Building DIY skyline diffusers but can't really figure out the heights

[kjell159]

Hello,

I would like to spend some time in making a diffuser out of simple wood for the rear wall of my studio.

But I don't know how long they need to be to make a good broad range (I'm talking midrange-high.).
I have this acoustic calculator but when I want to make a diffuser that scatters frequencies as low as 100 Hz it goes up to over 1,5 meters.
I do not have that much space in my studio and was looking at about a maximum of 35 - 40 cm height of the highest blocks.

Of course I do not need diffusion that low but if I somehow could get as low as 400 - 450 Hz I guess that would be an advantage as my studio room is rather small and more honky sounding.
I already use polyurethane absorption foam panels and I hope some diffusion will make it more complete. + I believe it will be a rewarding project and maybe can look pretty cool/professional.

So my 2 questions are:
- What kind of wood will give the most clear and consistent results?
- How low should I aim for?

My guess would be something like: the highest blocks about 35 cm to scatter at least to about 500 Hz and blocks of approximately 2 cm wide by 2 cm deep.

 

[robn]

0 3 4 1 2 3 3 1 4 2 3 3
3 0 1 4 2 1 1 3 3 2 1 1
3 1 1 3 1 3 2 2 1 0 2 2
2 2 2 2 0 4 3 2 3 2 1 1
3 3 1 1 3 1 1 3 4 3 1 3
2 3 2 1 2 0 3 2 4 2 1 0
2 3 2 1 3 1 2 2 3 1 3 4
2 0 2 4 4 0 1 2 1 4 2 2
3 4 1 0 1 3 3 1 0 2 3 3
1 3 3 1 2 4 1 2 0 1 3 1
2 1 2 3 1 3 3 2 4 2 3 4
2 4 2 3 3 1 1 2 0 3 1 0

Those numbers are in inches.

These plans are for 1x1" boards, so with 2x2" boards just double all the dimensions.

 

[mjbphotos]

Hello,

I already use polyurethane absorption foam panels and I hope some diffusion will make it more complete. + I believe it will be a rewarding project and maybe can look pretty cool/professional.

"Acoustic foam" doesn't do much for you. Do you have any broadband absorbers (bass traps)? I have also read that diffusors don't really work for small rooms.

 

[Ripthorn]

In case anyone else is looking for more general info now or in the future, those skyline diffusers and others like them are all based on the quadratic residue formula originally developed by Manfred Schroeder and commercialized by Peter D'Antonio. It is a basic equation that is based around the quadratic residue of a prime number. The higher the prime number, the more surface variation and greater overall diffusion bandwidth you get. The results are all relative values to each other, and are then scaled up or down based on lowest frequency of diffusion. I don't have the quadratic residue equation here in my lab, but I have it at my desk if anyone cares and Google doesn't turn it up readily.

Diffusion in small rooms is tricky. Diffusers are used to try to introduce the diffuse field condition, but small rooms have lots of early direct arrivals which make it very difficult to achieve a true diffuse field, but any diffusion is better than none.

 

[kjell159]

Do you have any broadband absorbers (bass traps)?

I will be making some bass traps in the near future but never got to it and haven't had the chance yet to find some light covering to seal the insulation.
I know I have a pretty serious room mode around 85 Hz and that helps as some kicks have much punch in that area.
I will probably be able to help soothen out that range a little with some big trapping.
So when mixing I know that that frequency deserves extra attention, especially that low but the harmonics are important to keep in mind aswell.
By that I mean around 170 Hz keeps drilled in my mind aswell.
I am an amateur and by that I do not have thousands of dollars, not even hundreds, so the studio monitors I have been using produce like no bass.
I use a PA speaker as a kind of reference, I use it as a subwoofer, I found a pretty nice spot for it, I can even crossover by just using a digital E.Q.
As I have two stereo outputs (thus 4 channels) I can send one stereo 'mix' to my little monitors and the other (with only bass content) to the big speakers that I can turn off when not needed.

The higher the prime number, the more surface variation and greater overall diffusion bandwidth you get.

Ok, thanks.

To robn: I already have that plan.

I guess I will first find out the wood I just like the most and then see how much it will weight at different heights and so find the right height as that sounds the most logical to me.
Anyway, thank you all.

 

[BroKen_H]

There is a diffuser calculator listed on THIS VIDEO (well, in the description).

 

[kjell159]

That is the program I was using.
On wine (program to run Windows software in a POSIX environment) though, I run Linux.
(Yes Linux!, I only can come up with 3 'professional' studios that use a Linux distro in their studio but I'm into open-source software and I'm not a professional so I have permition to choose.
The Focusrite USB interface I'm using works 'out of the box' on almost all Linux kernels I guess, I'm using it on Fedora 20 => Fedora is a Linux OS named after the hat: fedora.
That is because it works with USB 2.0 and so is class-compliant.)

