DIY acoustics - diffusors?

[hollywood_steve]

At least 90% of all the DIY acoustics threads on any of the forums seem to deal with bass trapping, and almost exclusively tube traps. I can't remember ever coming across a DIY diffuser article. The most well known commercial designs, (for example: http://www.rpginc.com/products/skyline/index.htm) seem like they would be extremely labor intensive to build. Are there any less demanding diffuser designs for the DIY builder?

I've got a nasty flutter echo occuring at the two parallel walls located on either side of my nearfields. As you can see on the cheesy Excel floorplan, I tried to keep the front half of the room symetrical, and that's what led to the empty parallel walls and the flutter echo. I've got to fix this before I can use the room.

Thanks

[Dave Martin]

Steve, why not just make the walls non parallel? I think that on my web site, there's a pictue of the control room that shows my approach to this. Essentially, it's 1x6's and plywood. Pretty easy (except that lifitng 4x8 plywoood that high is a pain in the butt). And it works fine.

The formula that I've read to eliminate flutter echoes is 1 inch per foot; by putting plywood on the wall with one side 5 1/2 inches out (the width of a 1x6 board) and the other against the wall, I get that. The only flutter I've got in my CR is floor to cieling (in one spot) and I've got plans to fix that as time and money permit.

[hollywood_steve]

If I wasn't already dealing with an extremely cramped space, the non-parallel wall idea would be a good option. But I'm using almost every square inch of floor space; if its not occupied by furniture or gear, its the very narrow access space to other gear or furniture. The diffusers would hang on what is now empty wall space (the ONLY empty wall space in the room, most of my furniture is floor-to-ceiling).

But now that I'm back, I've got another diffuser question. We all know that to either block or absorb sound, you need mass. But is mass really necessary for diffusion? When you picture the couple of standard non-foam diffuser designs, they all incorporate many pieces of different sized wood thereby providing a randomly uneven surface. Can these blocks be made of a very lightweight wood like balsa, or any of the other light "hobbyist" woods?

For example, something like this:
http://www.rpginc.com/products/qrd734/index.htm

Is the varied surface all that is important? (in which case balsa should be fine) Or does mass come into play somehow?

Thanks.

[Bob Olhsson]

Depends on the frequency that you want to diffuse!

Practically speaking, it doesn't require a massive structure to diffuse the only frequencies you can diffuse in most studio-sized spaces. Massive spaces can obviously benefit from massive structures.

I've always found simple things such as bookshelves full of books and records to do about as good a job as anything for broadband diffusion. If you want to get esoteric, diffusion can be used to alter the subjective frequency response of a room's ambient field but that requires serious calculations. To me it only makes sense in an isolated, soundproof floating room where you want to make it sound acoustically larger than it really is. Finally, simple flutter-echos can be easily tamed by a small checkerboard arrangement of acoustical ceiling tiles!

I recommend an empirical approach. Become a connoisseur of great sounding spaces by listening everywhere you go and noting what the good-sounding spaces have in common. A LOT of expensive studio designs sound really really bad despite the fortunes that have been spent on building them. This is because all acoustical design is an attempt to make a room sound like something other than what it really is. Great sounding rooms don't need acoustical treatment!

[hollywood_steve]

I'm in complete agreement with Bob's reply. Most of my space is covered floor-to ceiling with bookcases (3) and shelving units (2). I've always been a big fan of full bookcases as low tech, high efficiency diffusors. But on either side of my listening position, I have bare walls with no room for furniture. This space must be "fixed" via specific treatment that has to hang on the wall. The good news is that I have identified the problem as flutter echo strongly centered around 350hz. (350hz is the one frequency that is a multiple of all three of my primary modes, the calcs predicted this problem and testing just confirmed it.) So, in addition to all of the "natural" diffusion that I have stuffed into the room, I need to address this specific frequency in this specific location.

My first thought was a pair of 4ft x 2ft DIY diffusors, one on each of the two opposing walls. But if a 350Hz flutter echo can be fixed with just a dozen 12" x 12" tiles on each wall, that would be a MUCH easier, cheaper and faster solution.

[Ethan Winer]

Steve,

> if a 350Hz flutter echo can be fixed with just a dozen 12" x 12" tiles on each wall <

Absorbing completely down to 350 Hz is a bit optimistic for typical foam or fiberglass tiles, unless they're very thick.

--Ethan

[dbluefield]

Hi Steve,

Here's a thought. I can't find the link for the portable slats (sayers site is down), but they are easy to build, plus maybe 6 or so 6" 2X4 703 panels covered in fabric/burlap. easy to build, hang them cabinet style, mover them around to taste.

