homemade qrd diffuser

[Jakeman1086]

Well, I had a lot of fun making my broadband absorption panels, and though I still need to make more, I thought I'd break up the routine by making a qrd diffuser. I loved the sound Ethan Winer was getting in his video recording acoustic guitar with the diffuser. I want that.

I understand the concepts pretty well, and although not completely, I think I know enough to make the panels. I don't care so much about dimensions. I want the panel to be modular so I can tack on as many as I need. Using inverses in Barker's Sequence, I figure I shouldn't have to worry about this aspect.

Messing around with the QRDude calculator and referring to some sites, it looks as if these specs are best for frequencies diffused (correct me if I’m wrong):

23 wells
design frequency = 994 hz

This gives me a depth of 5.35 in, the only dimension I care about. The period will be 25.25 inches.
This only diffuses in the frequency range of 497-6880 hz. To go any higher, I guess I would have to use fractals (I probably won’t do this unless y’all know of an easy method)? For the low end, did I go low enough in your opinion. I plan to absorb the extreme bottom, but should I diffuse lower? Also, how do you absorb behind a reflective surface like the diffuser anyway? Do bass frequencies go through the reflective surface? This concept has never made sense to me.

Lastly, my ideas and questions on building materials and construction - THIS IS THE BIG ONE!:
I want to do it exactly like this guy - N23 Array Build
It seems the most economical (cheap), lightweight, and easy. However, I don’t understand how he put it together and I can’t find anything similar to his method anywhere else on the internet. Did he start with the frame, then just stick the fins in, layering them with the foam? Are the fins and foam actually attached to the frame in any other way than glue? I’d email him, but I can’t find his email address. Please tell me how he went about construction if you know!
Thank you!

[collo]

Great to see another build in the formative stages...

994Hz is right for a normal N23 panel of 5.35 inches, but the inverse will be deeper....

If you are going to use inverse panels as well as normal panels, your maximum build depth will be set by the inverse panel.

An N23 inverse with a build depth of your stated limit of 5.35 inches has a design frequency of 1265Hz. The normal panel needs the same frequency and gets a build depth of 4.17 inches


Now have a look at advanced mode with both "Auto-optimise build depth" and "Panel will be used in a sequence that includes its inverse" turned on.

You get a deeper N23+2 normal panel and a shallower N23-2 inverse. The maximum build depth of this pairing is more economical than the unoptimised inverse.

The inverse has a build depth of 5.35 inches as before, but the design frequency is improved to 1155Hz. The normal panel is the same frequency and has a build depth of 5.11 inches.


Regarding Xenon's build - find his thread and send him a PM.

The other questions I'll leave to the acoustic experts....

[Lupo]

Hei!

It's so cool that people are making diffusers! :)

Check out Xenons build thread, where he explains the pictures you linked to: QRD Prime + Period Question (Live Room)

As for absorption behind panels. A quick way to get a grasp of absorption is to think of the sound waves as water. Whatever material that will get wet when you throw water at it, will absorb. Throwing a cup of water on a diffuser will obviously not make the rearside wet. That's the equivalent to high frequencies hitting the diffuser. Low frequencies, on the other hand, are much larger than any normal size diffuser array. It'll be like dumping a helicopter load of water on the diffuser! The size of the diffuser will be so small compared to the impending wave that the water will wrap around and make the rearside wet too. That is, if the diffuser material is solid enough to block the passage of the low frequency wave. Chances are high that the LF content will partly go straight through the material. In most cases, it'll probably be a combination of diffraction around the object and transmission through the object.


Good luck with your build! Hope you post pictures! :)


Andreas

[collo]

Had a further play with the figures - A Normal N13+6 / Inverse N13-6 pairing gives an even better design frequency of 1069Hz.

[Jakeman1086]

Ok, I hope this doesn't seem ignorant, but I've had limited time to continue research on this project. I'd like to jump into construction as soon as possible (I have some vacation time coming up). I don't want to spend it still researching.

Thanks so far with the feedback. Everyone's been really helpful. I want to redefine the problem to get better feedback so I can get going quickly.

