Adhoc - I see the graph now but I did not see it earlier on when I made my post. Thanks very much. That link to your construction was great as well. edit: the photographs seem out of sequence but I think from your written description it is possible to see what you did.
I wonder if the grade of the rubber makes much difference? Do you have a link or some physical properties of the rubber sheet? Natural rubber is around 1,000kg/m^3 so 3mm thick = 3kg/m^2 but I wonder what Duro (hardness) it is.
KAsmira - the rubber sheet was fixed and sealed on 4 sides to make the enclosed volume airtight - it was just a brief discussion about how tight the membrane should be when it is fixed.
My pictures out of sequence? No way man! Didn´t you see them as most people would: To reach this goal, you first have to do this, but before that you need to do this, which of course means you have to start with this first.
The important thing for the absorber and its target frequency is a correct air space behind membrane versus its actual measured weight / area unit. I personally would not trust any catalogue figures on density, always measure the stuff you use. Wrong density entered => incorrect results from calculations.
Natural rubber sheeting, like Trelleborg #2645, #510 or #61490, has a stated density of 1000 kg/m², hardness is 40 +/-5 Shore A (= very soft , like a common rubber band). I hand picked some EPDM rubber and other synthetic rubber with higher density. Reason: With a higher surface weight I could use a smaller air space behind which saved me about 160 mm / 6,5" in room width and length. Hardness was 70 +/- 5 Shore A = very common for rubber gaskets in nitrile, EPDM etc but the hardness itself shouldn´t be an important point in this case.
I am interested in the hardness question. Tim has had success using vinyl-fronted boxes of relatively small dimensions (600x400mm). Voetmann and Adhoc have had success using SLAMs - natural rubber sheet fronting much bigger boxes (1200x1800mm). The natural rubber sheet is of course much more "rubbery" than a sheet of vinyl. In my part of the world it is also 3-4 times the price (natural and synthetic rubber is relatively very expensive).
Somewhere there was a link posted to a home depot sound mat that cost about $30 that could be used for one of these traps. For the life of me, I can't find that post, and my home depot searches haven't helped. Would someone mind reposting that link?
The pictures below show how I made the version for the longer side walls, with an internal height of 600 mm. The rubber sheeting has a standard width of 1400 mm which gave lengths 1200-1400 mm. Surface weight 5,4 kg/m² with 98 mm air gap behind the membrane.
1 Frame work of wooden studs 45x95 were screwed to the concrete walls and floor. Between wood and concrete some pieces of rubber, 3 mm thick, were inserted to avoid any direct contact wood-concrete, both towards the wall and the floor. The gaps were then filled up with expanding PU foam.
2 After cutting off PU foam excess, the joint was sealed off with flexible acrylic caulking
Some floor sections at the front ready
4 Dampening fibre glass, secured inside the frame work
5 Rubber sheet stapled to the frame work at a section in the back of the room. A string of silicone was attached between the rubber and wood before stapling (Above it is a BAD panel, Ternary version)
6 A frame, 10 mm thick, is nailed around the rubber sheet, so cloth later can be attached without touching the membrane.
7 4 ready sections at the front of the room. The middle section is a first reflection area and will have 95 mm of an equivalent to Owen Cornings 705 instead of membrane absorber.
8 Final result with cloth covering the rubber membranes. The slats on the last picture are 33x8 mm according to Boggys MLS-sqequence.
A note: This room is not intended for music production, only pleasure = music reproduction and some occasional TV- or movie watching. Leaving out the carpet, the acoustical stuff / construction should be quite OK I think also for a small home studio / control room. T60 is around 0,17-0,2 s from 125-4000 Hz, versus 32 m² / 74 m³. (Measured with a simple XTZ device) A bit on the dry side but not uncomfortable to stay in for hours. A forum member (Jens Eklund) came by on a short visit a couple of weeks ago and his reaction was: I had presumed the room was more dampened + I know studio guys who would be happy with this room. He proposed the coffe table could be made smaller and lower to avoid some SBIR, otherwise he didn´t have any immediate ideas for improvements. (He knew removing the carpet was a non option for me.) From his posts, I see Jens as a thorough and picky designer, so that was nice to hear.
