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Post by anonymous on Nov 29, 2009 8:01:00 GMT -7
I've made a DIY system with two Fostex 206E in Fostex-recommended horn enclosures and an IB sub with one 21" driver from Chinese company Senon. Well, it plays great, and I like it, but the room is untreated and its modes are very strong. I'm doing this on a tight budget, so the bass traps I'm going to DIY too I've read tons of FAQs, including everything that was written here on the subject. The obvious material is rock wool, the heavier the better. Everyone recommends to make columns or panels with it and cover them with just a single layer of acoustic cloth or something similar. I'm hesitant to do that - rock wool does produce some unpleasant dust, and I'd like to seal it in. So, here is my plan - please comment if it's viable: 1. Columns: I'm going to buy some rigid rock wool mats (5cm - 2" thick), they are packed together and the size of one pack is about 60cmx30cmx100cm - 24"x12"x40". I'll wrap them tighly in some thin plastic (Saran wrap?) and form columns 24"x24" floor-to-ceiling. Then I'll probably cover them in some thin cloth for visual appeal and plastic protection. There are two questions here: a) how could I estimate the frequencies at which the plastic and the cloth will interfere with absorption? b) is there any benefit in going to some very heavy rock wool? www.bobgolds.com/AbsorptionCoefficients.htm seems to show (right in the first table) that heavier 4" mats (96 kg/cu.m) seem to absorb _less_ at 125Hz than lighter ones (48 kg/cu.m). I could buy 75 kg/cu.m ones as a safe bet - and they are way cheaper - or go up to 125 or even 175 kg/cu.m - would there be any benefit? 2. Panels. Probably it would be best to use the available wall space (not too much of it, unfortunately) to install membrane absorbers to fight specific modes instead of trying to use rock wool panes, right? I'm thinking about making the membrane of 1mm thick steel. It's heavy enough to make an absorber for 43 Hz main mode and keep its thickness reasonable. A couple of 1.25mx1.25m square traps should do the trick. Is the steel a good material for this? 3. Carpets, drapes, etc - I think with two or three columns I'll have enough wide-range absorption for the lower frequencies, so I'll be able to get by with some thick carpets on the walls and heavy drapes in a few places (such as to cover the membrane absorbers). Am I right? I really don't want to make rock wool panels with only a cloth between the dust and my air, or to use the foam which disintegrates in a few years
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Post by jimcant on Nov 29, 2009 19:00:22 GMT -7
Your questions are way beyond my expertise to comment on, apart from suggesting dacron batting between the rockwool and outer material covering on your traps.
I used this material (as per Thomas W's sound absorbing panels thread) to cover my StudioTips Superchunk traps, and am quite confident that all the fibers are safely retained.
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Post by anonymous on Nov 30, 2009 10:11:00 GMT -7
Dacron will probably work as a filter for larger particles, but there is supposed to be constant air movement through it caused by the bass itself So smaller particles would eventually find their way outside. I'm not too afraid of that - there won't be too much dust anyway, but still, if it's feasible to replace dacron with airtight plastic without major performance problems, I'd like to go in that direction. Well, it seems no more replies are forthcoming :-( Could someone recommend another forum with a strong room treatment community?
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Post by weverb on Nov 30, 2009 10:26:49 GMT -7
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Post by ThomasW on Nov 30, 2009 17:59:48 GMT -7
Well, it seems no more replies are forthcoming :-( Excuse me for taking my first vacation in 2-1/2 yrs and that occurring in the middle of a desert with virtually no internet access. I could answer your questions or give you referrals but given your attitude I'm less than inclined to provide any assistance. Ciao, Thomas
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Post by anonymous on Nov 30, 2009 19:58:34 GMT -7
weverb, thanks a lot for the links - there are some very interesting measurements there. And, just as I noticed in the tables, the lighter materials are working better than the heavier ones when used in thick layers. Sounds weird, but seems to have pretty sound physics behind it. www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/177600-could-someone-help-out-interpreting-materials-gas-flow-properties.html comes to the conclusion that at my sizes (12" or 24") the best density would be something like 30kg/cu.m or even less. I could answer your questions or give you referrals but given your attitude I'm less than inclined to provide any assistance. That's OK - no one has to help if he or she doesn't want to. Sorry if I said something wrong - no offense was intended. I did wait a couple of days and the only reply was negative... Anyway, thanks to all IB people for help and the best bass for everyone.
