DIY QRD plans?

[damonrpayne]

DrWho & Others at some point have mentioned that these are easy to make and make them look nice. Having blown out my 2008 budget on certain other purchases I'm now willing to take a look at this. Anyone have links to plans/articles?

[damonrpayne]

Here's one in the style of the RPG skyline. I'm somewhat suspicious since the specific material is not mentioned, and I should think that difinately matters

http://www.bobgolds.com/DifuserKgveteran/home.htm

[damonrpayne]

This one looks interesting too, but I'm thinking that the longest column, 7 1/2" deep is wider than anything I personally have seen on a wall or ceiling.

http://www.pmerecords.com/Diffusor.cfm

Products like this ( http://www.silentsource.com/diffusors-rpg-hemiffusor.html ) are 4" deep.

[michael hurd]

Damon... the intent is to provide random scattering, so I really don't percieve an obvious advantage to using a certain material ( other than cost, aesthetics, weight, etc ).

I have thought of building my own out of 2" styrofoam board. Set up a table saw to rip in 2" wide and feed a few sheets in until you have a pile of 2" x 2" strips. Take these strips and cut them into shorter pieces... it helps if you have a miter saw and use a stop. Make a few different lengths and when you have a pile of these pieces, you can cut one end on a slight angle. Glue these together with wood glue, it works fine to glue stryofoam, and screw a panel to the ceiling.

You can also paint styrofam successfully with outdoor acrylic latex, it will adhere well and not eat the foam. I would personally spray them with a wagner or other like spray gun before installation.... probably semi or flat black.

[DrWho]

I'm mostly just replying so that this thread gets in "My Discussions", but I'm pretty sure I can get back to you with some plans.

Did you have any specific locations in your room in mind?

[colterphoto1]

I believe our painter friend is correct. You are diffusing the sound waves, not absorbing them. Any material should work so long that it is NOT absorbtive to any great degree. Note that Prof Thump has created some for his HT using wood scraps. The originals are molded polystyrene but you wouldn't want to start cutting that material, you'd have poly beads all over the neighborhood. I think the blue builder's foam would be appropriate.

Creating a pile of 2x2 strips, then cutting lengths completely randomly (ie, without stops on your fence other than perhaps a max length), and setting your chop saw at a slight angle right off the bat would speed production.

I wonder how the product edges would look as created on a saw versus the hot knife?

[damonrpayne]

Correct, I was saying the material matters since I wanted to make sure it was NOT absorbing.

DrWho: I was thinking intially a couple on the side walls and then moving to the ceiling with a larger # of these.

[DrWho]

One thing to keep in mind is that the low frequency limit of a QRD is when the well depth is 1/4 wavelength....so a 1ft depth is gonna get you down to 250Hz.

[DrWho]

Oh, I also wanted to mention that at the higher frequencies your room is "large" which means you will want at least some absorption up high...and you don't necessarily want to diffuse the higher frequencies.

[michael hurd]

Damon: I get what you mean about the material not being absorptive... gotcha.

Colter: handy dandy tip # 2.... after applying outdoor acrylic latex paint you can apply bondo or other product on top of latex paint to fill any small pores on the visible sides or tips. Repaint in the color of choice.Having a latex barrier prevents bondo from attacking the foam.

The edges cut on a tablesaw are really smooth.

[colterphoto1]

I've not tried this yet, but would imagine a fine-toothed, narrow-kerf blade, like for sawing metal, might give best results.

[michael hurd]

All I have tried is a combo rip/crosscut blade... and it gave very nice cuts, almost factory edge quality. If the blade kerf is too narrow, I would imagine that it would want to melt rather than cut.

[damonrpayne]

DrWho:

I've got some OC panels for bass right now and I'm about to put a huge wedge in there as soon as my wife sews the covering for it, I'm not looking to diffuse bass. I have some foam in the rear that has cut down on flutter echo from my side surrounds but need more, that's a project for another day. I'm looking to diffuse the mid range hitting the ceiling and perhaps the sides of the front walls and see how it affects my soundstage after I get used to my new speakers.

[DrWho]

I'm not looking to diffuse bass.

Yea, you would never want to diffuse the bass anyway (I hope that's not what I implied).

You do want to diffuse frequencies down to around 200-400Hz though. That's probably not something you can do with your ceiling panels, but on the side walls where you have more room you might want to consider going as deep as you can.