Redefined problem:

*make a QRD Diffuser

*no restrictions on any dimension (within 6'x6'x2'depth limit); depth is first priority if any for minimization. So, in essence, I want the ideal dimensions (if it exists) for diffuser performance within these modest limits.

*Ideally, have the diffuser dual-serve for low frequency absorption (similar to Ethan Winer's diffuser). However, the project I am referencing emphasizes the build is a LOT easier with the EPS "Blueboard". My understanding is that if this was replaced with fiberglass, I could get a lot more low frequency absorption at the cost of a major construction headache. Is there another way to achieve the same effect. I suppose I could literally put a bass trap behind the diffuser but that would take up a lot of space. Maybe I'm being too greedy. The blueboard route looks very safe.

To summarize, my questions are:

*What are ideal dimensions for diffuser performance within practical constraints (6'x6'x2')? (I'd still like it to be modular where I can use Barker's Sequence)
*Is there an alternative to blueboard that would make for an easy build but also be good for low frequency absorption?

Thanks

[collo]

Using a QRD to do absorption is beyond what we've discussed up till now. The RPG abfusor does this using a porus floor and rough walls, but the science is their trade secret. Certainly a porus floor will change the effect of the diffusion.

With the knowledge we are likely to come up with, you might be better to do a normal QRD diffuser flanked by absorption panels.

Your new found depth of up to 2 ft allows you to make the design frequency quite low, however I would be wary of going to 25mm wide wells if you use the maximum depth.

AFIK Xenon's build with the blue foam was more for ease of construction than for absorption, but could be wrong there..

I'm happy to be a code wrangler, but leave the application to those who have some experience in these matters.

Perhaps Terry, Lupo, Xenon or some of the trade pro's might chime in....

[terry j]

I'm assuming you worked out how xenon did his build.

two feet deep!? wow. most would kill for that.

Personally I would not bother with barker codes and repeats, others seem to think it is best to have repeats, I just don't get it myself.

So I would just use the available size you have and work from there, know what wall thickness you will be using, you have your depth (tho for any realistic build 600 mm is almost certainly NOT required so straight away you have the depth saving you wanted), have a rough idea of how high you want to diffuse and the design pretty well sorts itself from those parameters.

keep us informed.

[Jakeman1086]

Ok, I've thought things through a bit more...

Theoretical questions:

I couple questions I need answered before I can decide on dimensions:

I just realized that low frequency diffusers need to be placed very far from the seating position (around 180 inches or 15 feet for the plan I'm thinking about!).

Is there any point building diffusers that can handle low frequencies if you cannot place the diffusers the necessary distance away from the listening position? Also, I plan to record guitars and vocals in this room (the same room I mix in). I was planning on mic'ing these up in the back of the room in close proximity to the diffusers, say like 5 feet away or so. How will the "Minimum distance to seating position" affect this?

Also, I see that around 5 periods is suggested when doing Barker's Sequence. The back wall section that I intend to cover is 8'x6'. However, as I said, I intend to keep these diffusers for a long time, and I won't be living here much longer (a few years tops). I eventually want to construct a home studio, and I have no idea what the dimensions of that back wall will be. Therefore, I need flexibility. The dimensions of wood I can get will be a factor in what size I settle on, but I'm thinking around 2.5'x6'x1' panels. However, it was said that larger=harder build, so I might split this into 2.5'x3' panels. It would be nice to have the 2.5'x6' panel. It looks like larger panels have a tendency to flex, sag, and cause various issues though?

Construction:

I'm also reading up on a lot of y'alls construction tips in this thread:
QRD Prime + Period Question (Live Room)

There are some really good tips getting thrown around. I just want to compile it all into one fluid idea so I know what I'm doing and what mistakes to avoid. I see many of you have gone through this build process and know these mistakes very well.

Staining/Varnishing
I think I will do staining and varnishing with a sprayer on each assembled unit - something like Tool Force A-C1 50 PSI 2-in-1 HVLP Spray Gun.

Glue
As far as glue, it looks like a combination of white glue and liquid nails. Liquid glue to attach the faces to the blueboard, white glue for everything else. But, does liquid nails react with blueboard??

