and such tight time domain control is of benefit. How short is too short?
Too short is when you go below the Dolby spec. It is nearly impossible to design a small room acoustically correct on paper alone. I have seen this happen in about 1 in 200 designs. There are just too many unknowns to not have to take measurements and tweek it.
- furnishings acoustical properties
- low frequency sound loss
- impact from lack of uniformity
- installed acoustical performance versus published (fabric wall effect, AT screen effect, location efficiency, data accuracy...)
There are also numerous ways to measure RT60 which will generate wildly different values at these low reverb levels. I find T10 values of RT60 along with the Dolby spec results in a room that acoustically matches high end mix theaters like Sony which I have been in. Getting reliable T10 measurements is another challenge. You can buy many different products and get about as many different answers as the number you own.
It is very hard to meet this spec from 32Hz to 16kHz. Less than 10% of the theaters I work in do. Many fail because of sound isolation which can wreck reverb at 32Hz. It is also very difficult to pull down the frequencies in the trend shown. I had to make custom 1/4" panels for my theater to get 4-16kHz to drop without making 32Hz bad.
It is much easier to meet the Dolby spec if you keep the room reverb at 500Hz above the midpoint for your room.