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PostPosted: Mon Feb 26, 2018 7:46 am 
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Location: San Jose Calif
I'd like to confirm my bias on another potential use for laminate flooring. I have a studio under construction, room dimensions of 17' L x 11' W x 8' H project/commercial with soundproofing and electrical complete. I am considering putting a bunch of {6"} 703 on the back wall and side walls which is CMU block construction to give me some absorption and decrease the time decay in the room as well as increase the low frequency absorption. What I am after is a "range limited" type of bass trap effect built into the whole wall.This would be built in addition to several 34" cut super chunks of either 703 stacked, or safe and sound stacked with netting every third of the height.
The wall absorption/construction would be:
6" to 10" of 703 glued to the wall and then 7mm to potentially 12mm laminate flooring to cover the 703. The intended use is that the laminate, which is mdf/plastic covering, would act as a membrane to reflect high and mid frequencies...
How practical and effective would this be?
Or would that thickness of laminate be a hard stop barrier to everything below 100hz?
Budget is flexible, accuracy below 100hz and effectiveness is paramount & primary.
If this is not feasible, what would be more effective? Again, I want to range limit the absorption effect, at the scale of a whole wall, to maintain mid and high frequency energy in the room. Iv'e thought of using pegboard, or other hard thin laminates, hardboard, birch plywood all 1/8" thickness, but would have to route out either slots or B.A.D. type of patterns, and then cover it with fabric to get it to look decent. Laminate looks better.
Thanks,

RAD


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PostPosted: Tue Feb 27, 2018 2:42 pm 
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bumpity road
this musician thingy...

RAD


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PostPosted: Wed Feb 28, 2018 4:08 am 
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Quote:
What I am after is a "range limited" type of bass trap effect built into the whole wall. ... The intended use is that the laminate, which is mdf/plastic covering, would act as a membrane to reflect high and mid frequencies...
There's two sides to this: One is "membrane" and one is "reflections". Two different processes, really. Related, but not the same.

Thin flexible things, such as plastic sheeting, act as "foils", where the key is the surface density. They allow some frequencies through and reflect others, based on the density. You can feasibly cover the entire insulation-covered wall with plastic of a specific density, and get it to reflect back all frequencies above a certain point, while allowing through all frequencies below that point. It's not a "hard" cut-off point, of course: it's actually a gentle curve, where the "tuned frequency" is the mid-point of that range: reflection rises above that, absorption rises below that. Thicker plastic (higher surface density) pushes that "mid point" down the scale, towards lower frequencies. Thinner plastic raises the point, towards higher frequencies.

That's "foils", which are flexible and work based on density.

But laminate flooring is not light and flexible: it is rigid and fairly massive, so it does not act much like a foil. It acts like a solid reflector, mostly. There's also some diffraction going on around it, and other things, but mostly it is just a hard, flat, solid reflector. And you can tune it too! But not by varying the surface density. You tune it by varying the dimensions. There's a basic "rule" in acoustics, that sound waves are only affected by objects that are larger than there own wavelength. So a sound wave that has a wavelength of 1 foot won't even notice a wood slat that is only 1 inch wide, but it WILL notice a slat that is two feet wide. It will also notice a slat that is 1 foot wide, and to a certain extent, one that is 11 inches wide, and lesser still 10": it rolls of fast the smaller you go.

So you can tune your slats by varying the size. Larger slats reflect down to lower frequencies, smaller slats reflect only the very high end.

Here's a link to a thread where I am working through the tuning process with one of my customers right now, where you can see how I'm using these principles to tune a room exactly the way I want it. viewtopic.php?f=12&t=21324&p=144021#p144021 That's a "work in progress" that shows how this process is done, one step at a time. For the end result, here's a link to another thread, where the process is completed: the room has been fully tuned, down to the finest detail: www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=20471

Quote:
Or would that thickness of laminate be a hard stop barrier to everything below 100hz?
Not at all! In Frank's room, I'm using slats that are over 4" wide, precisely because I do NOT want to reflect stuff below 100 Hz. I only want to reflect parts of the mid range and the highs, to return some "life" and "energy" and "air" to the room. I want all the lows to get through to the back, an be absorbed.