Blueboard - AKA - STYROFOAM™ Brand Square Edge Insulation Extruded Polystyrene
Where can I buy this stuff? Is it at Lowe's or Home Depot? Can I buy it online cheap with shipping? There is a Dow Chemical Company about an hour away. Do I need to go there? Unfortunately, I don't know which depth I need yet (1cm, 2cm, 3cm, 4cm, 5cm...)

Wood
No specifics yet, but:

• Plywood backing - thin unless it will cause issues. In which case, thicker.
• Plywood faces - Lotta work cutting these and the blueboard. Any issues I don't know about?
• Wood outer frame
• Silicone sealer - do I need to use this to seal the diffuser air-tight?

Technique

I didn't follow Xenon when he was explaining how to build the ends of each module to connect with the next period on either end. It seems to me the best way would be to move panel half a well left to make the build symmetrical. Then, you could use the frame as the connecting '0 depth' well.

This picture N23 Array Build sums up my idea on how to build the unit. But what happens when you come across a full depth (or close to it) well. Do you fill the hollow well with a spacer or something to keep the well from collapsing?

I read it is best to install both the blueboard and the faces at the same time to avoid staining issues with glue.
________________________________________________________

I know this is a ton of questions. I could really use the help though. I've got the tools and a cabinet maker as a neighbor (lucky me). I think I can pull it off with help

[collo]

The distance to the seating position is 3 times the wavelength of the lowest frequency diffused. As you have found this can be quite large.
Ethan maintains that you can sit closer than this and still get worthwhile benefits. (I can't remember the exact post, but someone will)


When it comes to your solution, you have two main choices:
- What size panels
- How much work .. (how many fins)

The common size for commercial panels is 2 ft by 4 ft. This 1:2 ratio allows two panels placed side by side to form a square, which can be rotated 90 degrees if required for some modulation. I would probably stick with this ratio if it were me. Since 6 ft * 3ft is big, I would go for 3 ft * 1-1/2 ft. If you build 10 of these and stack 'em two high, you get 5 periods with coverage 6ft high by 7-1/2 ft wide. Make 8 normal panels and two invere panels.

How deep?
Fire up QRDude and reset to defaults. Switch to an inverse panel and adjust the depth of the deepest well until the period width is less than your desired panel width. Since the actual panel will be larger than the period width by 1 fin, build in a little slack - in your case, make the deepest well 8.5 inches. Now read off the design frequency - 797 hz. Round off to 800Hz

Whatever width you go with, this technique will give you the minimum design frequency that can be used for a given panel width.


If you want to use a true inverse panel in the mix, it will not have any zero-depth wells. This precludes using the half-width end wells for that nice build. A suggestion would be to use the fin material for the sides as well. Two panels side by side would then be out of sep by one fin thickness - not really an issue.


Now you have the design frequency, you need to look at how many wells.
Check the menu for "Advanced mode"
Check the menu for "Auto-optimise build depth in Advanced mode"
Check the menu for "Panel will be used in a sequence that includes its inverse"

Now step through the number of wells till your well width gets down one inch. You will see the following solutions
N7, N11, N13, N17
The N17 shows a period width greater than 1' 6"" so that needs to be left out.

As you go up in order, the bandwidth increases, but so does the amount of work.

The N11 has the same build depth for the standard and inverse panels , so that's nice

The N13 has different depths for the two panels. You are in the enviable position of having plenty of depth available, so turn off the auto-optimise. Rotate the phase wheel to the +2 position. This gives you a normal and inverse panel of the same depth - 7-13/16 inches.
This is your easiest to install, and best performing build in the 1:2 form factor.

So you see, there are compromises and decisions to be made along the way. I hope this little walk-thru shows how QRDude can be used to help make those choices.

[Jakeman1086]

Thanks a lot Collo. The advice really helped.

I followed your method for obtaining dimensions, but I altered my requirements a bit and came out with a good solution.

First off, let me say that I have 2'x4'x6" broadband absorbers. So, my plan is, since I want this in an 8'x6' area, I will just fill the bottom 2' with broadband absorbers and stand the diffusers on them (Only concern is how much these things weigh, but I'm definitely going with blueboard, so I doubt they weigh to much?)

