DIY Diffusors to the Max

[G. E.]

Originally Posted by asagaai

... My studio is very similar in some ways to what you are doing in respect of diffusors on ceilings- be very careful how you mount to ceiling-are very very heavy and if they fall they would crush skulls and kill people. ...

My wall-mount diffusors are made of cheap spruce, very heavy, especially the double-length ones (50cm [~20"] well depth, 45kg [~99lbs] each). The movable pairs are made of balsa and so will be the new ones (weighing 13kg [~29lbs] each with acrylic dividers and mounting plate). You're absolutly right to make sure they won't come down, ouch...

[G. E.]

BTW: I would probably neither build the double-length diffusors I've mentioned above again nor would I recommend building them. Reasons:

1. very inconvenient to move around because of the weight
2. the old BBC-design may easily be improved using more different well depths (remember: PRD theory suggests a different depth for each well)
3. their lower design frequency would need much more surface to provoke diffusion anyway

Right now they're mounted on the side wall near the speakers because I noticed that they "break up" some lower resonant frequencies and they definitely broaden the stereo-image in a very natural way (in my comparatively small room).

[G. E.]

If you watch closely you might have noticed that the double length diffusors "sit" near the first reflection points ... usually NOT recommended by the pros. But given the geometrics of my room and the sound field of my speakers this reflection simply didn't happen when I did some measurements (calculated impulse response) -- I guess the angle in this case is too big for higher frequencies. Soundwise I thought it was great (see above) so I kept it ...

On the other hand the first reflection from the opposite side wall was very strong (much smaller angle). Placing another set of diffusors there didn't work out, changed it to absorption first (quite ok) and in the end to angled reflection as seen in various renderings above.

Side-wall reflection point:

[G. E.]

Actually I had some reflections in the measurments that I tracked down just to find out that they were twofold reflections running via speaker-ceiling-wall-ear and speaker-wall-ceiling-ear. Because of the angles involved they were much more prominent than one would have expected and needed treatment (I use an absorber-cloud to break those sound-waves). Exploring those reflections you have to care of a total of eight! (2 speakers x 2 sidewalls x 2 sequences) possible ways a twofold reflection may run via ceiling/sidewalls! I mention this because they are not that obvious yet can get very strong depending on the room geometry.

Showing of 4 possible ways (one speaker), two of them actually exist in my room:

[G. E.]

Thought I should show another view of the two-fold reflections "red" and "green" to describe their path (red and green dot show the reflection points). Additionally you see "yellow" and "blue" running between speaker-sidewall-sidewall-ear in a horizontal plane. They are not strong enough that I could identify them while I did measurements -- for sure they contribute to the bass response but you won't see them watching the impulse response probably because the high-frequences get lost at this angle. Later I changed the reflection angle anyway with additional elements so they won't matter anymore. BTW: I use ARTA Audio Measurement & Analysis Software for my measurements - not the easiest interface but very powerful!

[G. E.]

Got hold of a copy of Trevor J. Cox's and Peter D'Antonio's "Acoustic Absorbers and Diffusors: Theory, Design and Application" today to check some ideas, takes me on a (hopefully good) read for some days though...

[BlueSprocket]

Originally Posted by G. E.

Got hold of a copy of Trevor J. Cox's and Peter D'Antonio's "Acoustic Absorbers and Diffusors: Theory, Design and Application" today to check some ideas, takes me on a (hopefully good) read for some days though...

That book is an incredible tool to have around. I can't tell you how often I end up referencing it. Actually, its open right now, as I'm designing some stuff for a church I'm consulting for. Your stuff looks quite cool.

[G. E.]

Found this great link to a Porous Absorber Calculator in another thread and had to throw in the numbers of my existing bass-traps filled with Isover Thermohanf to see how the calculator-results compare with my (may I say so: very good) experience with this kind of bass-trap. Seems plausible for my part... great to have 50% absorption down at 40Hz and such a smooth absorption-rolloff -- I guess its a good counterpart to the slow frequency-response rolloff of the non-vented design of the main-speakers I use (they're down -3dB @ 45Hz).