If you're feeling bold, there's no reason why you couldn't extend the QRDs into the wall either (unless you have sound transmission concerns)

[damonrpayne]

I've got insulation and such I'd rather not tear up so I don't think I'm willing to go into the walls.

[WMcD]

I have some personal issues with QRD.

Are they really better than poly-cylinders in a practical sense? Arto has his main room treated with polys and Dr. Who has treated a room at U of I. Results are good per these learned gentlemen.

It seems to me that it is possible to make, say, 100 square feet of polys without too much effort and expense. Yet the same square footage of QRDs is either very costly when bought from a manufacturer or very time consuming if home made. "Very" is an understatement.

I'm reminded of a short story by A.C. Clarke. (I'll have to find it.) One alien race make war using superior weapons which are high priced and cranky. These do not save the day against the inferior but reliable weapons of the other alien race which are mass produced.

The story is a parable on Nazi weapons versus mass produced Allied weapons. A.C.C. wrote that his story was used in a class at MIT.

Wm McD

[PrestonTom]

Some interesting issues are raised by Gil.

Certainly the poly-cylinders are easier to construct and may actually fit in with the existing decor (well, maybe...). The excitement about the QRD diffusers (and some others conceptualized by engineers associated with Bell labs) concerned their effective bandwidth. Speaking broadly, most designs are effectively diffusing only down to wavelengths that are about 2x the depth of the diffuser. That is, the diffuser would need to be about 6.5 inches in depth to tame frequencies down to about 1000Hz (lambda of about 13 inches). The new generation of diffusers (including QRD) only needed to be about 1/4 wavelength. So, with a 6.5in depth, you could get about the same degree of diffusion down from 1000 Hz to now about 500 Hz. If the diffuser depth is not an issue, or the extended bandwidth is not an issue, then the other designs could also be employed.

Just to deflect the criticism, I have intentionally painted this with a (simplistic) broad brush and have not defined a metric of diffusion or "effectiveness". It is an interesting story but not one that is quickly summarized.

Personally I have been toying with the idea of a poly cylinder that "hides" a Helmholtz resonator with in the "cavity". The idea may end up being ridiculous since resonator need to placed with the room to coincide with a low frequency node. I need to think this one through ...

Good Luck,

-Tom

[DrWho]

Are they really better than poly-cylinders in a practical sense? Arto has his main room treated with polys and Dr. Who has treated a room at U of I. Results are good per these learned gentlemen.

If I'm reading the literature correctly, polycylinders don't break up specular reflections...you just get more of them with lessor amplitude. The biggest advantage to QRD's is their ability to randomly scatter a specular reflection.

Btw, the room we treated at UIUC had nowhere near acceptable acoustics...it was just better than without treatment.

[damonrpayne]

You come up with any plans yet doc? Should I move forward with the blue foam for the ceiling?

[WMcD]

I'm not convinced that the skylines are effective QRD devices, based on theory.

It is my understanding that the original QRD's are a flat panel with hard walled wells in them, each having a hard bottom.

In an impulse analysis, an impulse hits the array, say, straight on. Then there are reflections from the bottoms of the very many well elements. But the wells are of different depths. Thus the impulse response of well elements is somewhat randomized (carefully pseudo random, or exactly uniform at all freqs), on axis, depending on the effect of hard wall and hard bottom.

If you look at the impulse response of, say, just two given wells, one is delayed compared to the other (different depth). Thus you have a phased array antenna elements which dictates an off-axis lobe. Hence, dispursion.

The magic of the QRD math is to make sure that dispursion is constant over some freq range where the delay can be affected by varying well depth. But, the implementation of delay depends, if I understand correctly, on a hard material for the well walls, and a hard bottom. (Some of the math should account for the off-axis excitation, too.)

It seems to me that the skyline devices fall short in implementation of the theory: They (1) only trace the well bottoms, (2) are not "hard" being styrofoam, and (3) don't have well walls (necessary for making sure that each well element causes a delay). Making they out of wood does not solve (3).

= = =

I've posted the articles on polys in the past. An interesting variation is to fill them with fiberglass and make them also serve as bass absorbers. Building in a Helmholz resonator is an interesting variation. I had thought we could put in surround speakers and/or lamps for indirect lighting.

Best,

Wm McD