Best,

Dave

[dbluefield]

pic won't post. sorry - I can get it in the the browse, but can't attach?

[Beauarts]

There's an article in the latest Recording magizine about a DIY absorber reflector.

Also I've heard of using the cross-hatch lathe available in builder supplys and pegboard as difussors. That would certainly be a cheap fix.

[hollywood_steve]

Steve,

> if a 350Hz flutter echo can be fixed with just a dozen 12" x 12" tiles on each wall <

Absorbing completely down to 350 Hz is a bit optimistic for typical foam or fiberglass tiles, unless they're very thick.

--Ethan

That was what I was afraid of, but I wanted to hear what you and others thought.

So, given a classic case of flutter echo due to parallel walls only 8ft apart, what is a practical, efficient solution that also looks nice and doesn't cost several hundred dollars. (I just spent a silly amount of money fixing up this little 120SF room, I really don't want to hang cardboard on my walls, but I've also just passed twice my original budget amount for room improvements - that's one very nice piece of gear that will never be purchased.) Please keep in mind that I have already crammed three seven FT tall bookcases into the room and a full height shelving unit; the room has no space for anymore "natural" diffusors. This one situation must be handled by specific acoustical treatment placed on the walls in these two locations

[Ethan Winer]

Steve,

> what is a practical, efficient solution that also looks nice and doesn't cost several hundred dollars. <

You want rigid fiberglass. It's a raw material that needs to be covered with fabric for appearance and to keep the fibers in place. But it's cheap.

--Ethan

[hollywood_steve]

You want rigid fiberglass.


For trapping and/or absorbtion, 703 is my best friend, but for diffusion, too? Please explain. My problem is a classic example of flutter echo and consists predominately of 350Hz and the first harmonic (700hz). Wouldn't a typical diffusor product (or design) work fine for that type of situation? Why would rigid fiberglass be the answer? (Knowing that I already have both specific and broadband panel bass traps being built for other areas of the room. Thanks.

[Ethan Winer]

Steve,

> For trapping and/or absorbtion, 703 is my best friend, but for diffusion, too? <

What I meant is you need absorption, not diffusion. That room is simply too small to take good advantage of diffusion.

> flutter echo ... of 350Hz <

Yeah, that too. You need a pretty big diffuser to handle such a low frequency very well.

--Ethan

[Bob Olhsson]

I think of flutter echos as being mid-range colorations. The 350 Hz. is a standing wave resulting from the room dimensions. Rigid fiberglass across the corner will help a lot. I'd find an industrial insulation dealer and buy the densest furnace insulation they'll sell you a reasonably small quantity of.

[Tim Farrant]

Yes, absorbtion on the sidewalls and ceiling around you monitors will remove the flutter and improve you stereo imaging no end. You want to try and reduce early reflections that produce comb filtering effects and a generally bad stereo sound feild. Reflections from sufaces with a path length of 10 metres or more can be considered useful - but looks like you won't be getting many of those in that room !!

Hope this helps
Tim.

[kurt]

Are there any guidelines about mixer positions? My control room is 22.5 x 18 x 9 feet.
At the moment I’m sitting in the middle of the room.
Where should I place my sitting chair?

[Ethan Winer]

Kurt,

> At the moment I’m sitting in the middle of the room. <

That's where the bass response is most lacking. The drawing below shows the best way to set up in a rectangular room.

--Ethan

[Joel DuBay]

Originally Posted by kurt

Are there any guidelines about mixer positions? My control room is 22.5 x 18 x 9 feet.
At the moment I’m sitting in the middle of the room.
Where should I place my sitting chair?

Kurt, a typical listening position in a rectangular room is set at about 38% of the rooms length from the front wall. You can see a good graphic of such here:

Studio Room Graphic


Get your head there, and be happy!


Good luck!

[Scott R. Foster]

Joel:

Where is the keg cooler... is that the box next to the desk thingy?



Just teasing, I love that graphic's transparency effect - a lot of core concepts well illustrated.

[kurt]

Thank you Ethan Winer & powerjoe for great advice.

More questions:

This is a 5.1 room in progress.
As you can se you can slide .L & .R speakers in 30 degrees axe so it’s possible to get perfect triangle with listening position.

38%. Is it from the window or surface of the speakers?

PS.
My new beautiful Portico 512 now occupies the empty space in the 19” rack.

http://gearslutz.com/board/attachmen...1&d=1141683886

[Joel DuBay]

Cheers Kurt!


We love our new Porticos Too! Amazing!

The distance percentage is from the front wall. Not the speakers. So, your head is 38% from the drywall (etc) on your front wall.


Alrighty!

[kurt]

Thanx.