Diaphragmal LF Absorbers (aka LMBT) damping and calculation

[FeddyLost]

Hello everyone

Now I'm more inclined towards making some diaphragmal LF absorbers with limp membranes.
But there are few questions that still unclear.

1) Can I trust to http://www.acousticmodelling.com/multi.php
and
if yes - which porous model is most realistic below 200 Hz ?

Just for explanation why am I asking this
Typical LMBT calc for damped inner volume uses just adiabatic/isothermal approximation
F = K/SQRT(MxD), where either
K=600 (adiabatic) for Tim Farrant formula ( https://www.gearslutz.com/board/bas...etc/743040-tims-limp-mass-bass-absorbers.html ),
K=500 (isothermal) for LFCA with damped light plywood panel membrane ( https://www.acoustic.ua/recommendations/567#4 )

And if we assume different damping in aforementioned calculator, it guves us slightly different results between those adiabatic and isothermal F with different width of central peak.

2) How damping factor can be considered in real world efficiency?
I mean, too heavy (thick and flow resistant) damping will make air spring loose, changing tuning frequency
and
also if total flow resistivity is too high, single short burst will not "pump through" rockwool and calculated efficiency will be available only with stationary resonance signal even if membrane is light and loose.

So, in graph high damping >2 looks nice (wideband and around 0,85 AC on peak) but if it's OK in real construction?

3) Which % of wall coverage will make effective modal disappearance?
I think it might be at least 25% if I imagine ideal tuned absorber like "hole in the wall" for central frequency.

If that matters, there are another current circumstances and thoughts about this
1) I agree with Tim Farrant's idea of combining differently tuned absorbers in room corners and on walls closer to corners. Because some of those frequiencies are close, if I use relatively wideband (= low Q) absorbers, I'll make i.e. "corner stack" working for few modes simultaneously with different efficiency.
2) I have around 30 cm gap between raw walls and frame for inner shell, so all absorbers must fit into this gap. With membranes 5 and 7 kg/sq.m it's acheivable with heavy damping by fluffy rockwool.
3) I understand that all our calcualtions, modal modelling and measurements are not very precise, so I have another one reason to make absorbers with low Q, otherwise with all possible errors I can just miss with all frequencies. When you have ~ 15 Hz width at 0,8 efficiency, it's much safer.
4) raw walls are made from aerated concrete, so I may presume it's surface mass might be around 60 Kg/sq.m, so it's not infinite as start equation means, but sufficiently heavier than membrane. Also, corners are more rigid due to construction.

Because my calculations lead me to not very cheap and easy construction, especially in terms of labour (around 50 pc of ~30x40x60 cm boxes) I'd like to get some feedback from experienced people especially with success stories of bass trapping.
I think it will be useful for any audio enthusiast without personal concert hall.

 

[ACHiPo]

Interested in responses as well.

 

[FeddyLost]

Interested in responses as well.

If you are still interested, I've done these recently.
They definitely work, but if you don't really need some improvement by physical acoustics, they are kind of not worth it. For studio or for "strictly no DRC with bad placement" (vinyl fans?) most probably it's the only one option.
Too much effort and materials expense and not much of measurable result.
Just for understanding - i've made 60 airtight boxes 60x40 cm of various depth (165 to 250 mm inside) from 18 mm (3/4") plywood with different membrane thickness. It's a lot of work ... for me at least.
I've finally done it according to original design by Tim Farrant, and placed all in vertical stacks in room corners and adjacent walls. Glued boxes with acrylic sealant to existing walls.
I have only one comparative graph just because difference is kind of ... obvious. And I had no way back after all that woodwork.
Still don't know how correctly I've offsetted these graphs (set equal 30 Hz as it's in pressure zone and below all tuning) , but even form of curve will tell. Main modes are 37(Y), 40(Y) and 54(XY) ...

Without offset, equal at midrange it looks like this and I can't understand HOW exactly I had real pumping and pressure feeling. Most probably, diagonal mode peak at ~ 54 Hz stimulated by R channel was unbearable.
Now all sounds "fuller" and "shorter" with better LF differentiation. Don't really know how to explain better.