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Post by ThomasW on Dec 1, 2009 5:52:30 GMT -7
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Post by anonymous on Dec 1, 2009 10:49:52 GMT -7
Thanks a lot - at www.johnlsayers.com/phpBB2/viewtopic.php?t=7278 I've found the elusive bit of information about the effects of plastic wrap on absorption: www.bbc.co.uk/rd/pubs/reports/1992-11.pdf contains lots of useful graphs and one of them gives 500Hz to 1kHz as the band where the difference begins for 0.14mm film. Anything below is not affected at all, and above the absorption coefficient falls rather rapidly. Well, 1kHz and above can be dealt with by "normal household means" - drapes and carpets. Columns from cheap low-density rock wool wrapped in very thin plastic seem to be quite feasible. Haven't found anyone who used steel for membrane traps yet - most people prefer thin plywood, even though it limits the lowest resonant frequency to 200+ Hz...
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Post by chrisbee on Dec 1, 2009 13:33:40 GMT -7
Here's the BBC A10 very low frequency absorber paper. downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdfSlightly off topic but my attic enclosure is lined on the sloping surfaces with 12-16" of rockwool bats in 4" layers. A naked, heavy polythene vapour barrier can be seen to flutter at times during programme material. I haven't heard any noise from this fluttering. I intended to board the enclosure but it was another roundtoit and the wadding is probably doing a good job.
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Post by ThomasW on Dec 4, 2009 16:25:39 GMT -7
most people prefer thin plywood, even though it limits the lowest resonant frequency to 200+ Hz... 3/8" ply will do a 87Hz trap, thicker ply will go even lower. I can't imagine using metal for a membrane trap since it's Fs is so high. It would be best if you used a software frequency response program to 'map' the frequencies of the room, so dedicated traps could be used where needed.
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Post by anonymous on Dec 5, 2009 16:59:04 GMT -7
3/8" ply will do a 87Hz trap, thicker ply will go even lower. I thought 10mm (3/8) is too thick to be effective as a membrane - it's rigid, the cross-directional wood layers are fairly effective dampers, so it won't really resonate as it should, it will not couple to the air properly, so the formula would be way off... I computed for 4 and 6mm - and even 6mm gives very thick traps for lower frequencies. Well, the numbers go like this. I've used dogbreath.de/misc/PlaneAbsorberResonance.pdf as my reference. The formula is Frequency=62*/sqrt(area density*trap thickness) where everything is metric - kg/m2 and m. Transforming, we get thickness=sqr(62)/sqr(frequency)/area density. For 10mm (3/8") plywood we have area density about 5kg/m2. So, for 87Hz we have thickness of sqr(62)/sqr(87)/5 = 0.10m which is OK (if 10mm plywood is OK). But for 42Hz it's 44cm even for 10mm. I can't imagine using metal for a membrane trap since it's Fs is so high. For 1mm steel its area density would be 7.85kg/m2, so the thickness for 42Hz trap should be 27cm - a bit too thick but still acceptable. And it _will_ resonate exactly at this frequency. It would be best if you used a software frequency response program to 'map' the frequencies of the room, so dedicated traps could be used where needed. I did that, I know my major room modes, I've read a lot of information on the topic. It was difficult to find solid data, not "general advice" from unknown sources and without any supporting numbers - but thanks to the Cult I've got everything I need. I still cannot find any hard data about steel as membrane, though, on all the recommended forums. Guess I'll just have to try it on my own :-) For the rest - well, wideband traps are a necessity, of course, so two or three columns in the corners should be a good start. Next I'll do membrane traps for the worst modes (measured with columns installed), then cover the walls with carpets/drapes/limp membranes where needed. Not an ideal solution, but hopefully acceptable.
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Post by chrisbee on Dec 6, 2009 13:44:42 GMT -7
It might be interesting to use a contact microphone and REW sweeps to find the natural resonant frequencies of different board materials. The sort of thing they use for amplifying acoustic guitars would seem likely candidates as microphones. Take the guesswork out of finding the true absorption frequencies for fine tuning. This might also be useful for checking the acoustic output spectrum of baffle walls.
