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Velocity based vs. pressure based absorbers Bass Traps
Old 9th November 2010
  #61
Gear Guru
Real World

I haven't read the BBC doc properly yet (thanks Andre). However they did operate very much in the real world of many many regional studios of all shapes and sizes. Many rented and so on, thus the modular thing. Immediately I note in Fig 3 that the biggest range of absorption at various angles vs normal is for example 0.35 to 0.65 vs 0.5.
3:5 as opposed to 2:8 from Whealy.
As I say a quick scan so I may be reading this wrong.

DD
Old 10th November 2010
  #62
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Dange's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
But at low frequencies (modal region), the sound field is not diffuse so the absorption coefficient predicted should be close to "real world". In a diffuse field it’s another story.
This very thought was on my mind at the time. Does absorption at low frequency perform more like the normal incidence abs. coeff. prediction and measurements from an impedance tube....?
Old 10th November 2010
  #63
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Quote:
Originally Posted by Dange View Post
This very thought was on my mind at the time. Does absorption at low frequency perform more like the normal incidence abs. coeff. prediction and measurements from an impedance tube....?
In an acoustically small space, the sound field is not diffuse at low frequencies (the modal region approx. below the Schroeder frequency). Consider an axial mode, it will only reflect at the walls at normal incident (a bit simplified maybe and only in acoustically rectangular space but still, it’s not a diffuse field).
Old 10th November 2010
  #64
Gear Guru
Primary

The sound in small rooms is indeed not diffuse, that is well accepted. Nor can it be properly called any other type of soundfield. It is certainly not a simple 'plane wave hits boundary at normal incidence' situation, even in the most extreme example of the primary axials.
That BBC document clearly states that Low frequency wavefronts in such small rooms are not even approximately plane. They also state that the normal incidence condition is rare. Their statistical absorption coefficient spread is designed to fit the real world of smallish studio control rooms. Andre's point is well stated. The wheel has been invented.
DD
Old 10th November 2010
  #65
Gear Guru
Different Resonant frequencies.

Could ye please check on my numbers here.
Jens has helpfully suggested actual numbers for a trap in my alcove.
I tried the same ones (unless I am reading something wrong) in Whealy.
I get a very different resonant frequency as can be seen here.
Different software yielding very different results?
A mistake in my input?
Velocity based vs. pressure based absorbers-picture-1.png


DD
Old 10th November 2010
  #66
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Dange's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
10mm panel with 6mm Ø, 50mm CC.
200mm wool.
DD, Jens has 6mm diameter holes, you have 6mm radius holes....
Attached Thumbnails
Velocity based vs. pressure based absorbers-dd_panel.jpg  
Old 10th November 2010
  #67
Gear Guru
Warming

Thank you Dange. Glad to see it was a DOH! and not my new Whealy toy. I am warming to the panel version. I have been playing around with slat versions, with the flutter diffusion thing in mind. Then don't show the same bang as the perf though.
DD
Old 10th November 2010
  #68
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Bjorn Omholt's Avatar
 

Can one put diffusers in front of perforated panels?
Old 10th November 2010
  #69
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Quote:
Originally Posted by bwo View Post
Can one put diffusers in front of perforated panels?
Yes. As a matter of fact this design concept is used in some temporal diffusers where the base of the slots has an opening the hollow filled with absorbent material. John Sayers is one of the most famous for using slats for spatial diffusion with absorbent materials behind.

Andre
Old 11th November 2010
  #70
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Quote:
Originally Posted by DanDan View Post
Thank you Dange. Glad to see it was a DOH! and not my new Whealy toy.
I'm used to debugging things like that, diameter/radius measurements would be the first thing I'd check for consistancy, then metres/millimeters (factor of 1000) etc. There are always short cuts to finding the problem, an accountant friend of mine told me a way to check columns of numbers had been copied correctly, something to do with factors of 9...., but I've forgotten it!
Old 14th May 2011
  #71
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Quote:
Originally Posted by Jens Eklund View Post
The normal use of perforated panel is broadband. It’s when you try to get to the lowest frequencies the devise gets a high Q (unless very deep). The pic above illustrates a standard single layer with 120mm wool and 120mm gap. To achieve a lower Q a low frequencies you need more depth but usually the lowest problem frequencies are very area specific so tuning to individual frequencies is usually not a problem. The problem with membrane absorbers is the difficulties predicting the resonant frequency.