 

[ACHiPo]

Feddy,
Interesting. I can empathize with measurements driving you crazy. I went through a similar exercise 6 years ago when I set up my room. I finally just took the best information I had at the time (ridgid 705 panels trimmed and stuffed into bottom cabinets with doors replaced by acoustic fabric, plus several GK soffit 440 traps in the top shelves of my book cases, 1 440 diffuser trap behind the primary listening position and 1 440 trap in the corner behind a floor to ceiling 6" DIY 705 absorber straddling the right rear corner).

My measurements went from a pretty atrocious +/- 20 dB to <10 below 100 Hz without subs and the room sounded much better.

I just remeasured with my Swarm subs running and I'm +/- 3 dB in the primary listening position (Swarm subs work!), but still about +/- 10 dB <100 Hz in my secondary listening position (a desk against the side wall). I have a 31 Hz mode and a 46 Hz mode with suck out in the mid-50s.

I only have a couple places on my walls in high pressure areas, and I'm trying to decide what to put there. Either 4' x 4' limp membrane absorbers broadly tuned for 31 Hz and 46 Hz, or a VPR/CPA (Composite Bonded Absorber)--either DIY or RPG Modex (if I can find them), or a spring suspended metal membrane with speaker gasket all the way around (not sure where I saw that idea, but it looks interesting).

I just don't know whether it's worth treating the only space I've got left. I do know that it was eye-opening to play a 31 Hz tone and walk around the room with an SPL meter!

Evan

 

[FeddyLost]

Either 4' x 4' limp membrane absorbers broadly tuned for 31 Hz and 46 Hz

In my opinion from experience such low tuning of small boxes with Tecsound membrane require significant depth for proper result.
I was limited with total 30 cm depth and IMO 10,5 kg per sq.m is too heavy and relatively stiff (compared to 5 kg/sq.m) when stapled on all borders for working well at normal SLP. It can be seen from small difference at 37 an 40 Hz.
And big membrane must be strong to keep its form, while Tecsound is more like very viscous window sealant between two film layers, I'm not sure about 4'x4'. Maybe MLV will be more suitable.
Regarding VPR and all such heavy units ... personally I'd saved another preset for subwoofer setup.

 

[ACHiPo]

In my opinion from experience such low tuning of small boxes with Tecsound membrane require significant depth for proper result.
I was limited with total 30 cm depth and IMO 10,5 kg per sq.m is too heavy and relatively stiff (compared to 5 kg/sq.m) when stapled on all borders for working well at normal SLP. It can be seen from small difference at 37 an 40 Hz.
And big membrane must be strong to keep its form, while Tecsound is more like very viscous window sealant between two film layers, I'm not sure about 4'x4'. Maybe MLV will be more suitable.
Regarding VPR and all such heavy units ... personally I'd saved another preset for subwoofer setup.

Feddy,
Which density of Tecsound did you use--it looks like it comes in 2 kg/m2 to 10 kg/m2? The heaviest should allow a modest (~100 mm) box depth for success (will need to run the calcs). I can imagine the 10 kg/m2 MLV material that I've found will be quite stiff. It would seem to benefit mechanically from a larger panel to minimize the effect of sealing/fixing the borders. This is the material I've found in the US: https://www.soundproofingmlv.com/details/2-lb-noise-grabber-brand-mlv-48-x-4-roll.

I found a YouTube video of an interesting construction technique. It used thinner/less-dense butyl rubber to sandwich a steel sheet for a mass/piston. This composite diaphragm was then framed with steel angle and placed about 1/2" from low-density fiberglass in the back of the "box", with addition insulation (not touching) in front of the diaphragm, the whole thing covered with acoustic cloth. Unfortunately there were no measurements.

Evan

 

[FeddyLost]

Which density of Tecsound did you use

From 3,5 kg/sqm to 10,5 kg/sqm. 10,5 was glued from two layers 7 + 3,5.
10,5 is quite thick and heavy.

It would seem to benefit mechanically from a larger panel to minimize the effect of sealing/fixing the borders.

It depends on membrane thickness and stiffness. 5 kg/sqm tecsound must work well in 60x40 cm box by touch. Almost no fixed borders effect.
Maybe single layer 10 kg/sqm would be also nice, but lead times was high and I have 2 glued layers have 4 layers of film, so stiffness is higher.

