My broadband absorbers - DIY

raven46 said:
its a great idea man,

i have two rolls of fiberglass im not using for anything so im going to try this,

is it ok/safe to use fiberglass in a small room, my room is about 13ftx10ft

i was thinking of making 4 frames at 90cmx50cm, wood 120mmx22mm
ant a 5th frame at 150cmx90cm, wood 120mmx22
with holes drilled out on all 4 sides

when these are packed with the fiberglass and only covered with weave frabic is that alot of fiberglass to be using in a small room that wont really be ventelated apart from opening a door or window when its not in use
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Well i'm not gunna say i'm a professional on the physiological effect of fiberglass. In fact, I'm not even sure if what i have - glass wool - is the same as fiberglass. I can tell you that you'll need to ventilate the stuff for a few hours (it was a windy day when we had it out..not sure if that makes any difference) as it smells pretty bad.

What's going to make a big difference in the frequency spectrum that the abosrber affects is the depth of the trap. I believe the ratio is somethig like the depth equals a quarter of the total wavelength of the lowerst frequency absorbed:


depth/4 = lowest frequency absorbed



You may also want to consider building some bass traps? It's quite easy to do soe, using the "pizza slice" method.
 
MessianicDreams said:
I believe the ratio is somethig like the depth equals a quarter of the total wavelength of the lowerst frequency absorbed:


depth/4 = lowest frequency absorbed
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Just an update to state that that is technically completely wrong, or so I've been led to believe...


:)
 
MessianicDreams said:
Just an update to state that that is technically completely wrong, or so I've been led to believe...


:)
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acutally, its not completely wrong.


The equation is as follows (I realised I quoted in differently above):

0.25 x depth = wavelength of lowest frequency absordbed.



What was missing is the fact that this does not meant hat anything below that frequency is not absorbed. It means that the material is working at its full effeciency until that point; below there it is still working to an extent, just not as efficiently as it should be :)
 
MessianicDreams said:
acutally, its not completely wrong.


The equation is as follows (I realised I quoted in differently above):

0.25 x depth = wavelength of lowest frequency absordbed.



What was missing is the fact that this does not meant hat anything below that frequency is not absorbed. It means that the material is working at its full effeciency until that point; below there it is still working to an extent, just not as efficiently as it should be :)
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It isn't the depth/4. It's the speed of sound/depth/4. So, in metres, it's 340/depth(in metres)/4 = Hz of lowest frequency theoretically absorbed. So 340/0.1(4")/4 = 850Hz, and for 16"(roughly the deepest part of a 4" panel stradelling the corner) 340/0.4/4 = 212.5Hz
 
What did you do about reinforcing the corner joints? I see you have no bracing in the middle, so I assume you did something special in the corner joints to help them retain thier shape and rigidity.
 
nice absorbers MD, good job. They are sure to make any room sound better that need treatment. In hanging them...I built 4 2'X6'X6" bass traps 5 years ago, hung them from corner ceiling with hooks like you are talking about, and simply pulled bottom of trap into wall and hooked making my angle and they are still hanging. I used a little diffrent design from yours that I think I got from JSayers website, and mine were all sealed...glued, screwed and caulked up tight, solid sides and front/back panels and 703 stuffed. Also made a mid-high absorber, same basic design/fabic wrapped X 4"depth except drilled 1" holes in front panel, dunno how many, alot haha...Had the values at one time... they make a world of difference in my little lab. Good thread!
 
pandamonk said:
It isn't the depth/4. It's the speed of sound/depth/4. So, in metres, it's 340/depth(in metres)/4 = Hz of lowest frequency theoretically absorbed. So 340/0.1(4")/4 = 850Hz, and for 16"(roughly the deepest part of a 4" panel stradelling the corner) 340/0.4/4 = 212.5Hz
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Whilst that's true for calculating frequency, the post you quoted was refering to wavelength.

however earlier on in the thread I did misquote the equation, gunna see if I can edit that up..
 
michaelst said:
What did you do about reinforcing the corner joints? I see you have no bracing in the middle, so I assume you did something special in the corner joints to help them retain thier shape and rigidity.
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No bracing or reinforced corner joints. I suppose you could add brackets, but with the size of wood and the dimension of the traps i build, they are pretty damned solid as it is and so I figured it would be pretty pointless to add'em in!

Thanks for stopping by :)
 
MessianicDreams said:
Whilst that's true for calculating frequency, the post you quoted was refering to wavelength.

however earlier on in the thread I did misquote the equation, gunna see if I can edit that up..
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Ahh right, sorry. But really, why would you just wanna kno the wavelength? If it's 4", then then lowest absorption, theoretically, is a 16"(0.4m) wavelength, which is 850Hz.

And also, it's depth x 4 = wavelength.
 
in practice, the porous absorbers are working on multiple velocity losses rather than solely on a single wavelength, so a 4" absorber 4" from the wall has effect all the way down to < 100hz. adding a limp membrane to the front changes the dynamic somewhat (as its now partially a panel absorber) and your now working on exciting a dampened membrane (which can go lower than porous absorbers if the mass is right) and relying somewhat on edge effects for velocity losses. while the ideal is to impede at the 1/4 wavelength point (highest velocity) the porous absorber is broadband by nature and the efficiency of it varies (as testified to by different absorption coefficients at different frequencies).
 
What material did you use to wrap it?
My dad get´s crazy when he hears about rockwool, so wrapping it in something reliable might ease him a bit :)
 
kubeek said:
What material did you use to wrap it?
My dad get´s crazy when he hears about rockwool, so wrapping it in something reliable might ease him a bit :)
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My mum's the same. But i just tell her that rigid fiberglass isn't carcinogenic(what she's most scared about, 'cause i had cancer a few years back), whereas sand is(we live at the sea). It is an irritant though, so best to wear protection. But once it's wrapped, then the fibers can't escape and you'll be fine.

Any fabric will be fine, as long as it's breathable(can breathe easily through it), and the weave is tight(?) enough to hold the fibers.
 
He is most concerned about the second..
I liked those non-woven textiles i saw somewhere, so I´ll try to find some seller.
 
kubeek said:
He is most concerned about the second..
I liked those non-woven textiles i saw somewhere, so I´ll try to find some seller.
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Just make sure whatever you wrap it in is a breathable material. I just went to a fabric shop and got the cheapest, least ugly fabric i could find. I'll try and remember what it's called..

It was funny seeing the look on their face when i came in and asked for 30m of the stuff!
 
gullfo said:
and remember to treat it with a fire retardent solution - spray on or dip.
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Whilst I'm not going to dis-recommend this, I'm not sure what the acoustic implications of it are? I\ve no idea what fire retardent solution is like, but if it makes the material unbreathable, don't do it!!


Of course if it's for a professional installation you may not have the choice due to legal issues..i'm just using mine at home so i'm careful :)
 
The fiberglass (or glass wool insulation...the nomenclature difference is geographical) is harmless until the fibers become airborne. Use respiratory protection while cutting or moving the product around. Once it's in place, vaccuum up the workspace with a good HEPA vac and you should be good to go...until your house cats get ahold of it and sharpen thier claws on your carefully-engineered absorbtion panels...
 
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