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Author Topic: Capping Polycylindrical Diffusors  (Read 2418 times)

mjgreeneaudio

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Capping Polycylindrical Diffusors
« on: February 19, 2009, 01:20:15 pm »

I have been looking all over for an answer on this and havent' found anything yet.  If I build a number of polycylindrical diffusors, what are the advantages and dissadvantages of actually closing off the top and bottom of them?  How does it affect the bass response?

Also if I mount 4 along a wall in a big room 20X24 is there any issues with building all of them the same size?  Does it skew or change the frequency response?

Thanks in advance.

Michael Greene

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Thomas Jouanjean

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Re: Capping Polycylindrical Diffusors
« Reply #1 on: February 19, 2009, 02:20:08 pm »

mjgreeneaudio wrote on Thu, 19 February 2009 12:20

I have been looking all over for an answer on this and havent' found anything yet.  If I build a number of polycylindrical diffusors, what are the advantages and dissadvantages of actually closing off the top and bottom of them?  How does it affect the bass response?




With or without cap, they behave Helmholtz-like. You can tune them! Calculate the size of the port vs freq, insert one, done.

mjgreeneaudio wrote on Thu, 19 February 2009 12:20


Also if I mount 4 along a wall in a big room 20X24 is there any issues with building all of them the same size?  Does it skew or change the frequency response?

Thanks in advance.

Michael Greene




Nah, not with polys (diffraction/diffusion wise). The resonator behaviour could be source of problems though (re-emission etc) but if you tune them properly with regards to the pressure points, they'll be very useful.


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Thomas Jouanjean
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mjgreeneaudio

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Re: Capping Polycylindrical Diffusors
« Reply #2 on: February 19, 2009, 03:01:18 pm »

Thomas,
Thank you.  That is very helpful.  I really appreciate it.

That picture looks great.  That is very nice construction.

Michael Greene

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Thomas Jouanjean

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Re: Capping Polycylindrical Diffusors
« Reply #3 on: February 19, 2009, 03:52:37 pm »

Glad I could help.

And thanks! The woodcrafters that work for us are doing an incredible job indeed Smile those polys turned out real nice once oiled. They were installed @ Amsterdam Mastering.
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Thomas Jouanjean
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Bruno Gouveia

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Re: Capping Polycylindrical Diffusors
« Reply #4 on: February 19, 2009, 05:28:52 pm »

Thomas, how do you guarantee that your polycylinders won't re-emit?

Thomas Jouanjean

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Re: Capping Polycylindrical Diffusors
« Reply #5 on: February 20, 2009, 02:17:17 am »

Bruno Gouveia wrote on Thu, 19 February 2009 16:28

Thomas, how do you guarantee that your polycylinders won't re-emit?


I could give you that information.

But I'd have to keeeeeeell you after...
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Thomas Jouanjean
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Bruno Gouveia

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Re: Capping Polycylindrical Diffusors
« Reply #6 on: February 20, 2009, 03:24:56 pm »

\stress

J.F.Oros

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Re: Capping Polycylindrical Diffusors
« Reply #7 on: February 25, 2009, 11:42:59 am »

Quote:

Thomas, how do you guarantee that your polycylinders won't re-emit?
I think its enough to just damp them on inside with enough mineral wool.

Those polys look awesome indeed. Thank you for sharing this Thomas, a very informative picture (including the type of phone u were using to take it, i love Sony Ericsson phone cams Very Happy )
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[ Flaviu Oros - acoustics engineer ]
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Bruno Gouveia

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Re: Capping Polycylindrical Diffusors
« Reply #8 on: March 01, 2009, 01:16:23 pm »

J.F.Oros wrote on Wed, 25 February 2009 16:42

Quote:

Thomas, how do you guarantee that your polycylinders won't re-emit?
I think its enough to just damp them on inside with enough mineral wool.



Yeah, but adding damping that way changes the ressonant frequency a little bit, that's the whole problem...

J.F.Oros

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Re: Capping Polycylindrical Diffusors
« Reply #9 on: March 01, 2009, 02:36:10 pm »

Well, if you damp them enough so that their resonant behavior and effects on the sound field will be minimized, wouldn't their resonant frequency become kind of irrelevant ?

Or you mean you want to use them both as diffusers and as tuned absorbers ? As far as i know, damping a resonant absorber to the point where it will not introduce reemission in the room anymore will generally make it less efficient for the intended absorbtion job too. So the reemission phenomena has to be used in a constructive way, tunning the device (on both the resonant frequency and Q) under continuous acoustical measuring until you get the best of the room response. But i'm hardly an expert on this part, so maybe others more experienced have better opinions to share on the subject...
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franman

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Re: Capping Polycylindrical Diffusors
« Reply #10 on: March 01, 2009, 11:14:18 pm »

Bruno Gouveia wrote on Sun, 01 March 2009 13:16

J.F.Oros wrote on Wed, 25 February 2009 16:42

Quote:

Thomas, how do you guarantee that your polycylinders won't re-emit?
I think its enough to just damp them on inside with enough mineral wool.



Yeah, but adding damping that way changes the ressonant frequency a little bit, that's the whole problem...


actually the damping inside these type of panels serves to broaden the Q of the resonant system. It is the right thing to do, to give you a better shot at efficiency. If tuned traps are too narrow (Q) then you have to hit the target frequency right on the buttom. ..FM
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andrebrito

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Re: Capping Polycylindrical Diffusors
« Reply #11 on: March 02, 2009, 06:45:00 am »

I was going to say the same thing as Fran but he anticipated me !

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