The dimensions I came up with after this decision seemed to click perfectly. With a design frequency of 525 Hz, the specs are as follows:

Deepest Well: 12 inches
Number wells: 29
Frequencies Diffused: 262-6880 Hz
Well Width: .98 inches
Period width: 31.84 inches
Fin width: .11 inches - as long as I keep below this, I'll be fine with how close I come to my goal period width
Minimum distance to seating position: 154.96 inches (12.91 feet) - this actually sounds pretty close to perfect but I haven't measured yet - I bet I have it though

With this period width, I fit just barely under 8 feet. Perfect!

So, I've now got my dimensions down. Rap up my 3 finals tomorrow, another next Monday, then the fun starts.

If anyone, particularly Xenon, has any advice on my construction questions from my previous post, I'd love to hear from you. I'm particularly worried about cutting the thin fins as it will produce nasty edges as was talked about in another thread.

EDIT: I just realized that .98 inches is quite small compared to the height of 12 inches. Will this result in viscous losses? QRDude gave this value to me when I checked "Auto minimize well width".

[Jakeman1086]

Ok, I did a little research on the glue. It looks like this is the best:

Polystyrene to Wood - Elmer's ProBond Polyurethane Adhesive

This to That (Glue Advice)

Unfortunately, the best deal I found was 6 gallons for $95.64 + $50 shipping.

I can probably use much less of the probond as it is stronger and more suited for the poly-to-wood application. The problem with PVA is it is only meant for porous materials (not polystyrene), but many people working with polystyrene say it works.

What should I do? I have no idea how much glue each of my panels will require, but I bet its a lot. They are 4'x ~3.5' x 1' with 1" wells.

[collo]

Originally Posted by Jakeman1086

EDIT: I just realized that .98 inches is quite small compared to the height of 12 inches. Will this result in viscous losses? QRDude gave this value to me when I checked "Auto minimize well width".

This is still something of an issue for deeper panels.

Most commercial panels are N7's and thus have their well width set by the need for the period width to at least equal the design wavelength. I've not been able to find much in the way of solid info on well widths for higher order panels.

One inch is about the smallest practical size for building. Some sources say this is fine for viscous losses, but none state what depth this is good for.

There was an N43 built by RPG that had 1 inch wide wells, 16 inches deep, but no measurements were forthcoming, and I don't know if it was a commercial or research model. Another site mentioned a 15:1 ratio, but I suspect this is guesswork based on the same diffuser.

QRDude is written using 1 inch as a lower limit, but with the expectation that most panels will be around 6 - 8 inches deep. There is some debate as to whether you need to go much deeper than this in the typical studio environment.

I suspect that for such deeper panels, it would be prudent to make the wells a bit larger.

I currrently have an email in to Trevor Cox, one of the authors of "the bible" on the subject, asking for his opinion on exactly this issue. If need be, his recommendations will be incorporated into a new release of the calculator.

[Jakeman1086]

Please put his response on here whenever you get it. I don't want to build the diffusers and then find out they have major flaws. I'm hoping to start building Monday.

Here's the design I've been working on. It's not finished, but it incorporates my ideas for how to handle the modular end-wells. The picture shown is for panel 1. The middle panels will probably have ends similar to the short end of panel 1.

I'm worried about the deep wells in this design. I'm thinking I might put wooden spacers (wood texture pieces in the sketchup) in the deep wells to stabilize these parts.

[mashuga]

Originally Posted by Jakeman1086

Ok, I did a little research on the glue. It looks like this is the best:

Polystyrene to Wood - Elmer's ProBond Polyurethane Adhesive

This to That (Glue Advice)

Unfortunately, the best deal I found was 6 gallons for $95.64 + $50 shipping.

I can probably use much less of the probond as it is stronger and more suited for the poly-to-wood application. The problem with PVA is it is only meant for porous materials (not polystyrene), but many people working with polystyrene say it works.

What should I do? I have no idea how much glue each of my panels will require, but I bet its a lot. They are 4'x ~3.5' x 1' with 1" wells.

Polyurethane glue is WAY overkill for what you are doing. Plus, the foaming action can really be a pain to deal with.

