I currently have treated my space via some purchased acoustic panels after I built my own (unattractively) with muslin etc and then handed those down to my cousin.

I would now like to add some diffusion. I will either build some 2'x4' 1-dimensional QRDs or a big polycylindrical. I am not interested in skylines due to size and ugliness.

For the QRD, I can model my work off of the common calculator online and http://www.soundcontrolroom.com/products/diffusion/qrd.htm

Question: What glue/caulking should be used to make sure all MDF components are 100% sealed? I presume any holes no matter how small will negate effectiveness ... True/false?

Additionally, is type of wood important here? MDF is used by the co above, but another offers all wood types.

On the other hand, I have considered going with polyclindrical (cheaper, but now seeming unlikely due to prospective difficulty in bending plywood and making the unit freestanding, transportable, and not just bracketed to the wall), ala this description:

How would the sound of each compare? I have never heard either before and have no way of listening.

I might go 2-3 QRDs vs. 1 polyclindrical along the longest wall. QRDs are more money and time of course but more attractive/modular and thus may be more likely ...

Any links re: 1D QRD or polycylindrical construction would be appreciated.

Thanks,
Mike

Mike,

> I will either build some 2'x4' 1-dimensional QRDs or a big polycylindrical. <

I'm not a fan of polys because while they may scatter sound somewhat, they don't reduce comb filtering nearly as well as the QRD types. This is especially a problem in smaller home-sized studios and control rooms.

> would I want to fill it in with rigid fiberglass and/or mineral wool, or does regular pink fiberglass work fine for this? <

If you make the diffusor from something massive like wood or MDF, filling the space behind with fiberglass won't do much other than maybe reduce resonances within the diffusor itself.

--Ethan

Ya I've heard that from you before re: comb filtering, Ethan. I do believe I will go with a few QRDs.

Last questions before I build:

1) Does wood type (Options: MDF, Pine, Birch) matter?

2) Must special glue/caulking be used and must it be perfectly airtight to work?

Mike,

> Does wood type (Options: MDF, Pine, Birch) matter? <

I can't imagine why that would matter.

> Must special glue/caulking be used and must it be perfectly airtight to work? <

Nor that either.

--Ethan

That's one place where Ethan and I disagree. Yes - poly's are not true diffusors but more dispersers. BUT, they work from a give frequency up based on size. QRD's have a top and bottom limit.

Also Polys yield a much smoother polar plot.

Bryan

Bryan,

> That's one place where Ethan and I disagree. <

That's fine with me. At least you don't call me an a-hole like some other people we know do.

As former New York City mayor Ed Koch used to say, "If anyone agrees with me 100 percent of the time, there's something very wrong with them."

> Yes - poly's are not true diffusors but more dispersers. BUT, they work from a give frequency up based on size. QRD's have a top and bottom limit. <

Yeah, but they don't reduce comb filtering as much as QRDs so who cares?

Seriously, I guess which type is better depends on the problem that needs solving. If a room is large and you need to avoid flutter echo across the room or between the floor and ceiling, I'd use a poly too. Those are the kind of situations Everest shows in his book. But in a bedroom studio it seems to me that comb filtering is the larger problem. Am I missing something with that distinction?

--Ethan

Yeah - it's kind of nice to have a free exchange of ideas without all the name calling crap. That's how we all learn.

Well, I honestly think that when you did your test on them, the poly that was used was nowhere near deep enough for it's width. I personally think that's why you don't feel they reduce comb filtering. And the soda can test, well sorry - again, not something that is functionally valid. Try an 8' wide sheet of 1/4" sometime bent to have about 12-16" behind it at the deepest point. I think you'd be very surprised at what that will do.

In any case, they can still only do any reduction or diffusion over a limited range while the poly can work from the bottom up. Even if it's comb filtering properties aren't as good (doubt this but for the sake of argument), it's still better above the point where the QRDs stop functioning like a diffusor and start acting like a plain flat wall.

Bryan

Bryan,

> Try an 8' wide sheet of 1/4" sometime bent to have about 12-16" behind it at the deepest point. I think you'd be very surprised at what that will do. <

One of the many things on my "to do" list is to build a poly like that and use ETF to measure the comb filtering side by side with a QRD. Now that I have a bunch of QRDs here I'm one step closer to being able to do that test.

--Ethan

Thanks for the discourse. It has been quite informative.

Although I actually think the poly would be better for my space based on your comments, I believe I will likely go with a few QRDs instead. The reason is I need the solution to be modular and transportable. A few 2'x4' QRDs can be taken out of my basement studio and hidden in my room when I'm away at university and my parents are hating what I have done to their house. A 8' poly might be a bit more difficult.

I'm surprised more people don't try to build these. RPG charges $500 USD for a basic 9" 2'x4' birch model. Mine should cost $80 tops for the functional equivalent, with not much work either (have access to a woodworking shop).

Hopefully all goes well.

btw. Ethan your diffusors look insane! Quite nice. Though I am struck by the seeming shallowness of the 6" depth. Most seem to go 9" these days, from what I see ...

Mike,

> I'm surprised more people don't try to build these. RPG charges $500 USD for a basic 9" 2'x4' birch model. Mine should cost $80 tops for the functional equivalent, with not much work either (have access to a woodworking shop). <

Not much work? Look again at the details.

> Ethan your diffusors look insane! Quite nice. Though I am struck by the seeming shallowness of the 6" depth. Most seem to go 9" these days, from what I see <

It's a balance between what's effective enough to be useful and what can be shipped by FedEx or UPS without incurring a $50 oversize charge. Most diffusors are much thinner than six inches! Auralex has a line that's only three inches deep, and a lot of the cheap stamped plastic and styrofoam stuff is only an inch or two thick.

Also, the diffusors RealTraps sells are also bass traps, though diffusion is the main feature.

--Ethan

I suppose I'll find out how much work once I actually try making them. I thought the muslin panels would be minimal work, and those were a time-consuming nightmare, so maybe I'll be wrong.

I'm sure YOUR diffusors take a tonne of work - 13 wells, gorgeously finished, mineral wool or fibreglass in the back ...

The RPG's are just wood and glue . . . Again, we'll see.

Just remember that no matter what you do, the depth of the cavities and the width of same are determining factors for both the bottom and top end of their effectiveness. Polys can be built smaller also with the same restrictions on their low end limits.

Absolutely a 4'x8' isn't very portable... Try a 2'x2'that's about 6" deep made from bent 1/8" material with fluffy fiberglass behind it.

One other thing with QRDs is that they do provide some absorbtion over their usable range due to intercavity cancellations. This can be a plus or minus depending on what your room needs.

Bryan