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post #361 of 376 Old 08-08-2014, 11:42 PM
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So I have a question related to one of these DIY QRDs or one of the other options mentioned. Is there a somewhat rigid material that would diffuse some of the sound while also being somewhat absorbent to certain, likely higher, frequencies? Just think some diffraction is good but so is some absorption. Thanks.
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post #362 of 376 Old 08-09-2014, 12:16 AM
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aackthpt - first I thought for sure you were making up **** when you mentioned tortuosity...so thanks for the education.

LOL... I kind of wish I was making things up. Acoustics can turn into a very deep rabbit hole.

Cool link from diyaudio - thanks. Pearlite etc makes sense too; I'll have to read the thread.

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So I have a question related to one of these DIY QRDs or one of the other options mentioned. Is there a somewhat rigid material that would diffuse some of the sound while also being somewhat absorbent to certain, likely higher, frequencies? Just think some diffraction is good but so is some absorption. Thanks.

You mean diffusion... diffraction is a whole other thing... and yes there are acoustical devices meant to do it too. Anyway there are ways to do what you ask but you're thinking about it the wrong way when you ask for it in one material - it's just done with clever combinations of materials. You can't put anything in front of a QRD or it doesn't work right, so QRDs are sometimes made with some absorption at the bottom of the wells. Another way to do what you want is a thin layer of absorption over a diffusing device, for example a half inch of fiberglass on top of a very large polycylindrical diffuser. Also perhaps the MyRoom concept contains some of what you're looking for. Also the BAD (binary amplitude diffuser) panels (the absorber with a hard facing perforated in a specific pattern) shown earlier in the thread have some of the combination effect you're thinking of I think.


Neither absorption nor diffusion (nor diffraction for that matter) are "good" or "bad", they are just physical phenomena that can be useful to some end depending on one's philosophy and take on psychoacoustics as well as the situation one finds oneself (e.g. speaker dispersion pattern).
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post #363 of 376 Old 08-09-2014, 12:28 AM
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LOL... I kind of wish I was making things up. Acoustics can turn into a very deep rabbit hole.

Cool link from diyaudio - thanks. Pearlite etc makes sense too; I'll have to read the thread.

You mean diffusion... diffraction is a whole other thing... and yes there are acoustical devices meant to do it too. Anyway there are ways to do what you ask but you're thinking about it the wrong way when you ask for it in one material - it's just done with clever combinations of materials. You can't put anything in front of a QRD or it doesn't work right, so QRDs are sometimes made with some absorption at the bottom of the wells. Another way to do what you want is a thin layer of absorption over a diffusing device, for example a half inch of fiberglass on top of a very large polycylindrical diffuser. Also perhaps the MyRoom concept contains some of what you're looking for. Also the BAD (binary amplitude diffuser) panels (the absorber with a hard facing perforated in a specific pattern) shown earlier in the thread have some of the combination effect you're thinking of I think.

Neither absorption nor diffusion (nor diffraction for that matter) are "good" or "bad", they are just physical phenomena that can be useful to some end depending on one's philosophy and take on psychoacoustics as well as the situation one finds oneself (e.g. speaker dispersion pattern).
Thanks for the info...yes I was thinking about combining materials but I have a living room theater that I want to perform as much like a theater as possible while looking like a normal living room. I like the challenge but obviously that means compromises and more asethetic treatments. My wife and I were looking at different treatments last night and discussing what we might put on the front wall and front ceiling. We've already picked out some art pictures that I will print onto fabric and make art treatments with Roxul Rockboard 60. (DIY Custom-Printed Movie Poster Acoustic Panels - cheap!). But that's just the start and I want to do more treatments so I'm really interested in where this thread goes. Thanks again.
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post #364 of 376 Old 08-09-2014, 04:47 AM
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I currently have several 2ft by 4ft panels that are constructed out of two layers of 2" think (for a total of 4" thick) OC703 that I have spaced 4" off the wall. As my theater room is nearing completion, and being that it is so much larger than my previous listening area, I need to make some new absorption panels, but, unfortunately I can not afford any OC703 as Home Depot wants way more than I can spend.

With that in mind, I can get Roxul Safe-N-Sound for much less at my local Lowes. I have read that Safe-N-Sound is very similar to OC703, and with it being significantly cheaper, I want to go with that. What do you guys think would be the most optimal thickness of the Safe-N-Sound that would work similar to the 4" OC703 + 4" air gap?