Sebg: In the SLAM document it says the membrane area can also be compartmentalized to i.e. 600x600 mm. A recommendation for a 50 m³ room is 6-8 m² total membrane area which can be distributed over several smaller ones. It does not say use large areas / membrane. What makes the rubber effective is its high internal damping.
Common vinyl for floor covering is usually on the low side regarding surface weight, around 2 kg/m², which would need a larger air gap behind versus targeted frequency. If a maximum room volume is seen as a premium, rubber sheeting could an alternative to keep in mind. There is also heavier special vinyl flooring for acoustical purposes (dampening foot steps) and public buildings from Forbo and other manufacturers. These usually have a foamed backside and are very close to 3 kg/m² in surface weight. An idea I had before I installed rubber sheeting was to use carpets with thick vinyl backside, those used for entrances and hallways. The ones I measured were close to 3 kg/m². Good air and moisture proof qualities were some from Berendsen. Actually, some had quite a good looking carpet side, having it towards the room would render some higher frequency absorbtion + the aesthetics, otherwise turn it towards the wall.
I have two questions. Is it correct that the dark grey site is the site with the membrane? How do you place the absorbers in the corners, do they overlap or is there a small gap? For example the 50Hz on the ceiling and the 120 Hz on the sidewall?
Is it possible to make square absorber with two mebrames facing two directions ? So for the 50Hz the the size would be 35*35*60, the sides facing the room will be the embrames? Just something that popped up :-)
I am interested in the hardness question. Tim has had success using vinyl-fronted boxes of relatively small dimensions (600x400mm). Voetmann and Adhoc have had success using SLAMs - natural rubber sheet fronting much bigger boxes (1200x1800mm). The natural rubber sheet is of course much more "rubbery" than a sheet of vinyl. In my part of the world it is also 3-4 times the price (natural and synthetic rubber is relatively very expensive).
The stiffer the panel, the less efficient it will be, ie lower absorption coefficient. Also, the design formula becomes less accurate due to the elasticity. The mass loaded vinyl sheets work OK. There is a product here in NZ called Acoustop, which is a barium loaded neoprene sheet. This is very floppy and heavy and is perfect for the job. Large areas are not necessary, this has been proven in practice by my installs. This means lower cost. It's better to make a series of smaller different depth boxes and alternate them.
__________________ "Opinions are like arse holes, everybody has one" Dirty Harry (Clint Eastwood)
Tim, across the ditch here we have Pyrotek's Wavebar and Acoustica's Mass Loaded Organic Barrier and probably Acoustop as well. They are all "floppy", at least as far as the lighter sheets go. Voetmann may also have tried MLV and found that rubber sheet was more efficient - perhaps I will email to ask him. However, one could go on trying stuff forever - certainly MLV (and MLO) is cheaper here than natural rubber sheet and your success with it and documentation of your builds is great for all the DIYers.
i remember some time ago reading about mass loaded vinyl. it got me to wondering could i simply hang the stuff and get good results. i mean build a frame and let it hang. maybe put a sheet on each side of the frame.
i've read about pressure devices, velocity devices...but somewhere back there i also remember reading about "limp mass". something like mass loaded vinyl makes a lot of intuitive sense to me.
don't know if i'll ever take it on as a blog post but i'd like to build something like this and test it against my gik corner traps and a fat bail of fiberglass insulation from home depot (still in plastic...returnable!). take some waterfalls from a few locations. it ain't a lab setting but if it works in my studio that's all the matter.
I have two questions. Is it correct that the dark grey site is the site with the membrane? How do you place the absorbers in the corners, do they overlap or is there a small gap? For example the 50Hz on the ceiling and the 120 Hz on the sidewall?
Is it possible to make square absorber with two mebrames facing two directions ? So for the 50Hz the the size would be 35*35*60, the sides facing the room will be the embrames? Just something that popped up :-)
Best regards,
Peter
Hi Peter, yes, the dark grey squares are the absorbers. The membrane needs to be exposed, so if you have one on the ceiling and another adjacent on the wall, you simply butt the edges of the boxes together to create a new room corner that is the two membranes. A gap does not matter, it's not really that critical. In the drawing the dashed lines show the outline of the units mounted on the adjacent surfaces.
Yes you could do it as you suggest to make it tidier. As I said, it's not that critical but the length and width of the enclosure should not be the same dimension (square).
