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Building some diaphragmatic absorbers - Advice wanted

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Hi,

 

I'm building some diaphragmatic absorbers to control some seriously horrible axial room modes in my studio. (I have several layers of Tontine acoustisorb as well as batts, but they do very little to the low end)

 

Has anyone built these and can provide advice? My first Modes are at 30 Hz & 55Hz. Not sure how big an absorber I'll need to reach 30hz (It may be an impossibly large box for me to build) but I think I can tackle the 55 hz one with some help.

 

Cheers

 

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I think I am a few steps behind you on the same journey and similar mode issues.  I am still at the stage where I am trying to tell myself that my room is OK even after some horrendous sweep data showing me otherwise.  Ill watch with interest.

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great thread on Gearslutz with design parameters for limp mass traps - I haven't managed to build any yet

 

https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html

 

cheapest place I've found mass loaded vinyl is here:

http://insulationbattsaustralia.com.au/store/advanced_search_result.php?keywords=vinyl&search_in_description=1&x=0&y=0

 

30Hz is quite a deep trap, even with the heaviest MLV - 50cm trap for 8kg MLV

 

I've not investigated the solid board and perforated board membrane traps - the BBC had some good papers, and Cox and D'Antonio has some design criteria for the perforated board (Helmholtz) panel absorber.

 

cheers

Mike

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Thanks for the links! I was just going simple - sealed, heavy MDF box with very thin MDF front to vibrate,  filled (lightly) with fiberglass insulation I have leftover. From what I can understand ( from the internet and my cousin who is a master of Acoustics) these may be the only way to fix my room's problems. I don't really understand how full the box needs to be (with insulation) or if the insulation is supposed to hang or just be stuffed in there... I guess I will experiment.

 

Stacked pink batts/ Acoustic foam would have to be 3 metres thick to absorb 30hz quarter wavelengths. My room is only 6m x 3.8 so that won't work. I've done hundreds of sine sweeps with different speakers and placements and they all give the same bad results. Mixing in here is a nightmare without room correction Eq (and that destroys the transients of everything).... the 30hz mode then creates a 60 hz mode and a 120. combine that with the 55hz and the 110hz mode and...its bad. :)

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My current planned dimensions are 150cm high x 80cm wide x 30cm deep. Trying to work out what frequencies that would catch.... I can easily make it deeper too.

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Try to find design criteria

Just building some may take lots of experimenting

Search for the bbc papers

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My current planned dimensions are 150cm high x 80cm wide x 30cm deep. Trying to work out what frequencies that would catch.... I can easily make it deeper too.

@@almikel links above gives a good summary of all of the frequency calcs and design criteria.  I'm definitely going to try some that can be hidden away in the corners behind my lp shelves.  The only additional comment regarding the design that I would add is that the membrane needs to not have any stiffness contribution...Barium-loaded vinyl is definitely a good material for this application  I.N.C. corporation were a great Australian manufacturer of this stuff...I used to work for their General Manager but cant find their online presence.  They may have gone the way of most automotive related manufacturers in Vic/SA

Edited by christosd

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Thanks Christosd. My main reason for using a thin wood front was that it is easy to find (and attach). Several designs use this -I'm not sure how the effectiveness compares to the limp mass absorber. The acoustic vinyl above only seems to come in 5 meter rolls which might be too much for me. 

I'm  trying to work out the limp mass absorber calculator - I don't really understand if it needs the mass of the whole front panel (limp mass) or the mass of the material used per square metre. If so, how do the height and width dimensions relate to frequency?

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My understanding is that once the front panel has stiffness the whole thing is a different animal and the calcs dont apply!  

 

The limp membrane method is based on there being NO Stiffness in the facing membrane....as natural frequency is dependent on sqrt(stiffness/mass), when the front is limp, it only has mass.  The stiffness is proportional to the sealed volume of the device which you would think depends on the height and width however, as the mass element is considered per unit area, then the area portion of the volume stiffness calculation disappears, leaving just depth!.  There is probably a better way of describing it but I've gotta pick up the kids from school so gotta run.

 

Also, having a large stiffness controlled skin over a volume is a completely different design and can work however, tuning of that system is difficult and requires careful control of the mechanical damping of the skin...the classic scenario in a gyprock stud wall occurs when the gyprock and wall cavity behaves as a reactive absorber and can notch out frequencies at the resonance quite effectively if damped from behind just the right amount..too little damping and the you have a lot of quite efficient radiators ...too much damping and they dont absorb much at all.

 

Maybe with some design sophistication, the method with a stiff membrane might work, I think the limp membrane has more scope for DIYers and for broad applicability however.  Please note...I have done neither yet as I am in an acoustic La -La land in my room at the moment!

Edited by christosd

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great info christosd! I think Ive found someone to share in my 5 meters of vinyl so I might go that way. I like the fact that the boxes can be a bit smaller. As the owner of 8 giant homemade floor-to-ceiling absorbing panels (using tontine acoustisorb - which sadly do almost nothing to the problem areas of my room) I really appreciate the ability to rearrange the room without playing furniture tetris and knocking out light fittings. 

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I still would have guessed that there would need to be a fair bit of face area to break a 30 Hz mode however.  I wonder if the gearslutz guys get reasonable benefit because of their relatively small control rooms.  I would love to see before/after measurements for an actual scenario...maybe I should dig deeper. I'd chip into the loaded vinyl bulk buy if I was in NSW....keep us posted!

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I still would have guessed that there would need to be a fair bit of face area to break a 30 Hz mode however. 

I agree - I think Tim's assumptions in the Gearslutz thread regarding size of trap could be off.