I was thinking about building the S-N-S panels to be 6" thick with a 3" to 4" air gap. Does that sound ok?
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post #365 of 376 Old 08-09-2014, 11:07 AM
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Originally Posted by aackthpt View Post

Neither absorption nor diffusion (nor diffraction for that matter) are "good" or "bad", they are just physical phenomena that can be useful to some end depending on one's philosophy and take on psychoacoustics as well as the situation one finds oneself (e.g. speaker dispersion pattern).
Well said, these are just tools. One thing I see coming up over and over, is a cheap 2" diffuser for example, and why we can't just use that vs an elaborate (read deep) diffuser. Just having diffusers is not the same as having broadband diffusers. It's like saying, why use a 1000 watt sub amp when I can get this 100 watt one cheaper. Different tools for different needs.

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I currently have several 2ft by 4ft panels that are constructed out of two layers of 2" think (for a total of 4" thick) OC703 that I have spaced 4" off the wall. As my theater room is nearing completion, and being that it is so much larger than my previous listening area, I need to make some new absorption panels, but, unfortunately I can not afford any OC703 as Home Depot wants way more than I can spend.

With that in mind, I can get Roxul Safe-N-Sound for much less at my local Lowes. I have read that Safe-N-Sound is very similar to OC703, and with it being significantly cheaper, I want to go with that. What do you guys think would be the most optimal thickness of the Safe-N-Sound that would work similar to the 4" OC703 + 4" air gap?

I was thinking about building the S-N-S panels to be 6" thick with a 3" to 4" air gap. Does that sound ok?
I don't have my chart of insulation acoustic specs handy - there is a great one that someone keeps up somewhere on the web (I don't recall where right now) where you can quickly look up and compare one type to another - if you can find one of those charts, or look up the acoustic absorption specs of both materials you can probably answer that for yourself. Chances are it's going to be pretty close to 1:1. All the quality materials are pretty close, and the placement and number of the devices you install will have far more impact than the exact specs.

Back onto the DIY subject - I picked up some foam last night. that pink stuff has gone way up in price since the last time I bought it. A 1" sheet used to only be $7-$8 and my local menards had it for $12! So I only got one sheet of that, and one sheet of the cheap plane white EPS which was still like $8. I'm working on some garage projects now, but once I get some space cleared I'm going to take a guitar string, some plywood, and make a 12" X 12" hot wire cutter I can mount to my table saw and turn big pieces of foam into little pieces. We'll see what becomes of that!
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post #366 of 376 Old 08-09-2014, 11:47 AM
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With that in mind, I can get Roxul Safe-N-Sound for much less at my local Lowes. I have read that Safe-N-Sound is very similar to OC703, and with it being significantly cheaper, I want to go with that. What do you guys think would be the most optimal thickness of the Safe-N-Sound that would work similar to the 4" OC703 + 4" air gap?

I was thinking about building the S-N-S panels to be 6" thick with a 3" to 4" air gap. Does that sound ok?
There are much better places to get OC703 than home depot. The industrial insulation contractors are usually much less since it's not really a special order for them. But SNS will work well too. Here are some posts you may find useful:
http://www.gearslutz.com/board/7529309-post7.html
http://www.gearslutz.com/board/7532995-post14.html
http://www.gearslutz.com/board/9172736-post18.html
and you can use this tool with that last bit of data to play with alternatives: http://www.acousticmodelling.com/porous.php
depending on your goals with said panels.
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post #367 of 376 Old 08-09-2014, 11:50 AM
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I don't have my chart of insulation acoustic specs handy - there is a great one that someone keeps up somewhere on the web (I don't recall where right now) where you can quickly look up and compare one type to another - if you can find one of those charts, or look up the acoustic absorption specs of both materials you can probably answer that for yourself
http://bobgolds.com/AbsorptionCoefficients.htm
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post #368 of 376 Old 08-09-2014, 12:08 PM
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Back onto the DIY subject - I picked up some foam last night. that pink stuff has gone way up in price since the last time I bought it. A 1" sheet used to only be $7-$8 and my local menards had it for $12! So I only got one sheet of that, and one sheet of the cheap plane white EPS which was still like $8. I'm working on some garage projects now, but once I get some space cleared I'm going to take a guitar string, some plywood, and make a 12" X 12" hot wire cutter I can mount to my table saw and turn big pieces of foam into little pieces. We'll see what becomes of that!
I've had half a plan to put a hole through one of my IKEA tables with the corrugated core, rig up some kind of arm, and use something like
http://www.harborfreight.com/50-inch...ide-66581.html
, or a modification thereof, for a fence. The object would be to able to cut nearly any depth desired... who likes limitations?


Unfortunately my theatre room is really too small to use diffusion with good bandwidth - both from the "distance from the diffuser" standpoint as well as from the "reasonable space usage" standpoint. I'd love to have the space and possibility to put a really deep QRD array at the rear of my room.
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post #369 of 376 Old 08-09-2014, 01:48 PM
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... I could design a diffuser that works from 272Hz to 13kHz. But it would be 12" deep, and roughly 36" X 36" with 1" wide wells. Being realistic. lets look at 6"-8" deep, 1"-2" wide wells. 24" X 24" - something like what I built for my ceiling, but deeper.
Do we really need them to work that low?