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Post by ThomasW on Dec 8, 2009 9:18:19 GMT -7
I still cannot find any hard data about steel as membrane, though, on all the recommended forums. Guess I'll just have to try it on my own :-) I've been doing this stuff for 40+ yrs and have never seen steel used as a resonant membrane. I've seen perforated steel (more like expanded metal lath) used to build movable traps. The most notable of these come from DaaD
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Post by anonymous on Dec 8, 2009 15:23:17 GMT -7
I've been doing this stuff for 40+ yrs and have never seen steel used as a resonant membrane. Well, that's my original question - is there anything wrong with it? It's reasonably priced, has appropriate area density, can be installed easily, and it's soft and springy enough to resonate as it should... I've tried to find any data - and I'm still looking... The only disadvantage I can see is a strong reflection of high frequencies - but it's not that much of a problem... BTW, I've also seen (but cannot find the link again) some traps made of a thin steel sheet - not perforated - bonded to some foam and attached to the wall. The company that sells them provides some measurements and they seem OK.
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Post by chrisbee on Dec 9, 2009 7:12:12 GMT -7
One would think that the size of the resonant panel was a large factor in the frequency of absorption. Large shallow panels might be easier to hide than small and deeper examples. My own attic listening room is lined with thin ply backed by very thick rockwool. I ought to try and discover the natural resonant frequency. It ought to be possible using just sinewaves and the fingertips. It might even explain my 150-200Hz trough!
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Post by anonymous on Dec 9, 2009 16:11:55 GMT -7
One would think that the size of the resonant panel was a large factor in the frequency of absorption. Nope In the case of membrane traps the size doesn't matter Well, larger ones are better - they collect more sound energy, but their resonant frequency is still the same; it depends only on area density and thickness. It's also possible to change their Q - to make the resonance more or less sharp - by adding more or less rockwool inside, but that doesn't shift the frequency either. Check out the link above with all the formulas - it's easy enough to follow the math there.
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Post by chrisbee on Dec 10, 2009 0:39:22 GMT -7
I don't need to do the maths. I just read learned forum posts to explain why the world of absorbers is any different from the large musical instruments required for making low frequency sound. I don't use a kiddy's toy tom tom to make a noise like a kettle drum or orchestral bass drum. Nor a double bass to play falsetto violin harmonics. The Rank Organisation would never have chosen a tiny breakfast gong for the start of their films even if they were comedies. Are the mathematical models wrong?
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Post by anonymous on Dec 10, 2009 18:32:10 GMT -7
Are the mathematical models wrong? Well, the problem with mathematics is... it's always right if used correctly So, if something seems wrong - it's usually time to rethink our assumptions. In the case of trap size - well, as I said before, its size does not change its frequency, but does affect its efficiency. There are two main reasons for that: first, the amount of actual sound energy that the trap encounters in its location (even 100% effective trap won't help much if it's very small or badly positioned), and second - the trap's coupling to the air is better for larger sizes (just like a speaker - you cannot replace its area with displacement and still get the lowest bass). Still, the size required for proper coupling is small enough - after all, most people are happy with subwoofers of 12"-18" diameter, so a trap bigger than that is good enough.
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Post by chrisbee on Dec 11, 2009 1:05:43 GMT -7
This goes completely against the grain of everyday experience. So I'll persevere in the hope we may learn something useful. Let's take a very simple example: A sheet of plywood we intend to use for our absorber membrane. As a full 8 x 4 sheet it is very floppy and has a very low frequency of resonance. I give it a rap and it responds with a very low, dull thud. Cut in half, it is only half as floppy. It responds to a tap with a higher note but still rather obviously low. Cut in half again it is getting stiff enough to support itself safely from its middle. The noise it makes when I rap it with my knuckles is very obviously higher in frequency than before. I can go on cutting until I have a range of xylophone keys.... doh ray me.... How small do you want to go?
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Post by ThomasW on Dec 11, 2009 9:20:50 GMT -7
This goes completely against the grain of everyday experience. So I'll persevere in the hope we may learn something useful. Let's take a very simple example: A sheet of plywood we intend to use for our absorber membrane. As a full 8 x 4 sheet it is very floppy and has a very low frequency of resonance. I give it a rap and it responds with a very low, dull thud. Cut in half, it is only half as floppy. It responds to a tap with a higher note but still rather obviously low. Cut in half again it is getting stiff enough to support itself safely from its middle. The noise it makes when I rap it with my knuckles is very obviously higher in frequency than before. +1
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