You should just get a small bottle of Titebond woodworkers glue and test it on a pc of the foam. Titebond is just a stronger formulation of the standard aliphatic resin glue, like Elmer's, that has been around forever. It doesn't have any solvents and that's what would cause problems with your styrofoam..

Once you determine that the Tite bond isn't causing any harm, then you can pick up a gallon at a woodworker's store.

While you are there, you can also pick up a handy roller bottle that you fill with glue and allows you to easily spread the glue around.. See here:Glue roller

They might also have some advice for you about materials and methods for the wood. Do you have a table saw? I'm thinking thin luan plywood for the dividers and faces. Then some 1/2" A-C for the outer frame and some 1/4" for the back.. Thinner will be lighter...

[collo]

Originally Posted by Jakeman1086

Please put his response on here whenever you get it. I don't want to build the diffusers and then find out they have major flaws. I'm hoping to start building Monday.

Here's the design I've been working on. It's not finished, but it incorporates my ideas for how to handle the modular end-wells. The picture shown is for panel 1. The middle panels will probably have ends similar to the short end of panel 1.

I'm worried about the deep wells in this design. I'm thinking I might put wooden spacers (wood texture pieces in the sketchup) in the deep wells to stabilize these parts.

We might not have an answer that quickly. He's a professor at Salford uni in the UK - with the end of term coming up fast, he's probably as busy as a one-legged man in an a** kicking competition!

I don't think you'll be looking at any major flaws - at worst it should be a slight tradeoff between equal-energy lobes vs bandwidth.

Some commercial designs use a bridging strut to support deep fins, such as this one from realtraps...

[Jakeman1086]

That bridging strut looks like a good idea. More aesthetic too. I'll probably do that.

Mashuga, can you explain your preference more. I looked up Titebond glue and its pretty much the exact same price for a gallon, around $16. It seems to me, the stronger the glue holds the two pieces together, the better. 'Overkill' doesn't seem applicable. When you say foaming action, do you mean stirring it causes it to foam? Because for the added bonding strength, I would be willing to treat it 'delicately'.

[SaSi_SiDi]

I've never built a QRD diffuser but I've made a lot of research - what you cannot afford the time to do now - so please take my advise with a bit of salt.

Diffusers are not meant to be used for absorbtion. They are meant to diffuse sound while keeping the acoustic energy. A very dead room isn't a good thing and for music halls, acoustic energy must be preserved and be heard everywhere as best as possible.

The principle of placing diffusers at least 3 wavelengths away from the listening position has to do with avoiding effects as similar as listening to a 3 way loudspeaker at a distance of 5". You get a lot of lobing that way and listening close to a diffuser tuned to a low frequency will have the same effect - actually it might make sound worse.

As many playback systems suffer from reduced high frequency playback level and high frequencies are more easily absorbed, it's best to tune an absorber to the 1kHz and above and enjoy the benefit it might provide in a large room. The back wall, close to the ceiling, is the usually recommended place to locate these.

QRD diffusers might cause "parasitic" absorbtion if they are not well crafted and seams are not airtight. These seams can behave like slat tuned absorbers and tune unpredictably. With a lot of research, calculations and probably trial and error, one might end up with something predictable and certain scenarios might even call for that.

But for small rooms (less than 30~60sqm) absorbption is actually what is needed and diffusers - although cool looking - might be an uncalled for overkill.

[Jakeman1086]

Hey Sasi,

Always happy to get more advice. My room actually falls right at 30 sq. meters. Pretty soon I'm going to have eight 4'x2'x6" broadband absorbers with no hi-frequency reflection. I will also have six more 7.4'x2.75'x1' broadband absorbers w/ no hi frequency reflection. I also have carpet. So, I'm pretty sure I will be lacking in hi frequency energy.

This room will serve as both my monitoring and my tracking room. I plan to put the diffusers on the back wall away from the monitors. This should give enough distance with my current diffuser design to prevent lobing.