[Balz]

G. E. those portable diffuser/absorbers that you made are absolutely beautiful. If you don't mind sharing, how did you make them look so damn good, I mean I guess that's fabric stretched over a wood frame with rigid fiberglass or the like on the back? It's hard to tell because it looks flawless...

[G. E.]

Originally Posted by BlueSprocket

... I can't tell you how often I end up referencing it. Actually, its open right now, as I'm designing some stuff for a church I'm consulting for. ...

Have you/someone else ever noticed a hint in the book about the influence of real dividers between the wells? I ran through the whole book yesterday but couldn't find a quick answer...

[G. E.]

Originally Posted by Balz

... how did you make them look so damn good, I mean I guess that's fabric stretched over a wood frame with rigid fiberglass or the like on the back? ...

It's a simple wooden frame covered with (cheap) fabric -- I "developed" a certain sequence that gives solid results, I take care to stretch with every staple. In the end there is a staple every 2-3cm [~1"]. For me the only difficult thing is the last step where you fold the fabcric inwards on every corner and do the final fixing, here's a quick step-by-step sketch:

[BlueSprocket]

Originally Posted by G. E.

Hi, good to see you here! Have you ever noticed a hint in the book about the influence of real dividers between the wells? I ran through the whole book yesterday but couldn't find a quick answer...

The dividers will help with specular reflections in one dimension. So if you need the soundfield to be more diffuse along one axis over the other, then the dividers would be ideal. If you put dividers in both directions then you'll end up with better diffusion all around.

I'm actually doing some ceiling QRDs for my studio live room. I chose QRDs just because I like visual symmetry. I chose to do a more skyline style design because I wanted even response in all directions. I could see the desire to want to "direct the duffusion" in a control room, that just seems like more work. Most rooms I've worked in have used non-divided QRDs in the ceilings if they used diffusion at all.

I'll rip through the book and see if either D'Antonio or Cox make a note. But I don't recall one off hand.

[G. E.]

After sitting in front of a big desk for years with all the acoustic disadvantages I thought an "invisible" desk would be great. Found a rusty old table and reworked it to hold keybord and monitor-controller:

[G. E.]

Another DIY detail in my room are the monitor stands made of:

• 4 layers of 12mm [~0.47"] plywood glued together,
• 1 layer of 10mm [~0.39"] rubber blanket, and
• 11 layers of 50mm [~1.97"] washed-out concrete panels
• each with 2mm [~0.08"] synthetic felt on top of it.

The weight is about 150kg [~330lbs] each but I can move them around because I've mounted 12 small rollers on the bottom of every stand. So its a high mass stand with internal damping provided by the felt layers. Its cheap and I feel it worked out fine thinking of the tight bass and great sense of width and depth I get from the speakers:

[G. E.]

Originally Posted by G. E.

After sitting in front of a big desk for years ...

See some images of the "big desk" on the "TOP 10 Sexiest Consoles"-Thread, it had to go though...

[G. E.]

Originally Posted by G. E.

... its a high mass stand with internal damping ...

Talking 'bout mechanics: I've never calculated the actual forced oscillation of the stand but I've noticed that knocking pretty hard against the stand/speaker-unit it "swings" 2-3 times within a second before it stops. Amplitude on top of the speaker may be +-1mm [~0.04"]. So the Eigenmode (in the bass-cabinet axis) of the mechanical system seems to be around a few Hz and is therefore subtancially lower and pretty much decoupled from the audible frequency-range of the speaker.

[G. E.]

Originally Posted by G. E.

... thought an "invisible" desk would be great...

Apart from the positive effect of having an acoustically "invisible" desk (no reflections from the desk) and lowered monitors it means that have to take extra care of the floor reflections. Let's have a look at the basic situation...

[johndykstra]

Checking in to say I love everything I see here, bravo GE.

With the exception of your posture at mix position. You're going to have some back and neck problems my friend

Seriously though, I really dig your documentation style, very informative.

[G. E.]

Originally Posted by dykstraster@gmai

Checking in to say I love everything I see here, bravo GE.

With the exception of your posture at mix position. You're going to have some back and neck problems my friend

Seriously though, I really dig your documentation style, very informative.