I found a YouTube video of an interesting construction technique

I've seen it. Idea looks reasonable but need a lot of effort to do.
It's still unknown how much matters resulting mass of piston vs SPL and suspension tension, so I'd go for LMBA as some proven concept.

 

[ACHiPo]

I have an email into RPG. I suspect their Modex is hard to beat with a DIY solution. Let's see what price they come back with.

 

[bo_knows]

I'm going to attempt to do Tim's Limp Mass Bass Absorbers and try to reduce some 50 Hz in my room.

In my MLP, I have a drop in the 50 Hz range which is telling me that I'm sitting in the zone where two waves are colliding. I've measured the SPL response of the 50Hz tone and most of the higher readings are at the bottom and top of the front wall. My plan is to update my existing treatments with Tim's boxes that will target approximately 50Hz.
I'm still going over the youtube videos, acousticmaterial.com calculator, and his PFD to figure out what is the best calculation and approach. I will be using mass-loaded vinyl (MLV) 1 lb for the membrane material.
One thing that concerns me is when I use his approach, the absorption coefficient number is very low for 50 Hz.
I've messed with some settings in the calculator and this yielded the best numbers. I'm not sure about the Flow resistivity number for the material I will be using so I was just guessing.
Any help, suggestion, advice, or feedback is greatly appreciated.

The current room FR response in MLP is attached.

Anyway, here's the calculation:

Result: almost 1.

 

[abdo123]

I'm going to attempt to do Tim's Limp Mass Bass Absorbers and try to reduce some 50 Hz in my room.

In my MLP, I have a drop in the 50 Hz range which is telling me that I'm sitting in the zone where two waves are colliding. I've measured the SPL response of the 50Hz tone and most of the higher readings are at the bottom and top of the front wall. My plan is to update my existing treatments with Tim's boxes that will target approximately 50Hz.
I'm still going over the youtube videos, acousticmaterial.com calculator, and his PFD to figure out what is the best calculation and approach. I will be using mass-loaded vinyl (MLV) 1 lb for the membrane material.
One thing that concerns me is when I use his approach, the absorption coefficient number is very low for 50 Hz.
I've messed with some settings in the calculator and this yielded the best numbers. I'm not sure about the Flow resistivity number for the material I will be using so I was just guessing.
Any help, suggestion, advice, or feedback is greatly appreciated.

The current room FR response in MLP is attached.

Anyway, here's the calculation:

View attachment 252364

Result: almost 1.

View attachment 252365

View attachment 252368

Something with 40000 flow resistivity doesn’t really pass through Air anymore.

Figure out a ballpark value for the material that you’re going to use, even better, use Caruso Iso bond since they advertise their flow resistivity numbers.

 

[bo_knows]

Something with 40000 flow resistivity doesn’t really pass through Air anymore.

Figure out a ballpark value for the material that you’re going to use, even better, use Caruso Iso bond since they advertise their flow resistivity numbers.

Hello Abdo123,

Correct, I just realized after posting this that I used the wrong/large number in that field.

Even if I lower that number to 20,000. the absorption coefficient still looks good.

I have to admit, I'm horrible with math and could use all the help. LOL

Found this on gearspace.com

Common Gas Flow Resistivity numbers. - Gearspace.com

Thanks Andre. It is odd that they put the figure right on the front page of a general info sheet, yet it is relatively meaningless. I suspect that 5KRa

gearspace.com

OC 703 has a resistance of 600 mks Rayls/inch (about 23600 mks rayls / m)
OC 705 has a flow resistance of 770 mks Rayls/inch (about 30000 mks rayls / m)
Bradford Fibertyex 650 (same density as OC 705) is 1270 mks Rayls/inch (about 50000 mks rayls / m).

P.S. I had to look up your recommendation (Caruso Iso bond) and I see it's available in EU.

I live in Dallas, TX.

 

[FeddyLost]

One thing that concerns me is when I use his approach, the absorption coefficient number is very low for 50 Hz.

According to graph, you'll have exactly 50 Hz tuned resonance system. I'd even widen it slightly, because you have a wide notch at MLP, not a narrow peak.