 

My "ideal" trap would be to have 2 x MLV traps on each side of a corner behind absorption straddling the corner - all floor to ceiling. The depth of the MLV traps determined by the modes requiring taming.

 

I've not delved into the maths or modelling for a plain resonator sheet like MDF - but I do have Cox and D'Antonio's text which includes the calcs for the Helmholtz version - lots of holes in the front sheet, and the air in the holes vs the depth of the trap provides the Helmholtz resonator component.

For a plain front panel look to the BBC papers.

 

Clearly 30Hz is a struggle for absorption, but @@royalston , don't get hung up on the 1/4 wavelength thing - you still get good absorption with traps at <1/4 wavelength whether via gapping or superchunks...

Also you mention above you don't like Room EQ at those frequencies (eg 30Hz) due to transient response - IME room EQ works well down there and kills the tails but not the attack as long as you only cut peaks with EQ.

 

Absolutely have a crack at tackling the lower modes with membrane traps - something I've been meaning to do for ages, but I don't see an issue with further fine tuning via EQ - down low (say below 150Hz) peaks are usually minimum phase, so cutting them doesn't hurt the transient response - and a tool like REW will show you what's minimum phase and what isn't.

 

cheers

Mike

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Hi Mike,

 

Have a look at Acoustic Fields website http://www.acousticfields.com/ . He sells plans for diaphragmatic absorbers.

 

I have built two to these plans to sort out bass problems. He will also analyse your room/studio for free if you send him your room details.

 

Attached are two photos, the first showing the cross-section and last is the finished product.

post-143090-0-11533800-1456817508_thumb.

post-143090-0-71268200-1456817514_thumb.

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My current planned dimensions are 150cm high x 80cm wide x 30cm deep. Trying to work out what frequencies that would catch

 

That will depend on the diaphragm (mass and stiffness).   Calculating what to expect is quite tricky.    So much depends on the material specifics, that I think the only sensible way is to use a  calculator to get a rough idea for somewhere to start ......  and build some prototypes, then measure/tweak.

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Posted (edited)

@Whites How'd this go? Did you take any measurements before and after by any chance?

Edited by JPete9

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@JPete9 no I didn't take any measurements. They definitely worked as you could play music with a lot of bass louder without it becoming "muddy".

 

The only drawback with the units is the size as they are quite deep. It's not ideal for my room as it means the speakers are further out from the wall.

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Posted (edited)

I am walking down this rabbit whole for a while now as well, and every time I think I have found an answer, I stumble upon reasons that contradict this. The biggest problem with taming low frequencies in small rooms is the large size needed for the trap, and the small size of the room. The trap itself becomes part of the room, and build from stiff and rigid materials introduces the risk of new and changed modes.

 

I believe that for diaphragmatic absorbers the minimum dimension of the trap (width or height) is 1/10 of the target frequency wavelength, usually multiplied by 1,5 for the other dimension. This has to do with efficiency, but it is not clear to me how much they are talking about. BBC papers have showed that smaller sizes can work, but their lesser efficiency needs more units to tackle a problem, claiming more real estate again. BBC units are all about a standard and interchangeable size unit though.

 

You should also ask yourself how much of a problem the 30Hz room node is in real live. As I understand it for music, it doesn't get activated much. This is something that I am trying to clarify for myself as well, regarding movies.

 

Edit:

Have a look at the porous acoustic modeling/calculators. It is not impossible to get a theoretical efficiency  of 0,5 at 30Hz with porous absorption, if you use a combination of different flow resistance products. Low flow resistance product used in bulk can be effective.

 

Edit:

have a look at Helmholz resonators as well, as they require similar volume as diaphragmatic absorbers but are easier to tune when you use a longer tube length that you can adjust to tune the frequency.

Edited by Primare Knob

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Posted (edited)
11 hours ago, Primare Knob said:

 

have a look at Helmholz resonators as well, as they require similar volume as diaphragmatic absorbers but are easier to tune when you use a longer tube length that you can adjust to tune the frequency.

I have the material to make one and was considering as an experiment at weekend, but anyone got any verification they work?

Edited by JPete9

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On 15/05/2018 at 9:06 PM, JPete9 said:

I have the material to make one and was considering as an experiment at weekend, but anyone got any verification they work?

do you mean a Helmholtz resonator with a box and tube, or a helmholtz panel resonator with many holes in front of a sealed cavity?

Every ported speaker works based on the box/tube type of helmholtz - so there's ample examples that they work.

Definitely worth an experiment to try - just make it so you can tune it (more stuffing should lower the Q, longer/shorter pipe will adjust centre frequency.

You may need more than 1 depending on size to pull down the particular frequency you want to target, but you won't know that until you try.

 

mike

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On 15/05/2018 at 9:06 PM, JPete9 said:

I have the material to make one and was considering as an experiment at weekend, but anyone got any verification they work?

https://www.gearslutz.com/board/studio-building-acoustics/630460-panel-helmholtz-resonators-studio.html?s=fd142976fdf788bd70c6a7880b4afb9d

 

Have a read through this thread and you will get a good idea about the best approach in tuning the helmholz resonator. Not sure why this guy uses so many ports compared to a single port solution. Probably has to do with the limited available sizes in PVC tubing.

Edited by Primare Knob

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I meant the box. Gear slutz has a LOT of controversial discussion on these. I have the stuff to make one, a big one or a few smaller ones and i have the calc. But for now i will finish making my absorption panels, and reserve the Helmholz for tuning anything 'special' once i have solved the main issues.

 

 

 

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