... thanks a lot! Well, I lowered the monitors (2 x 20" in portrait mode) half a year ago and have been working that way since then, got used to it quickly. And you know, it helps the acoustics...

Maybe the 2D-guy in the sketchup-renderings has been misleading, exchanged it for a 3D-model (thanks Google 3D Warehouse):

[G. E.]

So I've finally ordered the balsa and the acrylic glass for four diffusors as proposed in the beginning. Still not sure whether it's the perfect design for a ceiling diffusor (because of the "missing" dividers in one direction) but confident enough that it's a good working diffusor at all that may get used in another application (thinking of another pair of mobile diffusors for the recording space). The acrylic glass will be clear (92% transparent) without any color, I tried to match the 3D-model as close as possible:

[johndykstra]

Originally Posted by G. E.

Oh, thanks a lot! Well, I lowered the monitors (2 x 20" in portrait mode) half a year ago and have been working that way since then, got used to it quickly. And you know, it helps the acoustics...

Maybe the 2D-guy in the sketchup-renderings has been misleading, exchanged it for a 3D-model (thanks Google 3D Warehouse):

Is your floor carpeted?

Reason I ask is that there is still a first reflection point, it's just not a desk anymore, it's the floor.

[johndykstra]

Never mind, I just re-read.

[Nordenstam]

Hi Gernot!

Am still curious about the unidirectional well dividers too. Bet it'll work great anyway. As the PRD's will be ceiling mounted, having the dividers across the room length will probably be a good idea? So the sideways scattering is less than the front/back scattering. That way less early reflections will reach the sweet spot. If the diffusors does turn out to be more effective in one direction, that is.

It's an impressive effort you've laid down in this! Big cudos for all the work. Got me inspired to finally get around to build some. So I've been re-reading the Cox and D'Antonio book and the PRD patent. As for the math, the formulas are pretty straight forward - the one thing that confuses me is how to find the prime root for a given prime number. Is there any way to get a results directly, or any tables to look it up in? Or do I have to try every possible number from 0 to N-1 for any prime number I choose to use, finding out if there is a suitable prime root the hard and long way? Or is five a magic number that works with them all? (seems that way when using the online calculator, but I'm a bit sceptical to such a quick solution!)

Am also wondering about oblong shaping. Most of my damping is ~2x4 feet and I'd like to construct some drop in replacement diffusors. A 23x12 array with 5cm "wells" gives 115x60cm, close enough for me. This will avoid the repetition of doing two 13x12 instead. There's also two spots that could do with some 16x7 PRD's. Is there anything wrong in doing oblong shape?

Hope it's alright to ask in this thread. It's not my intention to threadjack! But this seems like the right place to ask.


Cheers,

Andreas Nordenstam

[G. E.]

Originally Posted by dykstraster@gmai

Is your floor carpeted?

Reason I ask is that there is still a first reflection point, it's just not a desk anymore, it's the floor.

The room has PVC-flooring on 30cm [~1'] solid concrete and you're right, there would be measurable reflection though not as strong as a desk reflection because the reflection angle is larger (regarding the radiation pattern of the midrange driver and the tweeter that means there's not so much sound radiated in this direction). Still experimenting which treatment makes up for the best sound, didn't like total absorption (bass trap on the floor), right now I use partial absorption with loose gravel in the reflection points (see "The Sound-Absorbing Properties of Some Common Out-Door Materials" by Kaye et al 1940). I propably won't overdo treatment because -- sorry, can't remember who came up with this "wisdom" -- I understand that as human beings we are used to hear reflections coming from the floor=ground (as opposed to the ceiling=sky)...

[SoundWeavers]

fantastic looking diffusors. I'm sure those will surely improve the sound of any room you put them it. congratulations!

[G. E.]

Originally Posted by Lupo

... As the PRD's will be ceiling mounted, having the dividers across the room length will probably be a good idea? So the sideways scattering is less than the front/back scattering. That way less early reflections will reach the sweet spot. If the diffusors does turn out to be more effective in one direction, that is...

