Acoustical Treatments Master Thread - Page 51

Originally Posted by myfipie

Isn't that like learning how to build a car but not telling you how to put the wheels on?

Glenn

Originally Posted by pepar

It's only a tree-day course; they can't go into depth on everything. And, IMO, a more apt analogy is that it's like learning how to build a car, but not telling you how to spin balance the wheels. The car will still run, but it's not "optimized."

Originally Posted by Dennis Erskine

A couple of minor points: diffusors do not affect RT60 in the manner described nor will they have any impact on standing waves.

Originally Posted by Terry Montlick

I have observed, and the literature strongly suggests, that RT60 is lowered with added diffusion, at least when the absorptions of different room surfaces vary.

Originally Posted by pepar

Hi, Terry, will you please expand on that, especially your qualification with different room surface absorptions? And, if possible, perhaps couch it in layman's terms?

Originally Posted by pepar

It's only a tree-day course; they can't go into depth on everything. And, IMO, a more apt analogy is that it's like learning how to build a car, but not telling you how to spin balance the wheels. The car will still run, but it's not "optimized."

Originally Posted by Ethan Winer

> We are not told how to BUILD bass traps, we are told to buy them. <<br />
That makes a lot of sense. Of course I'd say that!

Seriously, unless someone values their time at $7 per hour, or just loves to get their hands dirty for the fun of it, building bass traps is not usually cost effective. And certainly not for a professional installer doing custom jobs in homes where people care about appearance.

> I'm trying to figure out how to build them. <<br />
It's all right here:

www.ethanwiner.com/acoustics.html

--Ethan

Originally Posted by myfipie

So would you like to go back and forth and insult each other about this for about 4 pages?

Glenn

Originally Posted by Terry Montlick

Sure!

The notion is that when you have a range of different absorption coefficients on surfaces in a room, the surfaces with less absorption have greater influence on the reverberation time. These "more-live" surfaces make the reverberation time longer -- longer, in fact, than would be predicted by a reverberation time formula such as the Eyring equation. [With a mix of absorption coefficients, some of them being large, the Eyring reverberation time formula should be used instead of the Sabine formula.]

But when you add diffusion, you mix up the wave directions a lot. This causes waves that might otherwise get stuck reflecting only between less absorptive surfaces to hit and be absorbed by surfaces with higher absorption coefficients. The reverberation dies out quicker. The results conform much better with the Eyring equation, which in fact presumes a diffuse sound field. And that's just what diffusion provides.

- Terry

Originally Posted by CriticalListener

It seems that, in my area, 705 & 1212 are nearly impossible to find. So I took Ethan's suggestion on his website and found a Knauff FSK board that is 6lbs/foot and ordered that (not a great price $1.71 sq ft) but with nothing else available I took it.

Normally I would buy the bass traps, but I'm having my carpenter build 7' x 2' frames of stained wood that match the other carpentry work in my showroom. I'm going to cover them with my left over 1" Owens Corning Black Acoustic board, and I hope they will come out looking very sharp.

Originally Posted by pepar

Thanks, I see and understand the premise. Is this always the case? Isn't it just as likely that sound that would have been reflected onto an absorptive surface instead is reflected onto a reflective surface?

Originally Posted by Terry Montlick

In the papers I referred to earlier, the lowering of RT60 with increased absorption was consistently shown. The rooms ranged from simple boxes to complex concert halls. This doesn't mean that one could not come up with an artificial example room which exhibited the reverse.

Originally Posted by pepar

You mean "diffusion" there, right?

I'm thinking of adding diffusors - Skyline LPs - to my theater to make it a bit livelier. I will run some RT60 tests first to be sure it's needed, but my ears and experience (live sound and a bit of studio work) lead me to think that I do. Some of the Skylines would be mounted onto parts of my false wall, behind which is 2" Linacoustic, and soon some superchunks. As this will, for above-bass frequencies, be "replacing" absorption with diffusion, is it likely that this *will* increase my RT60?

Other spots I've eyed are directly fired at by my side dipoles and are now plaster. My thinking was that this would contribute to a greater sense of envelopment by the surround fields. Am I on the wrong track?

Thanks again.

Originally Posted by Terry Montlick

. . The studies I cited kept the absorption the same while changing the diffusion coefficients only. And a few small diffusers on the walls shouldn't measurably affect RT60, while they may signficantly enhance the sense of spaciousness.

Originally Posted by pepar

That seems to be a THX-driven issue - envelopment. And personally, I like that feeling. But I rarely read anything on that. Home theater acoustics seem to be limited to RT60 and frequency response. CriticalListener mentioned his HAA Level 1 course touched on "sound waves become omnidirectional", but I'm not sure that could be contrued as envelopment. Your comment on spaciousness is one of the few on that topic that I've read from an acoustician. Am I just missing that discourse?

Originally Posted by CriticalListener


However we all discussed in detail the important aspects of sound quality, which I remember by using the acronym FRED C. [Focus, Response, Envelopment, Dynamics, Clarity].

Originally Posted by bpape

Having the ONLY sub in the room behind me is, to me, very distracting - even with an 80Hz xover. May just be me.

Originally Posted by nowandthen

Whew! I just finshed reading through this entire thread.

One area that has not been discussed: Using a riser for a base trap. I have seen this mentioned but have not seen any discussion in this thread. Assuming I have a 12" riser that is roughly 12' x 9', can this be used for a base trap? If so how would that be done? Also, I do plan to add buttkickers to the risers if that matters.

Thanks,

Todd

Originally Posted by Ethan Winer

Terry,

> I have observed, and the literature strongly suggests, that RT60 is lowered with added diffusion <

Also, many diffusors absorb as well as diffuse. RPG has absorption data for all of their diffusors, and the amount of absorption is not insignificant.

--Ethan

Originally Posted by Terry Montlick

Yes, the added absorption for QRD-type diffusers which RPG publishes is over and above the diffusion-absorption effect I described.

- Terry

Originally Posted by pepar

Are they absorbing frequencies that they are not diffusing? Are they absorbing any significant amount of the frequencies that they are diffusing?

Originally Posted by mschiff

Hi Ethan,

I downloaded your Real Traps Test Tone CD yesterday, and did a quick check of my home theater space. I set the level to 70db using the pink noise, and then just did a quick check at the beginning of each track. I had nothing below 60db, and nothing significantly above 80db. From 100 Hz up, I don't think it varied more then 5db either side of 70. Does this mean I'm in pretty good shape, or do I really need to check each tone?

-- Martin