Omnidirectional QRD

[jeep]

I've decided to design and build an Omni directional Quadratic Residue Diffusor to place on the back wall of my studio.
I have used the quadratic residue formula and made use of this website - HiFi Speaker Design © mhSoft 2007 to assist me with the overall measurements of the diffusor
If anyone has some knowledge or experience with omni diffusors or just acoustics in general, could you please check out my design and pick out any faults and / or give me some advice as it would be greatly appreciated

here is my design:



cheers!

[Sui_City]

There is a link to an Excel spreadsheet on the page link here:

Acoustic Calculation Tools

Download the "Control Room Calculator" spreasheet and take a look at the Schroeder Diffusor tab. Should help a bit.

Busy working on this myself. Have fun.

[bit mangler]

You've said Omnidirectional QRD but your picture shows a 3d diffuser I think.Am I missing something here??

[Jason Jones]

Originally Posted by jeep

I've decided to design and build an Omni directional Quadratic Residue Diffusor to place on the back wall of my studio.
I have used the quadratic residue formula and made use of this website - HiFi Speaker Design © mhSoft 2007 to assist me with the overall measurements of the diffusor
If anyone has some knowledge or experience with omni diffusors or just acoustics in general, could you please check out my design and pick out any faults and / or give me some advice as it would be greatly appreciated

here is my design:



cheers!

Jeep,

Glenn told me to head over hear and give my input. Looks like a fun project you've got going.

But.........your design won't work very well as it is. To make a 2-D diffuser that works evenly, you need to use a more sophisticated method to "translate" your sequence from a 1-d to a 2-D. Chinese remainder theorem is one option, for example.

I would be happy to help you work through this. But, before we dig through the math give me a better idea of what you want the end product to be. Here are some questions:

1) Do you want a device that has very little absorption, or is moderate absorption OK?
2) Are you planning on making a 2-D well diffuser like the Auralex space array? or something without wells like the RPG skyline?
3) Are you making many of these or one?
4) What are you planning to make them from?

Once we get these things covered..and anything else you want to add, we can cook you up a top quality sequence in a flash!

Jason

[jeep]

thanks so much for your input jason


to answer your questions:

1) Do you want a device that has very little absorption, or is moderate absorption OK?

less absorption would be preferable, but if that makes things much more complicated, then moderate is fine

2) Are you planning on making a 2-D well diffuser like the Auralex space array? or something without wells like the RPG skyline?

originally I had a 2D diffusor like the RPG Omniffusor in mind, but the Space Array looks like a great design too.

3) Are you making many of these or one?

just one for my back wall

4) What are you planning to make them from?

idealistically I would like to make it from balsa wood because I will need to remove and transport the diffusor in the near future when I move.
one problem with balsa wood is that it is rather expensive and not that easy to obtain where I live but I still would like to use it


Also, I want to get the best amount of low frequency diffusion out of this diffusor keeping in mind I can't make my wells much deeper than 20cm or 8" due to my small room size. I realise that the low-frequency limit is determined principally by well depth but I see commercial products claiming diffusion down to as low as 125Hz with well depth of only 4"!
how is this so?

a few other things desired are broadband diffusion and an overall dimension of approx. 1x1m or 40x40"

thanks again!
JP

[Jason Jones]

OK. Here is your trade-off:

An omni style diffusor will have better low frequency performance than an equivalent skyline style diffusor when one panel is placed as you describe. But, they have LOTS more absorption. An 8" deep omni type may have an absorption coefficient of .8 or .9 at 250 hz and around .5-.6 across the upper band. For a skyline, you can cut those numbers in half.

Below 500 hz or so, all of the scattering is coming from the exposed edges. The omni has better edges for that. This is also the answer to your 4" diffuser with specs claiming effectiveness down to 125 or so. With the edges exposed a flat 60 cm x 60 cm board 10 - 15 cm deep will scatter the low end reasonably well. That is why we recommend spacing between our D1 diffusers: it takes full advantage of this.

The other thing to keep in mind is construction. If the wells are not airtight, you will get lots more peaky absorption. So, which design you feel more comfortable with the construction should also factor in.

Assuming you definitely want a 2-D diffuser, I would recommend the skyline type. But, think it over. Also, I don't know how much distance you have behind you, but a 2-D array of 1-D diffusers works very well if you have some distance to let them work.

Jason

[David Rick]

Originally Posted by Jason Jones

Jeep,

Glenn told me to head over hear and give my input. Looks like a fun project you've got going.

But.........your design won't work very well as it is. To make a 2-D diffuser that works evenly, you need to use a more sophisticated method to "translate" your sequence from a 1-d to a 2-D. Chinese remainder theorem is one option, for example.

