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In Reply to: RE: really? do the math (it's easy!) posted by Tre' on March 01, 2017 at 15:06:23
Hey - what happened to you guys? I just made some more popcorn!
But Siriusly, a studio's echo/reverberation chamber has a pretty long decay time, even though they're pretty small rooms. On the other hand...
It's pretty hard to measure ANY notable reflections in an anechoic chamber of the same size.
Reverberant field, or not?
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"Still Working the Problem"
Hey Tre', thanks for that link. This is part of what I've been talking about for decades! I don't remember if Leo Beranek first analyzed and named the phenomenon, but the "critical distance" is an important measure of the soundfield at any position in a room (AND outdoors!), and is an important contributor to achieving good intelligibility and musical detail where speech and instrumental detail is important. (Be aware that the Critical Distance varies somewhat with frequency and with angle from the source.)
It is important to note that the explanations on the linked site are a bit over-simplified in a few areas, but overall, this is a very good basic explanation of what's going on in a room, sound-wise. It would be good if the author would state such, so that the uninitiated reader won't take it as "the gospel truth".
Even though we know several fundamental factors, the soundfield in a room is not easily described. For the purposes of discussion here, even "typical" living rooms are anything but "typical". Although they do often share some common traits with regard to absorption characteristics, as soon as we start talking about architectural plan design, things go way off the rails very quickly! An "open plan" house is acoustically very different from a more "segmented" house, even though they may share similar "room treatments".
Back to "reverberant field, or not". Clearly, in any "normal" home room where a high fidelity system is employed and some degree of ambient sound is present, there is a reverberant field, however "dry" it may be. Whether or not it achieves a state which can usefully be measured as an RT60 is another matter.
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.
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.
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