Attachment 201785

/Jens
Jens what program did you use to generate this graph?
Old 14th May 2011
  #72
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Quote:
Originally Posted by gtzack4 View Post
Jens what program did you use to generate this graph?
AFMG SoundFlow.
Old 27th June 2011
  #73
NLP
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Quote:
Originally Posted by avare View Post
It is related to the manufacturing process. The raw material and binder is mixed together and then compressed to the desired thickness. With thinner panels, the compression gradient acros the thickness of the panel does not get a chance to equalize, causing greater density.

I am certain the explanation is not technically correct. I hope you get the gist of it.

Andre
yes + this:
https://www.gearslutz.com/board/1424911-post29.html
https://www.gearslutz.com/board/1921360-post38.html
Old 14th April 2014
  #74
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Quote:
Originally Posted by Jens Eklund View Post
Is it ok if the Rockwool touches the perforated sheet or should it be like a few mm away?
Old 14th April 2014
  #75
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Quote:
Originally Posted by Jolida View Post
Is it ok if the Rockwool touches the perforated sheet or should it be like a few mm away?
Not only ok, but beneficial.
Old 14th April 2014
  #76
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Quote:
Originally Posted by Jens Eklund View Post
Not only ok, but beneficial.
Great. We have 48kg rockwool available here as opposed to the 30kg u have modelled. I hope there is no downside to using that..?
Thanks
Attached Thumbnails
Velocity based vs. pressure based absorbers-screenshots_2014-04-14-17-25-46.png  
Old 14th April 2014
  #77
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More important than density, is the flow resistivity value.
Old 14th April 2014
  #78
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Quote:
Originally Posted by Jens Eklund View Post
More important than density, is the flow resistivity value.
The simulation u did shows just the density. So for the 5 inch absorbant plus 5 inch airgap u modelled, what is the preferred flow resistivity?
Also, does the panel needs to be sealed on all the other sides like one would do with a membrane absorber?
Old 17th April 2014
  #79
Quote:
Originally Posted by Jolida View Post
The simulation u did shows just the density. So for the 5 inch absorbant plus 5 inch airgap u modelled, what is the preferred flow resistivity?
Also, does the panel needs to be sealed on all the other sides like one would do with a membrane absorber?

flow resistivity affects the Q at the cost of effective absorption at the resonant frequency.
Higher FR lower Q less peak absorption, lower FR higher Q more peak absorption-

so, it really depends on you room, if you have strong modal behaviour and very specific, use lower FR. If modes are distant from each other and spread evenly in the Freq Response, then use higher FR.

AAAAAND this works with preasure, so if you put them where the spl is low, dont expect much of them!

yes it needs to be fully sealed (exept for the perforations) if there is a crack were the preasure may scape efficiency is decreased. the mass spring mechanism is the same principle. spring is the air in the cavity and the mass is the air in the port (perforation)
Old 17th April 2014
  #80
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Quote:
Originally Posted by Sebastian Vera View Post
flow resistivity affects the Q at the cost of effective absorption at the resonant frequency.
Higher FR lower Q less peak absorption, lower FR higher Q more peak absorption-

so, it really depends on you room, if you have strong modal behaviour and very specific, use lower FR. If modes are distant from each other and spread evenly in the Freq Response, then use higher FR.

