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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
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From: Dan Nelson <dprimary@x??????xxxxx.xxxx
Date: Wed Dec 1, 1999 5:28 am
Subject: Re: new article

I would like to add that although RT60 is a useless measurement in small
room acoustics, doing the reverberation calculations for rt60 is useful in
determining a rough amount of absorption materials needed to treat the
room. The calculations will also help determine at what frequencies more
absorption is needed. Which would be dictated by the frequency that has the
greatest amount of absorption in the room. The selection of absorption
materials is then based upon what is needed to have roughly equal
absorption at all frequencies except for maybe a slight rise in the reverb
time in bass frequencies.

Dan Nelson
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From: Brian Cassell <bcc2a@x???xx.xxxx.xxxx
Date: Wed Dec 1, 1999 6:08 am
Subject: Re: new article

Why the slight rise in 'verb time for lower frequencies?

Dan Nelson wrote:
>
> From: Dan Nelson <dprimary@e...>
>
> I would like to add that although RT60 is a useless measurement in small
> room acoustics, doing the reverberation calculations for rt60 is useful in
> determining a rough amount of absorption materials needed to treat the
> room. The calculations will also help determine at what frequencies more
> absorption is needed. Which would be dictated by the frequency that has the
> greatest amount of absorption in the room. The selection of absorption
> materials is then based upon what is needed to have roughly equal
> absorption at all frequencies except for maybe a slight rise in the reverb
> time in bass frequencies.
>
> Dan Nelson
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From: Dan Nelson <dprimary@x??????xxxxx.xxxx
Date: Wed Dec 1, 1999 6:22 am
Subject: Re: new article

It is not required but a rise in the bass time has been thought of as desirable
in rooms designed for music and the BBC did some research and found it could be
tolerated in voice recording as well, although flatter is preferred in voice
recording.
The average living room has been found to have a rise in the bass verb time so
that could be part of it as well.

Dan Nelson
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From: Brian Cassell <bcc2a@x???xx.xxxx.xxxx
Date: Wed Dec 1, 1999 7:11 am
Subject: Re: new article

This makes sense. Recording in a space that is acoustically similar to
the end listening environment could help lend to a better image for the
listener that the performance is occurring in the space that they are
in. And mixing in a space similar to the typical listening environment
would also have it's own advantages.
This is interesting, though, in that many people avoid using 'verb on
low freq. elements of music to avoid a "muddy" sound.
My \$0.02, B C Cassell
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From: SRF7@x??x.xxx
Date: Wed Dec 1, 1999 7:25 am
Subject: Re: new article

In a message dated 12/1/99 2:08:35 AM Eastern Standard Time,
bcc2a@f... writes:

<< This is interesting, though, in that many people avoid using 'verb on
low freq. elements of music to avoid a "muddy" sound.
My \$0.02, B C Cassell
>>

One thing about a touch of low end resonance is that it will tend to keep you
from overdoing the bass in your mixes which is probably a good idea.
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From: "Ty Ford" <tford@x????xxx.xxxx
Date: Thu Jan 1, 1970 4:59 am
Subject: Re: new article

>From: Brian Cassell <bcc2a@f...>
>
> This makes sense. Recording in a space that is acoustically similar to
>the end listening environment could help lend to a better image for the
>listener that the performance is occurring in the space that they are
>in. And mixing in a space similar to the typical listening environment
>would also have it's own advantages.
> This is interesting, though, in that many people avoid using 'verb on
>low freq. elements of music to avoid a "muddy" sound.
> My \$0.02, B C Cassell
>

Brian,

I think the point here may be that the low end reverb from an added reverb
circuit is an effect (like chorusing) and therefore is different than what
you might encounter in an expected playback area.

If you add enough LF reverb to make it sound muddy, it's certainly not going
to sound any better in the expected playback area even if there's a suck out
of LF in general. In that case, you'd have less LF, but it would still be
muddy.

Years ago I ran into the term "CBS reverb", which was used a lot while
producing records for CBS. It seemed to imply rverb that had the bottom
rolled off and the top tipped up a bit, in order to keep it cleaner sounding.

Regards,

Ty Ford

Ty Ford's equipment reviews and V/O files can be found at
http://www.jagunet.com/~tford
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From: Brian Cassell <bcc2a@x???xx.xxxx.xxxx
Date: Thu Dec 2, 1999 6:01 am
Subject: Re: new article

Yeah. I frequently do the same thing to give it a cleaner sound, but
this thread is getting off topic, so back to room nodes I suppose...

-Brian