I would be happy to help you work through this. But, before we dig through the math give me a better idea of what you want the end product to be.

<snip!>

Once we get these things covered..and anything else you want to add, we can cook you up a top quality sequence in a flash!

Hi Jason,

I once used the Chinese Remainder Theorem to build a stereolithography prototype of a small 255-cell, 2-D well diffuser. I also built its complement so the combination could be binary-modulated into larger arrays. But we never spent the money to have molds made from the prototype parts.

I have a rudimentary array selection spreadsheet (Excel) and a case-specific design script in Matlab. One needs to know a smattering of number theory and have read the relevant AES preprint to benefit from these, so I'm not posting them here for general consumption. P.M. me if interested.

-- David

[jeep]

Originally Posted by Jason Jones

OK. Here is your trade-off:

An omni style diffusor will have better low frequency performance than an equivalent skyline style diffusor when one panel is placed as you describe. But, they have LOTS more absorption. An 8" deep omni type may have an absorption coefficient of .8 or .9 at 250 hz and around .5-.6 across the upper band. For a skyline, you can cut those numbers in half.

Below 500 hz or so, all of the scattering is coming from the exposed edges. The omni has better edges for that. This is also the answer to your 4" diffuser with specs claiming effectiveness down to 125 or so. With the edges exposed a flat 60 cm x 60 cm board 10 - 15 cm deep will scatter the low end reasonably well. That is why we recommend spacing between our D1 diffusers: it takes full advantage of this.

The other thing to keep in mind is construction. If the wells are not airtight, you will get lots more peaky absorption. So, which design you feel more comfortable with the construction should also factor in.

Assuming you definitely want a 2-D diffuser, I would recommend the skyline type. But, think it over. Also, I don't know how much distance you have behind you, but a 2-D array of 1-D diffusers works very well if you have some distance to let them work.

Jason

Well at the moment I only have about 1.2m behind me, which isn't a lot at all. I have read that Skyline type diffusors are the best type for small rooms. can you confirm this?

I currently have 4 Wave Panels corner traps installed and I plan on building 2 polycylindrical diffusors filled with rockwool for low frequency absorption. so i guess additional absorption ontop of this is not something i really need or want.
If I make the well depth of the omni 10-15cm like you suggested, would this lower the amount of absorption?

The construction of an omni Vs. a skyline would be a little bit harder IMO as i would have to be far more accurate in making sure everything is airtight. But generally the amount of effort required to construct the diffusor doesn't bother me. I just want to get it right

So I guess like you said, the Skyline probably would be a good idea. But then again, one of my main aims is to create the best low freq. diffusion I can in order to reduce the problem frequencies in my room


hmm.. I have a lot to think about!

thanks everyone

[Jason Jones]

Originally Posted by jeep

Well at the moment I only have about 1.2m behind me, which isn't a lot at all. I have read that Skyline type diffusors are the best type for small rooms. can you confirm this?

I currently have 4 Wave Panels corner traps installed and I plan on building 2 polycylindrical diffusors filled with rockwool for low frequency absorption. so i guess additional absorption ontop of this is not something i really need or want.
If I make the well depth of the omni 10-15cm like you suggested, would this lower the amount of absorption?

The construction of an omni Vs. a skyline would be a little bit harder IMO as i would have to be far more accurate in making sure everything is airtight. But generally the amount of effort required to construct the diffusor doesn't bother me. I just want to get it right

So I guess like you said, the Skyline probably would be a good idea. But then again, one of my main aims is to create the best low freq. diffusion I can in order to reduce the problem frequencies in my room


hmm.. I have a lot to think about!

thanks everyone

Skylines are great for small rooms. However, if low freq is a concern a 1-D diffuser will be MUCH better.

Jason

[jeep]

Okay.
after much thought, i've decided to go with the skyline.
what next?

thanks jason!

Originally Posted by Jason Jones

Skylines are great for small rooms. However, if low freq is a concern a 1-D diffuser will be MUCH better.

Jason

[tonymite]

[jeep]

after about 6 months of procrastination due to well.. lack of effort, time and determination, i have finally decided to get this project over and done with!!

check the photos


p.s. the "studio construction and acoustics" forum wasn't available when i first started this thread










i should have some more photos up tonight

[jeep]

not that anyone really cares, but i'm moving this thread to the studio construction and acoustics section..
updates will be posted there

[Tomer1]

Originally Posted by jeep

not that anyone really cares, but i'm moving this thread to the studio construction and acoustics section..
updates will be posted there

I care!!

Please post the link,
I cant seem to find it...


Tomer.

[jeep]

here it is:

Home-made Skyline Diffusor


thank you