Before you start to cut plywood, I'd recommend you to
1) investigate your room with REW RoomSim and see if you can move listening position (your graph looks like some absence of modal support)
2) calculate again your boxes with actual data and approximate prices including labor
3) check out if you can make multi-sub solution within more or less same price
4) if you by some reason don't accept easy and cheap multi-sub augmentation (i.e. have turntable and need your audio chain fully analog), I'd try to consult experienced acoustician before purchasing materials

Because I've tried to do LMBAs and it's not the best soultion (at least with my embodiment).
They work,
but for good result you'll need A LOT of them, and it's expensive.

Also, the lighter the membrane (and deeper the box) the better they perform in fact.
Maybe it's just energy of resonance, maybe ligher membrane excites more easy, maybe I did something wrong or placed them incorrect, IDK.
50 Hz within 260 mm of total depth will not be very effective IMO. 75-80 Hz will be OK, 50 Hz is troublesome.

 

[bo_knows]

According to graph, you'll have exactly 50 Hz tuned resonance system. I'd even widen it slightly, because you have a wide notch at MLP, not a narrow peak.

Before you start to cut plywood, I'd recommend you to
1) investigate your room with REW RoomSim and see if you can move listening position (your graph looks like some absence of modal support)
2) calculate again your boxes with actual data and approximate prices including labor
3) check out if you can make multi-sub solution within more or less same price
4) if you by some reason don't accept easy and cheap multi-sub augmentation (i.e. have turntable and need your audio chain fully analog), I'd try to consult experienced acoustician before purchasing materials

Because I've tried to do LMBAs and it's not the best soultion (at least with my embodiment).
They work,
but for good result you'll need A LOT of them, and it's expensive.

Also, the lighter the membrane (and deeper the box) the better they perform in fact.
Maybe it's just energy of resonance, maybe ligher membrane excites more easy, maybe I did something wrong or placed them incorrect, IDK.
50 Hz within 260 mm of total depth will not be very effective IMO. 75-80 Hz will be OK, 50 Hz is troublesome.

Hello FeddyLost,

I really appreciate your feedback.

1) investigate your room with REW RoomSim and see if you can move listening position (your graph looks like some absence of modal support). Yes, If I move my chair 10 inches back towards the rear wall, 50 Hz comes back a few dBs.
2) calculate again your boxes with actual data and approximate prices including labor. I'm thinking to build 6 boxes on the upper ceiling line from one corner to the other. I would need to buy more wood ($200) and should have other materials in-house. Not really expensive but yes, it will take some time for me to build them.
3) check out if you can make multi-sub solution within more or less same price. I have one KEF KF92 sub for home theater duties and I don't use it when listening to two channels. This sub is 2K (not cheap) and my room is really small. It would be nice to have another sub but that expense needs to be justified. I rather upgrade my center channel speaker vs buying another sub at this moment.
4) if you by some reason don't accept easy and cheap multi-sub augmentation (i.e. have turntable and need your audio chain fully analog), I'd try to consult experienced acoustician before purchasing materials. This is a hobby for me and I do enjoy building a DIY room treatment. If it works, great, if it doesn't oh well. I tried...

Because I've tried to do LMBAs and it's not the best soultion (at least with my embodiment).
They work,
but for good result you'll need A LOT of them, and it's expensive. According to Tim, 4 boxes should make a difference if placed in corners.

50 Hz within 260 mm of total depth will not be very effective IMO. 75-80 Hz will be OK, 50 Hz is troublesome.
I agree, 50 Hz will be tough to tame. Funny, when I decrease the air space cavity inside the box via the calculator, the absorption coefficient goes up?! Tim is mentioning 300 mm and I'm getting better results when using ONLY 200 mm including the 100mm worth of insulation. This makes me question the accuracy of the calculator. Hmmm. What to do, what to do? Also, the guy that did the same experiment in the below youtube video, end up using a shallower box and he said it worked.

 

[FeddyLost]

Yes, If I move my chair 10 inches back towards the rear wall, 50 Hz comes back a few dBs.

I'd start to optimize FR <100 Hz with relocation of speakers and LP.

I'm thinking to build 6 boxes on the upper ceiling line from one corner to the other.

