[nodiak]

To clean up and simplify this thread I've deleted this post about sonotube diffusers since I came across another type I link to in a post below. Bryans post is in response to the post I deleted.
I just received bass trap material from Bryan too and am working on 1st reflection point material too. So finally will get some room problems treated.
Over the next several days I should start getting things hung on the wall. I'm hoping to get a digicam soon to share the adventures.

Don

[bpape]

Well, good luck.

Just a couple of things.

They're not really poly's - they're half rounds.  Polys are a continuously varying curve - kind of like part of a french curve.  That's why they disperse so well.  Half rounds will still do some good but don't expect miracles and they do have their own set of issues.

Also, you should likely consider putting some strips of absorbtion where 2 of them meet at the wall to absorb the focusing effect that this little nook can give.

Lastly, if you want to make them be some kind of absorber, I'd recommend not cutting holes in them but instead, stuffing them lightly with insulation and sealing them air tight to act like a variable cavity membrane absorber.

Good luck.

Bryan

[nodiak]

see below.
Don

[nodiak]

Spurred on by Bryans comments I've googled down that long and winding road and found unending links to polycylindrical diffusers, wavey panels, semi cylinders, sealed, open ended, suspended from ceiling, aperiodic orientation, etc., etc., and some actual design ideas!
I found a couple suitably simple ideas by admirably intelligent men who know how to speak in numbers.
 
One idea is to use a 48" x 96" sheet of 1/8" tempered masonite. Compress the 48" side down to 46.5" and it takes on an ellyptical form. He talks of it's resonant frequency (63 hz) when left open ended, and so useful as an absorber (debatable?). I am hoping/expecting this could be scaled down to smaller sizes like 24" x 48" or 32" x 72". Construction notes are about halfway down in this link:

http://malcolm.bignoisybug.com/rsdp/fuzpoly.txt

To stuff, close ends, or leave open, each has supporters.

Another idea is maybe the same basic shape in a smaller size, with multiples and put into use in a large space. He talks about spacing them apart with 2x width between each unit. This post is at the bottom of page in the link. Has photo of room and drawing of diffuser, but no dimensions - maybe further on in the reading asI didn't finish it yet. This is the same discussion the above link came from, lots of great discussion with further links.

http://forum.studiotips.com/viewtopic.php?t=2472&postdays=0&postorder=asc&start=0

Now I will probably discard the sonotube units if the 1/8" masonite or similarly simple diffuser can be worked out. After seeing the photo in the 2nd link I am thinking of similar spacing and so needing about 6 on the walls. Possibly same units on ceiling or 2-3 large 48" x 96" ones.
Would like to get a pattern for a French curve type shape, like Bryan mentioned, for wavy panel for ceilng.

Don





 

[nodiak]

I've got 2 diffusers (maybe better called reflectors?) built and 2 glued up that will be dry tomorrow. They are ~ 30" x 44" with about a 4.5" peak to the curve. They are made following the idea of the malcolm.bignoisybug link (above post). These and maybe 2 more are for the side walls. I'm making a 48" x 96" with 6" peak for the ceiling, possibly one more.
For the rear walls and ceiling corners I may use curves from this 1/8" masonite material too as it's cheap ($8 per 48"x96" sheet), light, and easy to use. I have 3 skyline type diffusers I made with 2" x 2" fir. Two of them are 18" x 36" and one is 18" x 48". They're heavy and not so sure i want them overhead where they were innitially planned to go. I think they'll work on a wall where we don't go often.

I'm going with lots of diffusion to see how well it works at making the playback more intelligible. There's alot of confusion from all the reflections in this room - hard plaster walls and a big window on the front wall (that is getting curtains soon).
The room is fully carpeted and I don't feel the need to lose much treble, so I want to get reflections dealt with before I dial in absorption - except I will be doing first reflection absorption between speakers and my sitting position right away as everyone suggests, which makes sense.

I'll just say I'm new to this and expect to make plenty of mistakes/adjustments and try different treatments as I learn. Most things I put up will be easily removeable so I can get some clue as to what is doing what. Also, at some point I'll run out of time and this surge of focus and will live with whatever is up.