So is tuning just down to box volume? Would it be possible to make a tunable absorber by having some sort of sliding rear panel inside the box (assuming edge sealing can be taken care of)?
I guess so, but to make one that goes from 40Hz to 150Hz is quite a large difference in depth. With 4kg membrane, the depth would need to vary from 4cm to 55cm.
An adjustable airgap (air tight one that is) is tricky due to practical mechanical reasons.
The membrane absorber needs to be exposed to the room as Tim wrote. This does not mean it has to be visible though.
To broaden the frequency range of the membrane absorber (well, the space / area it occupies), the easiest would be to add a frame to the front of the absorber. -So you get a gap in front of the membrane and the membrane can still vibrate freely. To the new frame one can add resistive absorbers and if one wishes, one can add wooden slats on top of the resistive absorbers to keep the room more live for higher frequencies.
=>
The membrane closest to the wall would affect the lowest frequencies.
The back of the resistive absorbers will be at something like 100-250 mm out from the wall and would absorb higher bass / midrange pretty well beacuse of the distance to the hard back wall. High frequencies will be 100% absorbed.
The wooden slats, say 30-100 mm wide x 6-12 mm thick, in a binary pattern like Boggys has a 50% open area, meaning absorbtion of lower frequencies should not be affected but reflect higher frequencies back into the room. There will be some diffusion as well because of the binary series.
if you aren't doing the sealed pressure device at the boundary thing. if you are just hanging limp mass from a frame you should be able to tune this i think. adjust the distance from the boundary. yes?
i found something about this in mike seniors book. i'm very tempted to put this on my "projects" list. is anyone here doing very independent tests and posting results?
I have ordered wood, fluffy glasswool, 70kg/m3 rockwool and 5kg/m2 EPDM to make 8 bass traps and test some configurations with or without back panel and rock wool facing.
I'll test results in 2 or 3 weeks if everything's ok.
I have ordered wood, fluffy glasswool, 70kg/m3 rockwool and 5kg/m2 EPDM to make 8 bass traps and test some configurations with or without back panel and rock wool facing.
I'll test results in 2 or 3 weeks if everything's ok.
i'd be interested in your test methods. seems like you'd have to leave the test mic in one position or figure out how to move it super consistently. and the traps will need to be consistently positioned too. anyway that's how i imagine it. :D
Nice groove here in this thread now. Thanks for cleaning it up Tim.
A mic in the centre of the room will be in the null point of many modes.
I reckon it would be best to place the mic in a floor or ceiling TriCorner.
Probably best to place the speakers in an opposite Tricorner.
Nice groove here in this thread now. Thanks for cleaning it up Tim.
A mic in the centre of the room will be in the null point of many modes.
I reckon it would be best to place the mic in a floor or ceiling TriCorner.
Probably best to place the speakers in an opposite Tricorner.
DD
i was thinking about 6' out from a corner. you could find a trouble spot before hand and put the mic there. i figured keeping it near the traps you are testing would be a good idea.
a null should be improved with bass treatment. after all it's modal too right? so it might work in the middle.
by my thinking the the most important thing is to hold everything except the treatment constant. that's science! (kinda)
6' or closer to the corner is fine, very similar, in the context of very low frequencies. But think about PZM mics and Soffit mounted speakers. Almost touching is almost PZ, at LF. I frequently place mics for recording at the boundary.
DD
When I said centre, it's not exactly center, but at the listening position, about 40% of the length of the room. I'd like not to move the setup of the room a lot cause it's a pain, and moving SM9 alone is nearly impossible...
Placing treatment behind speakers will act on modes and SBIR. It should be the best improvement.
It's a totally non treated room for the moment. There's a difference in side walls construction (plaster vs concrete) which is obviously audible. Hope it would help with that.
I am interested in the hardness question. Tim has had success using vinyl-fronted boxes of relatively small dimensions (600x400mm). Voetmann and Adhoc have had success using SLAMs - natural rubber sheet fronting much bigger boxes (1200x1800mm). The natural rubber sheet is of course much more "rubbery" than a sheet of vinyl. In my part of the world it is also 3-4 times the price (natural and synthetic rubber is relatively very expensive).
AFAIK SLAM is polyolefin material (kind of plastic) like floorsheeting vinyl. Not rubber. In the building information (from Voetman) you can make appartments 60 x 60 cm (2 x 2 ft) if you like. Or separate boxes. A roll of SLAM is 1200 mm wide. 3 kg/ m2 (Delivered from Denmark)
The trick is to build as effeciant absorbers as possible.