AAAAAND this works with preasure, so if you put them where the spl is low, dont expect much of them!

yes it needs to be fully sealed (exept for the perforations) if there is a crack were the preasure may scape efficiency is decreased. the mass spring mechanism is the same principle. spring is the air in the cavity and the mass is the air in the port (perforation)
Cool. So if its not possible to physically mount these on a hard wall for some reason, is it ok to seal the back with some 18mm plywood & space it a few cms off the wall?
And I assume these work best on a flat wall as opposed to corner straddling?
Old 4th May 2014
  #81
I dont see why its is imposible to mount. You will need to use caulk or some other sealent to avoid air to scape from the cavity. It may be difficult to construct but not imposible.

yes 18mm is ok, there is no theorical inconviniece in using less thickness. Thinner backs may produce some sort of resoncance so keep it thick. I made some of this resonators using 15mm mdf and i did not measure any unwanted or unspected behaviour.

Yes, flat on the wall you will get higher preasure levels, but pick a mic and see for your self how much it change, and figure out if it may become vital in your case.

In my expirience I have notice 2 to 3 db changes in 20 cm from the boundary down 100 hz, I guess that higher freq would change more due to wave length. but in general a few cm distance is meaningless. hope this become usefull
Old 1 week ago
  #82
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Quote:
Originally Posted by Jolida View Post
Is it ok if the Rockwool touches the perforated sheet or should it be like a few mm away?
Quote:
Originally Posted by Jens Eklund View Post
Not only ok, but beneficial.
Sorry to bump this old thread. But I was doing my homework about HH and came across this great discussion.

I trust you Jens.
But I don't understand why the simulation is telling the opposite i.e not to applied any air gap between perforated panel and mineral wool


When I model it with Acoustic Modelling:

The differences between

-A gap between the perforated panel and the mineral wool (green curve)
and
-No gap (blue curve)

is huge:

Multi-layer Absorber Calculator




If I understood correctly, the air gap in this graph is between the perforated panel AND the mineral wool:


Did I misunderstand your answer?


Btw, anyone has some sketchup file of HH resonator? ( @gullfo ? )

Cheers
Attached Thumbnails
Velocity based vs. pressure based absorbers-capture-d-ecran-2018-11-06-19.00.10.png   Velocity based vs. pressure based absorbers-capture-d-ecran-2018-11-06-18.16.02.png  
Old 1 week ago
  #83
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Acoustic Absorbers and Diffusers, Third Edition: Theory, Design and Application - Trevor Cox, Peter D’Antonio - Google Bocker

Acoustic Absorbers and Diffusers: Theory, Design and Application - Trevor Cox, Peter D’Antonio - Google Bocker

I don´t get the results you got when testing it in SoundFlow. I assume the SF model is a bit more complex than the free online tool. Soundflow predictions are in line with what the theory says.
Old 1 week ago
  #84
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Quote:
Originally Posted by Jens Eklund View Post
Acoustic Absorbers and Diffusers, Third Edition: Theory, Design and Application - Trevor Cox, Peter D’Antonio - Google Bocker

Acoustic Absorbers and Diffusers: Theory, Design and Application - Trevor Cox, Peter D’Antonio - Google Bocker

I don´t get the results you got when testing it in SoundFlow. I assume the SF model is a bit more complex than the free online tool. Soundflow predictions are in line with what the theory says.
tack tack Jens
Old 1 week ago
  #85
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Quote:
Originally Posted by JayPee View Post
tack tack Jens
Ingen orsak.
Old 1 week ago
  #86
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Ok, here is why...


"Perforated Panels
The software will now model perforated panels. The default model (which I have named Ingard/Allard) correctly models the case where there is a porous absorbent directly behind the perforated panel and may give different results to some other calculators. I would like to point out that the model is very sensitive to the distance between the panel and the absorbent. A gap of just 1mm can often make a significant difference to the result. I have included Links to two graphs to illustrate this point. The first uses the Ingard/Allard model. The second is the same calculation performed using a more conventional transfer matrix approach. I will allow you to draw your own conclusions.


Multi-layer Absorber Calculator

Multi-layer Absorber Calculator

by prairiedog

New Porous Absorber Calculator
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