Feel free to offer your experiences.

Don

[woodsyi]

Hello Don,

I do think you are making deflectors rather than diffusers.  From the readings I have done, you need well depth of at least 8" in a proven quadratic well or skyline type to get true diffusing down to 100 Hz -- waves scattered enough not to form standing waves or delayed arrival.  Multiple deflectors I think works in a very large room but may not be as good in a normal household room.  Real Traps, SRL Audio and RPG makes diffusers that I know works.  There are also DIY formulas on the web for making them out of wood or other material.

[nodiak]

woodsyi, I agree they can be called deflectors, tho in the (bignoisy) link above they are called polycylindrical diffusers, and he considers them broadband - as far as having high end dispersion and low end absorption. I think disperser is a better term, but oh well a little late to change maybe.
Why do you say works in big but not small rooms? I'd like to know the idea behind that doubt. I know there's no way I'll get it perfect with this method. But the improvement is there, and this endeavor will give a reference to learn from. Maybe that's the most important thing. Definitely worth taking steps instead of staying frozen with indecision.     
My plan has been to go with diffusers/deflectors first to see how much they will cut down on echoes/reflections without changing the amount of treble in the room.
Then put in full length corner absorbers and side wall absorbers.
The flaw (fatal to some) is that all this will innitially be done without measurements as I don't have the tools now. Mostly it's a lack of time and I'm choosing to get some treatments up while I have time. I hope to get measurements soon, but in all honesty time to focus may be a problem.
I like this approach (in my situation) as corner absorbers are recommended by everyone, and dispersion of itself doesn't seem to add or subtract from frequency level in the room, except from reflective peaks and dips I assume (assume).
So far so good, with four of them the echoes and playback confusion have been reduced alot, showing me they're effective. Almost no more "sproing" remains from the clap test. The soundstage is more stable (even the tv sounds better).
In searches I came across all types of diffusers and deflectors and arguments pro and con for each. The only thing to do was to try something myself in this room. These are cheap and easy to make, for better or worse.
In time I'd like to replace them with nicer wood ones, possibly using slats instead of flat panels. BTW I used the unfinished textured side of the masonite exposed to the room because it's "less ugly" ! If they're keepers they'll get covered later.
Trying to decide which ideas to try and speculations on what does/doesn't work can go on forever. This may seem a weird or stupid thread, could've been presented better, but still I would like to hear from others on their room experiments, failures and success.     
Room treatment can be mysterious and daunting. Any room treatment device or piece of furniture can be moved and change the balance, or more or less people in the room will do the same.
So I choose to go with a more general plan.
As with all things audio room treatment is room and taste dependent. I probably like a livelier room than some.
My room has hard plaster walls and ceiling and full padded carpet. It's 11.5' wide, 7.5' high, and 26.5' long. 2nd story apartment on suspended wood floor. Two channel system on short wall. Speakers 6' out. Listening position 17'5' from front wall/11.5' from speakers. 9' space behind listener, and 4' opening on rear wall to another room. They're all different.
Don 

[woodsyi]

Don,

I just remember concluding from the readings -- I was overloaded with information -- that poly diffusers in single period did not scatter enough to work in a small room and that it was not good in oblique angles which, to me, eliminated it from being used on side walls, eg. early reflection points.  That's why I went with QRD design with 8" depth and a full period.

[nodiak]

woodsyi, I think the room treatment division of audio is in it's infancy, as far as we the the masses go. I didn't come across any idea/device that didn't have arguments from differing sides.
These things on my walls have helped reduce the echoes I hate. The room's better for music play back. I'm going to go on with the corner bass traps and absorption on walls and ceiling between the speakers and listening seat. I'll probably stop there for awhile.
I would expect there are definite ways to measure a room and correct it's problems. And like with choosing the rest of the audio gear even adjust it to a persons tastes. I don't have the needed tools now, maybe some later. 
The issues of audio in a room becomes more and more complicated and complex the more you get into it. There's always the dangers of being armed with only partial knowledge and little experience.
I remember a day when all I wanted was music in the room  ...
It's all interesting but since it's not a business or graduate thesis for me I can only do general things. I think that will still be far better than doing nothing
Don