I will try both ways but I guess it'll not be easy to "hear" a diffusors contribution to the sound of the room instantly. One thing I've found though is that I usually didn't like diffusors in strong first reflection points -- often caused to much of an "stereo-effect" to be useful.

Originally Posted by Lupo

... As for the math, the formulas are pretty straight forward - the one thing that confuses me is how to find the prime root for a given prime number. Is there any way to get a results directly, or any tables to look it up in? Or do I have to try every possible number from 0 to N-1 for any prime number I choose to use, finding out if there is a suitable prime root the hard and long way? Or is five a magic number that works with them all? (seems that way when using the online calculator, but I'm a bit sceptical to such a quick solution!)...

I've been using the computed result for the least primitive root for a given prime number for various calculations without finding any downside -- the math behind all this doesn't allow a fixed formula for primitive roots, it's more about "searching" (for the very reason this math is strongly related to encryption theory as well...); you might enjoy further reading "Least primitive root of prime numbers". So "5" isn't a magic number as it's for sure not always the smallest primitive root for a given prime number (check 1559 for example, usable for a 41x38 grid).

Originally Posted by Lupo

... Am also wondering about oblong shaping. Most of my damping is ~2x4 feet and I'd like to construct some drop in replacement diffusors. A 23x12 array with 5cm "wells" gives 115x60cm, close enough for me. This will avoid the repetition of doing two 13x12 instead. There's also two spots that could do with some 16x7 PRD's. Is there anything wrong in doing oblong shape?...

In theory there's nothing wrong with 23x12 but I'd rather use two 13x12 instead as they give more flexibility in positioning.

BTW I appreciate your interest!

[G. E.]

Happy to show some real images of the diffusor material (balsa, acrylic glas, MDF):

[G. E.]

Originally Posted by G. E.

Another DIY detail in my room are the monitor stands made of:

• 4 layers of 12mm [~0.47"] plywood glued together,
• 1 layer of 10mm [~0.39"] rubber blanket, and
• 11 layers of 50mm [~1.97"] washed-out concrete panels
• each with 2mm [~0.08"] synthetic felt on top of it.

...

During a listening session a week ago an (audio professional) friend of mine suggested to remove the rubber blanket at the bottom, so we moved 300kg of solid concrete to the side and back again afterwards. I was not sure about the result because it somehow changes the stand-system from decoupled to more coupling with the ground but we found out it helped sub-bass. Next thing would be to replace the rollers with spikes...

[G. E.]

Originally Posted by G. E.

... right now I use partial absorption with loose gravel in the reflection points (see "The Sound-Absorbing Properties of Some Common Out-Door Materials" by Kaye et al 1940). I propably won't overdo treatment because -- sorry, can't remember who came up with this "wisdom" -- I understand that as human beings we are used to hear reflections coming from the floor=ground (as opposed to the ceiling=sky)...

Right now a pair of flowerpots (diameter 43cm [1.4']) filled with loose gravel "sit" in the reflection points on the floor and help to -- partially -- reduce reflections:

[Nordenstam]

Hi!

Thanks for the answers, especially the link to the least primitive root calculator! Exactly what's needed.

Originally Posted by G. E.

In theory there's nothing wrong with 23x12 but I'd rather use two 13x12 instead as they give more flexibility in positioning.

Sure! But if you know for certain that a given area will be filled, isn't it better to avoid repetition? Or am I missing something?

Another question: would it be much point in building symmetrical arrays? Mirroring one of them, so the L/R sides of the room is identical seen from the sweet spot.


As for reflection points on the floor - that's an excellent place for the otherwise useless acoustic foam! Have two patches with 6" acoustic foam there now. The foam started out as a mixing board silencer for listening session. The big desk is gone now, have about the same rig as you with a modded table frame holding a computer inteface and the 2 outboard pieces I use for mastering. Unnatural or not, the foam on the floor seems to help a bit! I subjectively prefer it. Such foam is so cheap that it's at least worth the try to see if it suits each individuals preference.

The gravel pot idea is interesting. Perhaps some plants in the pots could do the same job with added aesthetic appeal?


Thanks again for the help! Am hoarding tools and materials now. Looking forward to get building.


Cheers,

Andreas