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In Reply to: RE: really? do the math (it's easy!) posted by Inmate51 on March 05, 2017 at 13:52:59
Hi
The issue is "where does the reverberant field begin".
Picture a low frequency one below the lowest room mode, at these frequencies your room is a leaking pressure vessel and this "containment" of the bass pressure is the only free lunch in audio, "room gain".
Be aware too that a flexing wall or floor etc is also a diaphragmatic absorber and a sink hole at that frequency.
AS soon as you increase the frequency past the first mode, one is in the modal region where a measurement easily identifies large peaks and dips in the response in the room.
The larger the room, the lower the first mode is and the more numerous the modes are as you go higher up. With a large enough, live enough room one can reach the critical distance and the reverberant field at that point no longer has large peaks and dips, the modes are so dense they have averaged out. There is zero voice intelligibility as well as the time information has been completely scattered and all is left is a noise with the same power spectrum as the acoustic power of the source.
That condition is pretty normal in very large spaces and when public address announcements are barely intelligible as a result of the direct sound not being high enough over the "noise" or reverberant sound.
At the dimensions of a living room and typical loudspeakers it is usually the specular reflections close to the loudspeaker which harm imaging the most, and the side walls are often the source.
One way to "see" where absorption will help is to place a mirror on the wall where you can see the tweeter of the closest speaker from your sweet spot and then replace the mirror with a sound absorbing panel, do both sides..
Remember that critical distance and rt-60 are all terms that were originated for and apply in large scale acoustics and do not apply where the modal density is sparse.
Follow Ups:
Thanks for your thoughful and useful post.
Room modes are definitely an issue. Here's a related story: Back in 1982, I wrote a BASIC program to calculate room modes and print the results. Whoa, dude, 1982?! Yeah. I was sick and tired of calculating and plotting everything by hand! Hey, remember slide rules?! Fortunately, they were past their prime when I was in high school.
"At the dimensions of a living room and typical loudspeakers it is usually the specular reflections close to the loudspeaker which harm imaging the most, and the side walls are often the source. "
This is common "wisdom" and is total bullshit.
But then, I'm a recording engineer and a musician, so what the heck do I know?
:)
:)
"But then, I'm a recording engineer and a musician, so what the heck do I know? "
Well think about it, in a real studio, which end is dead and close to the loudspeakers when traditional front sofit mounted speakers are used?
When near field / meter bridge mounted monitors are used, how far away in time and level are the closest wall reflections?
To preserve stereo image, suppressing any strong reflected sound within say 0 to 10 ms of the source is usually audibly beneficial, something like directivty in the way it increases the nearfield where the direct sound dominates late reflections.
Actually, I have thought about it. A lot. :)The room is an integral part of the sound, and, early reflections contribute to the sound.
But, speaking of soffett-mounted speakers, when I moved to Dallas decades ago, my (now) old buddy Russ Berger invited me to his home, and guess what?... He had Altec 604's soffett-mounted in his house! Just like they had at the Mastering Lab in L.A. back in the day.
Anyway, here's a recent pic of a room where the stereo imaging was absolutely EXCELLENT without any absorption on the rear or side walls.
Edit: The guy on the right is Siegfried Linkwitz. I don't know who the guy on the left is.
:)
Edits: 03/07/17
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