Unless the speakers are huge, those freq are more or less omnidirectional.

But that raises a question I've never considered - what effect does diffusion have on room modes? Though I guess the 300 Hz range is around where they stop being an issue.

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I had a book shelf in my room (before clearing it out to make the house ready for showing according to realtor!) that actually had a very noticeable diffusing effect. ... It might have been a case of "better than nothing" maybe, but quite effective subjectively.
How did you determine the benefit was from diffusion, as opposed to absorption or interfering with a first reflection that might have been problematic?


Re the charcoal, most interesting.

I just read a little from the links, but it sounds like it works by essentially "solidifying" (adsorbing means they stick to the surface of the carbon) some of the air molecules so that they don't contribute to gas pressure.

What I don't understand is how that would work on both halves of the compression/rarefaction cycle.

I can see how pressure increase would be reduced by the molecules being impressed into the carbon surface, bur what about rarefaction?

Ah, it must be that at atmospheric pressure there is a portion of adsorbed air,some of which is released on rarefaction.

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post #370 of 376 Old 08-09-2014, 02:25 PM
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Do we really need them to work that low?

As low as 272 Hz? Well, presumably yes. Because a diffuser is more or less by definition something you use in the specular range, which as you noted more or less happens above 300Hz in the average room. And if you believe Toole, who more or less says all devices treating the specular range should extend to the bottom of it for the given room.

Toole, p. 507 "All absorbers, wherever they are located, must be not less than 3 to 4 in. deep." And later when discussing diffusers "To assist listeners in the perception of immersion or envelopment, engineered surfaces should be not less than about 8 in. deep, and of the type that disperses sound horizontally. Many curved and multifaceted convex geometric shapes will work, so considerable artistic freedom exists, as long as they are not less than about a foot deep."


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Ah, it must be that at atmospheric pressure there is a portion of adsorbed air,some of which is released on rarefaction.

All reasons why I'm more interested in test results more than understanding the mechanism. Tortuosity etc is just geometry, and that I understand. But I'm completely not qualified to judge the veracity of any claims of adsorption etc. It's not as though one is concerned about how fiberglass works when deciding how to design an acoustical device with it either - just about being able to predict what it does to the sound waves.
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post #371 of 376 Old 08-09-2014, 07:35 PM
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Fair enough.

Re fabrication, I find that a bandsaw works great for foams of all types and OC 702; the narrow blade gives less waste and particle generation, and lower blade speed doesn't throw it as far.

Also, had an idea of perhaps questionable merit and fabrication practicality -a QRD made from rigid fiberglass.

It would give some absorption at freq below the diffusion range.

To keep it from absorbing in the diffusion range the face could be sprayed with something goopy that would set relatively hard; maybe just thinned drywall compound would do it.

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post #372 of 376 Old 08-09-2014, 07:48 PM
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That's the one I was thinking of - thanks!
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post #373 of 376 Old 08-09-2014, 08:39 PM
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To keep it from absorbing in the diffusion range the face could be sprayed with something goopy that would set relatively hard; maybe just thinned drywall compound would do it.
Actually latex paint does this. Jens Eklund (from Gearslutz) sells an EPS diffuser (QRD-ish but without fins, designed with AFMG Reflex, made on a CNC hot wire machine) and he's tested the amount of absorption reduction based on painting. It was not small, but I don't have the reference handy nor the will to find it... should be easy to Google.
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post #374 of 376 Old 08-10-2014, 07:50 AM
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Actually latex paint does this. Jens Eklund (from Gearslutz) sells an EPS diffuser (QRD-ish but without fins, designed with AFMG Reflex, made on a CNC hot wire machine) and he's tested the amount of absorption reduction based on painting. It was not small, but I don't have the reference handy nor the will to find it... should be easy to Google.
That's another guy that has a great grasp of room treatments.
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post #375 of 376 Old 08-10-2014, 05:07 PM
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Actually latex paint does this.
I was concerned that too much might get sucked up by the porous fiberglass.

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post #376 of 376 Old 08-10-2014, 05:16 PM
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I was concerned that too much might get sucked up by the porous fiberglass.
Ohhh durr. Somehow "rigid fiberglass" translated to me as something more like "fiberglass layup" at that moment.


Yeah the combination devices I've seen have the typical frame, wooden bottom, and then use the rigid fiberglass to set the well height but I think they have some sort of plastic or other membrane on top of the rigid fiberglass at the well bottoms.
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