[woodsyi]

Don,

I agree that acoustic stuff is in it's infancy.  Just think, Real traps and SLR audio just released their diffusers this year.  RPG had them for years but their commercial price was beyond the reach of most consumers.  I understand that even now there is no concensus on how to derive a scattering coefficient, which is necessary for most equations that will compare different rooms.  You have a plan and it's working.  Kudos for you.  My personal method is broadband absorption at various bi-wall and tri-wall corners for bass, absorption of mids and highs in the front and diffusion in the back and side.  I am not quite done yet, but I like what I got so far. 

[nodiak]

We're doing some things with the same approach - broadband/bass bi and tri corner absorption. And both doing diffusion in back and on side walls. I think the taste difference might be I lean to a touch more reverb/liveliness than some folks. Must be from my kid band years playing in concrete basements  . Probably where this arthritis started too come to think of it...
The full carpet absorbs some highs so I'm careful to add much to that, so deflectors on ceiling too. I may end up adding reflective material to corner absorbers.
I'll see if I can get a camera soon and post some pics.
It's funny because I hesitated soooo looong to choose an approach. But once something is tried you can then use it as a gauge for what needs improved or what's acceptable. Can't expect to hit it perfect the first time.
I also have some skyline diffusers (max depth 7.5") to compare to these deflectors. The rap against skylines has been their depth causes some absorption. At what frequencies I don't remember. They turned out pretty heavy and I'm not sure I want them up where we walk (earthquake country).
Also want to determine if these masonite deflector/diffuser/ellipses/etc. (couldn't resist) absorb bass as claimed.
Don
 
 

[Ethan Winer]

I understand that even now there is no concensus on how to derive a scattering coefficient, which is necessary for most equations that will compare different rooms.

Here's my theory (uh oh):

After spending quite a lot of time listening and experimenting with different distances and placements, it seems to me the main point of a QRD in a smaller room is to avoid comb filtering and flutter echo. Let's say for discussion that a QRD doesn't even "scatter" at all, and all reflections come straight back at you. Just having the differing well depths means the reflections are less coherent, and return with different delay times. That alone reduces comb filtering by a lot, and probably flutter echo by some amount as well. Any scattering you get is just icing on the cake.

--Ethan

[ooheadsoo]

Has anyone dared yet to write a faq on when you should use diffusion compared to absorption, and in what ratios?

[woodsyi]

Has anyone dared yet to write a faq on when you should use diffusion compared to absorption, and in what ratios?

I don't know anything well enough to write about what should be done.  Next month, however, I will do something where sound will be compared where back wall and side relection points will be covered with absorbers and then with diffusers.  I suppose I should do bare wall as well.  It will be fun to see what happens. 

[ctviggen]

I would also mix and match.  Try for instance diffusers on the rear wall and absorbers at first reflection point. 

[woodsyi]

I would also mix and match.  Try for instance diffusers on the rear wall and absorbers at first reflection point. 

Good point.  I think I better draw up a combination chart. 

[Ethan Winer]

Has anyone dared yet to write a faq on when you should use diffusion compared to absorption, and in what ratios?

Diffusion is useful on the rear wall if it's more than, say, 6 feet behind you. It might be useful at reflection points in a large room, but in my 25 by 16 living room absorption sounds better. In very small rooms absorption is a better choice.

--Ethan

[nodiak]

Ethan, is your point about absorption working better than diffusion in a small room because, diffused or not, the sound waves still don't have enough distance to cause the needed time delay? And absorption will interrupt that pattern (damn, someone say it better).
Don

[bpape]

I think that's part of it. Also a part is the fact that in smaller rooms, one needs proportionately more absorbtion to bring decay times in line than you do in a larger one.  In addition, smaller rooms have less air - which is a surprisingly decent absorber from a couple thousand Hz up.

Bryan