And that was the first video that inspired me to go try to build it myself.

 

[kjell159]

I went for plain pinewood as it is the cheapest and usually well available.
The board I used was a decently thick pinewood board aswell, I read that some others who did a similar DIY project used a MDF which I guess is ok but I prefered the exact same material for the blocks as for the the board/plate because it has the same density (kg/m³) and so the same acoustic properties?
I chose to cut wood pieces of 5 cm (2 inch), 10 cm (4), 15 cm (6) and 20 cm (8).
The wood pieces were 2,7 cm in width so my diffuser should work as high as +6 kHz if the calculator is right.
I ended up with a plate of about 33 cm by 33. (I guess the wood pieces were +/- 2,75 cm instead ? )
picture 1
picture 2

There were 2 layers of rosewood stain (The difference between plain pine and rosewood finish is amazing!) and a layer of varnish.
I just glued the pieces on the board with wood glue.
I had to restart because half of my pieces fell off after doing the first 40-50 pieces.
I also had to redo some later as I had my board/paper wrong. (I should do even more checking the next time.)
I like the way it ended but I still have some questions:

- Does it really matter if there are a couple pieces which aren't cut perfectly like in the picture? (That one is the biggest mistake, the others are usually less than 3 mm off.)
picture 3
picture 4

- The circular saw I was using was actually already kind of dead but my we haven't had the chance yet to get a new one.-

- Is it a bad thing that some of the 0's (Where the sound should hit the plate rather than a block, so kind of 0,0 cm high.) are not completely covered as I made a mistake while cutting the board from a bigger plate and now the 33 x 33 is not completely straight?
picture 5
picture 6

- What is the best way to hang this board on a wall as it is very heavy?


In the end it will still diffuse a lot better than a random bookcase right although it is not even 99% perfect? (At least I hope so as I put hours into making just this one diffuser and I really like the almost tropical darker look of it, much more esthetically pleasing than dark foam.)
I hope to make 4 more of these, one probably won't make a big difference.



+ I made two bass traps, one about 30 cm as I can remember and another about 20 cm (8 inch).
I'm not sure they are really working though, I made them with some insulation I still had laying around and some almost thrown away/used as rags - cotton bed coating for covering it. (and same pinewood)
I heard a bass trap needs to be at least 1/4 of the wavelength of the frequency you're trying to absorb, that would mean my bass traps should be over 0,6 m!
At least it will press down the modes a little I guess, of course it won't absorb say a frequency of 70 Hz for 100% that is just logical to me.
Otherwise you wouldn't hear the bass coming out of a club and almost all neighbours would be best friends.
My neighbours don't complain but that is because you can only hear the bass coming out of my studio and when you walk for about 10 meters away from the door you almost can't hear anything anymore.
My studio is also located in my garden and that gives me quite some room and isolation from the houses in my street.

 

[DM60]

That is the program I was using.
On wine (program to run Windows software in a POSIX environment) though, I run Linux.
(Yes Linux!, I only can come up with 3 'professional' studios that use a Linux distro in their studio but I'm into open-source software and I'm not a professional so I have permition to choose.
The Focusrite USB interface I'm using works 'out of the box' on almost all Linux kernels I guess, I'm using it on Fedora 20 => Fedora is a Linux OS named after the hat: fedora.
That is because it works with USB 2.0 and so is class-compliant.)

And that was the first video that inspired me to go try to build it myself.

I tried using Ubuntu and the Presounus USB (two channel) and it worked out of the box as well, but for my portastudio I have a Tascam 1800 and there was no luck with that interface. Decided to purchase a Win system and use it for my purpose.

But a true USB 2.0 compliant interface looks like will work with Linux OOTB. (Just confirming, not hijacking)

 

[BroKen_H]

They should have Z clips where you are. Google "Z clips" and see what comes up on your local sales. They're designed (by using 2' on walls to hit studs and 4" on your wood construct) to hold large weights on walls.
If you have a 1-2% error overall in your design (probably what you're dealing with), you will get a 1-2% error factor in diffusion. Mathematically, it should be fine. Acoustically, I don't know.
Make sure you don't get coatings (especially the varnish) where you're gluing as that will cause cohesion problems.
Your bass traps are 8 and 10" thick? Did you use regular insulation or high density rock wool (stiff stuff)? Did you go floor to ceiling?

 

[kjell159]

But a true USB 2.0 compliant interface looks like will work with Linux OOTB. (Just confirming, not hijacking)

Did you use regular insulation or high density rock wool? Did you go floor to ceiling?

I used regular insulation for one and for the other I used all of the acoustic insulation we still had one package laying around off (looks like a mix of all colors, some pieces were soft and others were hard(er)) and I put them only in a corner resting on the floor in an angle of approximately 45°.