But there's more going on with that latest part of the thread (the curved slat wall): In addition to their sizes, those slats are also tuned Helmholtz resonators. The size of the slots between them, and the depth of the cavity behind them, and the thin insulation on their rear farces, create a tuned
resonant system that absorbs some very specific frequencies. Plus, the overall curved shape is designed to do two other things: deal with the very low frequency SBIR issues and modal issues in that room, and also NOT produce a "lobing" pattern to the reflections, which you would get from a flat wall built the same way. Each slat is at a different angle, so the high frequencies (above the 4.5" wavelength) are all reflected in different directions: In other words, it acts as a diffuser as well. To be more precise, it is a "tuned resonant absorptive slotted poly-cylindrical diffuser reflector". And if you can say that ten times fast after a few beers, then you win the Internet for a day! :)

Quote:
Budget is flexible, accuracy below 100hz and effectiveness is paramount & primary.
First, do a REW test of your room, to find out what is wrong with it. Then you will know what specific problems need treating, and you can design the treatment accordingly. Here's how: viewtopic.php?f=3&t=21122 .

Quote:
Iv'e thought of using pegboard,
That's also an option. Technically, that's a "perforated panel" or just "perf panel" device, which is also tuned to a specific frequency, given by the diameter of the holes, the separation between them, then thickness of the wood, the depth of the cavity behind it, the insulation damper, and a few correction factors for the hole pattern and mouth correction. I used some of those in the Studio Three room, but I tuned them to specific ranges of frequencies by varying the sizes and spacing of the holes across the panel: Some holes are larger than others, some are spaced closer together than others. There's a pattern that extends the frequency range across the zone where I needed it.

Quote:
but would have to route out either slots or B.A.D. type of patterns,
Those are two different things: Slots are resonator/reflectors, while BAD panels are combination diffuser/absorbers...


- Stuart -

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PostPosted: Wed Feb 28, 2018 12:54 pm 
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Thanks,
That's quite the thread and device, multi-tasking at it's finest!
As usual I have more to chew on than I can swallow...
Just offhand, any idea what thickness of plastic would be usefully able to block everything above 200hz and allow everything else below to pass through? I found plastic sheeting in a 6mil thickness, then it goes to the category of tarp @ 14 mil up to 16 mil...wish I knew the frequency transmission threshold of it...
I could always cover that with fabric... to make it look sharp....
RAD


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PostPosted: Wed Feb 28, 2018 2:16 pm 
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I wrote this a while back: it might be helpful...

---


There is a reflection curve for foils (plastic is a foil too, acoustically) that defines the point where foil of any given surface density will be 20% reflective (80% transparent) to sound (allowing 80% of that frequency through to the other side, reflecting back 20%, or coefficient of reflection = 0.2). The curve rises to about 99% reflective above that (1% transparent), at about 6 dB/octave, and falls off to practically 0% reflective (99% transparent) below that, at the same rate:

F = 90 / m

F = The frequency at which the foil transmits 80% of the sound
m = The surface Mass of the foil in kg/m2

THEREFORE:

F*m=90
m=90/f

0.5 mm is 500 microns
1mil plastic. = aprox 25 microns. = .025 mm
In USA 'mil' is a measurement equal to 1/1000 of an inch

Thickness of common items in mils:
Dime: 53 mils
Credit Card: 30 mils
10 pieces of paper: 10 mils
Plastic grocery bag: 2.25-3 mils


Thin plastic is measured by weight : gsm, = grams per square metre

Polyethylene density: Aprox 970 kg/m3. = so 1 square meter at 1mm thick = 9.7 kg.

THEREFORE:
--- 1mm = 0.97 kg/m2
--- 1micron = 0.00097 kg/m2
--- 1.0 mil = 0.0242 kg/m2
--- 1.5 mil = 0.0360 kg/m2
--- 2.0 mil = 0.0485 kg/m2

--- 6.0 mil = 0.108 kg/m2

------------------

Not very detailed or clear, but hopefully it gets you in the right track. So 6 mil plastic is good for reflecting about 830 Hz and above.


- Stuart -

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PostPosted: Thu Mar 01, 2018 1:18 pm 
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Not to be anti-pedantic, but if I were to double the thickness of the 6 mil plastic to 12mil would that effectively lower the threshold of frequency transmission by half
to 415 hz? Or tripling the thickness to say 18 mil, would that get me the 207.5 hz ?
In other words I'm not sure I get the formula.
That would be in the ballpark I seek to play in;
I appreciate the help but I'm not very metrically inclined.
So according to your formula, a light duty single ply cardboard about 1/16", roughly equivalent to eighteen sheets of paper, would also work?
IE any 18 mil material .4356 kg/m2 ?
Thanks

RAD


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