From: "Greg" <gfreitag@x??x.xxxx
Date: Wed Nov 24, 1999 4:18 am
Subject: Re: new article
>From: "Ty Ford" <tford@j...>
>>address. Several people have refered to RT60, reverberation time
>>in a acoustically small room does not exist. By definition the RT60
>>is a statistical measurement that is made at some point beyond the
>>critical distance. The "critical distance" is the point in a room
>>that the level of the reflected sound is equal to the direct sound.
>>In a acoustically small room the critical distance does not exist.
> How does that work? I would think that if the room were small and
>reflective enough that the reflected sound could easily be almost as
>large as the direct sound (minus standard reflective losses).
Almost as large is not good enough, you have to be in the field
where the reflected energy is larger.
> It would sound like your typical bathroom. Not an ideal recording or
>monitoring environment, but certainly possible. What am I missing?
The terminology "RT60" has a very specific technical meaning that
was originally derived by Sabin. It was further refined by Fitzroy
which takes into account rooms that have different absorption
coefficients on different surfaces. The key here is that the sound
must be well mixed in a way that the angle on incidence on the
reflecting surfaces is statistically random. Inject a steady state
sound, like pink noise, into a room. If you are beyond the critical
distance you can wander around with a sound level meter and the
sound pressure will not change significantly. The ability of a
material to absorb sound is quantified with a value called the
"absorption coefficient" and is statistically based. It is really the
average absorption coefficient since the amount of energy absorbed
by any material depends on the angle of incidence of the wave.
In a acoustically small room the angle of incidence is not random
and any attempt to calculate RT60 will be futile. Any attempt to
measure it will depend on where you are in the room and will
yield wildly varying results. In a small room we are dealing
with modes, Don Davis calls the decay "Modal Decay Rate",
which is a technically more accurate description.
Greg