(Where have I heard that before... )
An optimum combination of limpness / mechanical lossfactor/ surface weight/ depth of the box/porous absorber (damping).
So thats why the SLAM foil is especially selected by Voetman et al. to form a working combination.
If the foil is tensioned the resonant frequency will go up.
If the box is shallow the airspring is too stiff and that will damp the movement of the foil to much (low efficiency)
Low efficiency = more traps (area) needed.
Of course you can use carpet w rubber backing with approximately the same surface weight and end up with LF absorption, but you don't how good they are. Probably cheaper.
You might still benefit from measurements.
Slightly OT. Specially selected probably goes for steel plates and porous absorbers also and that is the practical and economical problem.
Availibility in different countries, bulkyness, cost of freight etc.
AFAIK SLAM is polyolefin material (kind of plastic) like floorsheeting vinyl. .......
From wikipedia, i find for polyolefin:
Quote:
Originally Posted by wikipedia
Polyolefins are impossible to join by solvent cementing because they have excellent chemical resistance and can only be adhesively bonded after surface treatment because they have very low surface energies. They are also extremely inert chemically and exhibit decreased strength at lower temperatures.
AFAIK, vinyl/linoleum flooring material can be easily glued with neoprene glue.
And:
Quote:
Originally Posted by wikipedia
Polyolefin is used for blown film and heatshrink electrical insulation sleeves for crimped wire terminals, as well as rash guards or under garments for wetsuits.
I'm uncertain if it may be thermally unstable (tensioned) after some time and higher temperatures in studio, especially where we have warm equipment.
AFAIK, vinyl/linoleum flooring material can be easily glued with neoprene glue.
And:
I'm uncertain if it may be thermally unstable (tensioned) after some time and higher temperatures in studio, especially where we have warm equipment.
Important comments!
I asked mr Voetman about temperature stability and glue fairly recently and his reply was "no probem" and "you can use ordinary contact-glue to fasten the membrane to the box and then staple it".
RE; polyolefin... I could have been mistaken...
I should have written "If I remember correctly... I'll try to find that paper.
It was 15 years ago and I used SLAM in control room build.
If the product is exactly the same to day I'm not sure.
When I contacted Voetmann about the membrane 3 years ago, he referred me to a Danish flooring company. I got the type number of the brand /manufacturer of the sheeting and when I checked the catalogue sheet and MSDS (Material Safety Data Sheet) the compound was described as natural rubber (NR). Actual hardness in Shore A of the material was not stated, surface weight was 3 kg/m².
As I wanted to keep as much of internal room width as possible, I picked a heavier synthetic rubber (from work) instead though, NR+SBR with 70 Shore A hardness. => Air gap behind membrane could be reduced from 180 mm with 3 kg/m² to 98 mm with 5,4 kg/m².
70 Shore A means the NR/SBR-rubber is a bit stiffer than for 100% NR with about 40-45 Shore A, and probably for this reason has less internal loss but that was a price I was ready to pay. (Total membrane area was also increased from recommended 20% to 25% which should make up for some of the material properties.)
If any polyolefin starts to shrink in your studio. Get the hell out of it, –it is probably on fire! When I use polyolfine tubing for insulation around electrical wires, I use a hot air gun to shrink it.
AFAIK SLAM is polyolefin material (kind of plastic) like floorsheeting vinyl. Not rubber. In the building information (from Voetman) you can make appartments 60 x 60 cm (2 x 2 ft) if you like. Or separate boxes. A roll of SLAM is 1200 mm wide. 3 kg/ m2 (Delivered from Denmark)
......
We have Tarkett here... so I checked recently, and vinyl and linoleum flooring material is easily available with wide range of surface densities and thickness
Could I consider that glueing 2 layers of linoleum of 2,5kg/m3 with neoprene spray will be equivalent to a single 5kg/m3 layer ?
I could buy some to include in my tests if you're interested...
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Didn´t you see them as most people would: To reach this goal, you first have to do this, but before that you need to do this, which of course means you have to start with this first. 
) 
When I use polyolfine tubing for insulation around electrical wires, I use a hot air gun to shrink it.