It's better to check out via REW RoomSim if this frequency is really dominant in this upper corner.
Tuned pressure absorbers work only if pressure at tuning frequency is present, they will not work i.e. placed in the middle of the room.
Even if it's peaked in planned location, most probably you'll have to measure and investigate to see results.
IMO perfect tuned absorber it's a "virtual hole" for sound wave with a given frequency. You need enough holes in your wall for wave to sink in.
So, just 6 units ... IDK.
I've made 4x4 per given frequency - stacked 4 boxes 16"x24" in each corner.

If it works, great, if it doesn't oh well

Good point of view.
Just be ready to make few times more if they work, but not good enough.

ccording to Tim, 4 boxes should make a difference if placed in corners

In case of empty room and only one annoying peak - most probably yes, but subjective difference might be like +12 Db and 1200 msec decay vs. +20 and 2000, so it's kind of little optimistic promise.

Funny, when I decrease the air space cavity inside the box via the calculator, the absorption coefficient goes up

Calculation is a calculation.
In fact, it's modelling like "if we make full wall of this absorption and put pressure generator of stationary soundwave on the opposite wall, stationary absorption might look like this".
And you have some dynamic sound. You need your absorbers to excite fast and settle down fast. So, it's better to make them as light, limp and supsceptible as possible.
IMO.
Just compare MLV of different weight and feel it. Too shallow box will require more mass and you'll have more inert membrane.

 

[bo_knows]

Quick update.

Tim Farrant Simple Limp Mass Bas Absorbers are built.

I built two boxes to start and see if they work before I start sinking more money into this project.

The bad thing about this project is that there are no real measurements showing that the boxes work.

A few DIYers did take some REW measurements and posted them on Gearspace but the results were inconclusive.

Tim himself is not big on measurements and uses his ears. I basically built these on faith, his plans, provided formula, and his word.

I did deviate from his plans by putting the fiberglass panel closer to the membrane vs in the rear. It just made more sense to me.

It's late here and I'm tired to place them in their new location.

Tomorrow, if I have time, I'll try to place them in the upper corners of the front wall and see if the SPL of 50Hz goes down a dB or 2. The build wasn't hard but trying to keep the enclosure airtight is not an easy task.

A few pictures are attached.

 

[FeddyLost]

The bad thing about this project is that there are no real measurements showing that the boxes work.

I need to check for mdats that I've done before and after installation of LMBAs.

They work, but as I've said, you need some working area, and I think that just 2 boxes are not enough.
Most probably, you'll see some effectiveness on waterfall plot unless your peak is extremely sharp.

Regarding audition, I'd recommend to use Jesse Cook's album Gravity (1995). It have extremely thick and mellow bass lines.

but trying to keep the enclosure airtight is not an easy task.

A lot of sealant will work.

A few pictures are attached

Your MLV is keeping rolled form without any force applied. I'd say it's not a good sign for something that must be really limp.

 

[bo_knows]

I need to check for mdats that I've done before and after installation of LMBAs.

They work, but as I've said, you need some working area, and I think that just 2 boxes are not enough.
Most probably, you'll see some effectiveness on waterfall plot unless your peak is extremely sharp.

Regarding audition, I'd recommend to use Jesse Cook's album Gravity (1995). It have extremely thick and mellow bass lines.


A lot of sealant will work.


Your MLV is keeping rolled form without any force applied. I'd say it's not a good sign for something that must be really limp.

Hello FeddyLost,

Tim never used MLV in his design. I believe he lives in New Zealand/Australia where different materials for membranes are available. He said that MLV is OK and wording in his document is referencing MLV. What you see in the picture is MLV bought on Amazon (Soundsulate™ 1 lb Mass Loaded Vinyl MLV, Soundproofing Barrier 4' x 25' (100 sf) and it comes in a roll hence being ROLLED. Once it was placed on the table, it straightened out.

 

[FeddyLost]

I've used Tecsound SY by Soprema. It lays like soft rolled dough and don't keep any form at all.

 

[bo_knows]

I've used Tecsound SY by Soprema. It lays like soft rolled dough and don't keep any form at all.

Probably closer to Tim's original recommendation for the membrane than MLV I'm using.
I just played a 50Hz tone and when I place the hand on the membrane, it moves the most on 50Hz so maybe tuning and build are right. I did mess up with the measurements of the enclosure so now I can't place them where I wanted them to go.

 

[bo_knows]

Just saw this video:

Hmmm, that is totally different material than MLV I used.