I'm primarily making these diffusers for tracking purposes. In my mind, they will make the acoustic guitars have more life. I plan to lay some plywood on the ground on the side of the room with the diffusers (maybe 5'-6' away) and mic my acoustic guitar and vocals there. Since 5'-6' is not the minimum distance away from the diffusers, this may cause problems. But from Ethan Winer's video, where he shows himself mic'ing acoustic guitar about 1' away from his diffusers, that sounded pretty freaking good. So, I'm thinking (correct me if I'm wrong), I will significantly improve the sound of my acoustic guitar recordings doing this.

Lastly, you mention that this diffusion only needs to take place at high frequencies (above 1 kHz) because this is the acoustic energy that is lost in the room. Is this true? It just seemed to me that more diffusion, the better. And maybe it would help randomize energy in the low-mids and break up room modes a bit. Am I wrong about this?

Should I make thinner (higher frequency) diffusers? Should I make diffusers at all?

Help.

[terry j]

Originally Posted by Jakeman1086

That bridging strut looks like a good idea. More aesthetic too. I'll probably do that.

Mashuga, can you explain your preference more. I looked up Titebond glue and its pretty much the exact same price for a gallon, around $16. It seems to me, the stronger the glue holds the two pieces together, the better. 'Overkill' doesn't seem applicable. When you say foaming action, do you mean stirring it causes it to foam? Because for the added bonding strength, I would be willing to treat it 'delicately'.

that polyurathane glue works by absorbing moisture. when it does, it also foams up automatically. It dries hard, and can only be removed by mechanical action.

If it foams in the 'wrong place' then you could be up shitcreek. think of that insulation stuff you see, they spray a little into the cavity and it expands 'a thousand times' if you follow. 'same' with that glue.

one inch width on twelve inch depth would not 'feel right' to me. NO data to back up that gut feel tho!!! so I could be completely wrong. Do you need twelve inches is maybe the question.there seems to be general consensus that diffusion itself is not required that low, but it does carry with it the added advantage of a low design frequency (diffusors work best at DF * integer, think it is part of collos scoresheet).

Engineering, always pick your compromises! BUT, we do not really have data that tells us WHERE in the FR it is best to have diffusion, that would help a lot in being able to pick our compromises!!

(hope you asked trevor that question bill)

[Jakeman1086]

Seeing as how the diffuser's well depth has no impact on the other dimensions of the unit and only changes the low frequency cutoff, I can change it to anything. What would you suggest? Its application will primarily be for recording acoustic guitar and vocals. Speaking of which, I found this very promising quote from GIK Acoustics:

"The QRD really holds its own sonic imprint: warm, non-directional yet sonically cohesive. A great acoustic guitar played into a single quality QRD just feet away will sound like an even greater version of same guitar."

This coincides with what I heard from Ethan Winer's diffusion video and sets my mind at ease a bit with some issues raised in this thread. I suppose for most of my songs, I am primarily concerned with the acoustic guitar sound 400 hz and up. So, maybe if I dropped the well depth down to 8 inches?

You're statement about the polyurethane glue makes sense. Would you suggest Elmer's White? My only concern with this is that it is not 'supposed' to be used with non-porous material.

Thanks.

[terry j]

gee, sorry.would feel wary about making recommendations to anyone.

There is a bit of wiggle room in some of this. eg, flow in qrdude is given as half/design frequency. Going on that, it suggests a DF of 800 hz which equals 195mm.

However, Flow from that is also a bit of a guesstimate, 'just better diffusion than a bare wall' rather than TRUE diffusion, if you follow.

I guess eight inches is a reasonable compromise?? One thing I think that is agreed upon, the deeper the better if you can (within reason) as that gives the lowest possible design frequency (hence closer/more effective diffusion points in the range).

white glue is cheap and common, do a series of tests to see how good it is. I don't know personally.

[Jakeman1086]

It seems diffuser designs are marred with compromises. Let me ask this.

If you (anybody willing to answer) were building diffusers:

*for an 8'x6'x12" back wall area
*designed to record acoustic guitar and vocals (and make the mix position sound better)
*and were focused more on performance and less on design difficulty (but not insanely so)

what specs would you pick?

Btw, you (I) also have the option of reducing the 6' height to 4' and putting 2' broadband absorbers underneath.