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post #31 of 58 Old 07-25-2004, 03:14 PM
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Quote:
Originally posted by kgveteran
Then cut it from one end to the other in a 2" swath pattern and it yeilds a mirror image of it self and you now have two 7"x4' sections that can be hot glued together
Sorry for sounding ignorant, but I guess I would have to see how the foam cutter works to understand what you mean here..

They look nice as well by the way, my room needs some serious attention along these lines also.

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post #32 of 58 Old 07-25-2004, 03:31 PM
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Jim, the cutter is a wire stretched between two steel rods and it heats up enough to cut but not enough to cause smoke and such.It cuts like a hot knife through butter.It is tedious to measure it out on the foam with a marker but it helps to cut strait lines. I did my whole room for about 600.00 ( RPG's , Tube traps ,Acoustic panels ). Keep reading on other web sites.
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post #33 of 58 Old 07-27-2004, 08:45 AM - Thread Starter
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kgveteran,
Would you know if the bass trap you build is more efficient then flat 6†thick 703 fiberglass covered with 1†foam? For the given space if tube trap is more efficient then maybe I should build 4 of those.
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post #34 of 58 Old 07-28-2004, 09:47 AM
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Xavier,

> Would you know if the bass trap you build is more efficient then flat 6†thick 703 fiberglass covered with 1†foam? <

There's nothing magical about tube shape. Low frequency absorption is all about mass/thickness and placement. A tube shaped trap based on one-inch rigid fiberglass - or fluffy fiberglass squished down to one inch - can't possible compete with six inches of rigid fiberglass, assuming the same corner placement. I'll also mention that adding one inch of foam to six inches of rigid fiberglass will have a negligable affect.

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post #35 of 58 Old 07-28-2004, 10:18 AM - Thread Starter
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Thanks Ethan for the info. The one inch foam is to keep the fibers inside. Kind of like a pillow cover. Now is it much easier to move the panels around. Since they are 8" thick they can even stand on their own :) As long as the foam has no negative effect I am good.
Guilford of Maine FR701Style 2100 is $15 a yard which I will get in the end. I did not like burlap, too much loose fibers.
I got the "Master Handbook of Acoustics". It is a great book but it has no mention of building Bass Trap by just using fiber glass.
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post #36 of 58 Old 07-29-2004, 06:27 AM
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Xavier,

> The one inch foam is to keep the fibers inside. <

Gotcha. Yes, that's fine. One-inch foam doesn't absorb low frequencies very well, but whatever is not absorbed passes through to the fiberglass.

> I did not like burlap, too much loose fibers. <

Go to any fabric store and you'll find all kinds of nice looking fabric. You do not need fancy acoustically transparent fabric to put over a bass trap! It's not like speaker grill cloth you put in front of a tweeter, where you really do need transparency. With a bass trap, if the fabric absorbs a little on its own no harm is done.

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post #37 of 58 Old 07-29-2004, 06:45 PM
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Sorry, I lost my own reply post.I have only experience with tube traps.The ones I made are black holes for bass.I went back and retweaked my DIY subs by removing insulation because the room was ringing not the sub.I'm not about to make any more.They have added balance to my room acoustics and brought it all together.By absorbing and diffusing only causes some real funky irregularities,the bass is never right.The reason is with out bass traps you are only treating 500hz and above.Or so. Good luck.
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post #38 of 58 Old 07-29-2004, 06:50 PM
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Ethan, is there something about the void inside the trap.I thought the shape of the trap gave it an edge in pressure absorbtion.I would love to see acoustically what my traps are doing.Like walk them in a room with sweeps going and move them from corner to corner.Some guys are using unfaced rolls all piled up as traps,what do you think of that.
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post #39 of 58 Old 07-30-2004, 11:19 AM
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KG,

> is there something about the void inside the trap. <

There's nothing magical about a round shape. The two main things that affect absorption at low frequencies is the thickness of the absorbing material and its distance from the room boundray. A round shape does enforce a minimum spacing, but one inch of fiberglass is too thin to absorb low frequencies as well as thicker fiberglass.

I'm not disputing that your traps made an improvement! But a better design, or thicker material, or both, would make an even larger improvement.

> I thought the shape of the trap gave it an edge in pressure absorbtion. <

A tube trap is a porous absorber and so works on wave velocity, not wave pressure. I've measured tube traps in a lab, and they're not substantially different than an equivalent thickness of fiberglass in a panel shape with the same air space.

> I would love to see acoustically what my traps are doing. <

You can determine their effectiveness in a round-about way by measuring the change in frequency response in your room. A much more direct way - the way labs measure absorption - is to see by how much the reverb time is reduced in each 1/3 octave band.

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post #40 of 58 Old 07-30-2004, 05:52 PM
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I thought a standing wave is low velocity , and thus higher pressure. Nulls are high velocity and low pressure ? How they absorber pressure was a mystery to me.???????
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post #41 of 58 Old 07-31-2004, 12:42 PM
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KG,

Standing waves and absorption are entirely different concepts.

A porous absorber like rigid fiberglass absorbs sound waves as they pass through it. Friction converts the sound energy into heat. In order to pass through there must be some velocity - miles per hour, if you will. As a wave approaches a wall it is travelling fast, but right when it hits the wall it stops cold and there's no more velocity. On the other hand, a wood panel bass trap works on pressure, and is basically a shock absorber. It needs to be at the boundary to receive the full impact.

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post #42 of 58 Old 07-31-2004, 03:58 PM
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Quote:
Originally posted by Ethan Winer
As a wave approaches a wall it is travelling fast, but right when it hits the wall it stops cold and there's no more velocity.
I'd describe this differently. The air particles slow down slowly and smoothly as they approach the wall. They must of course reach zero velocity at the wall, since they don't continue on through it. But anywhere near the wall, there is very little velocity, and a great deal of pressure change. In a standing wave there is a "pumping" of air back and forth, to and from high pressure areas as the wave oscillates.

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post #43 of 58 Old 08-01-2004, 09:56 AM
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Terry,

> The air particles slow down slowly and smoothly as they approach the wall. <

Thanks for clarifying. Do you happen to know what sort of distances are involved? Feet? Nanometers? :D

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post #44 of 58 Old 08-01-2004, 10:19 AM
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These are some of my favourite animations about sound.

The first one shows the pressure change near a wall, about 30% down the page.
http://www.isvr.soton.ac.uk/SPCG/Tut...ter-reflex.htm

This one shows exaggerated particle motion:
http://www.isvr.soton.ac.uk/SPCG/Tut...ding-rooms.htm

http://physics.nad.ru/Physics/English/int_txt.htm
http://www.gmi.edu/~drussell/Demos.html
http://www.kettering.edu/~drussell/D...s/driving.html

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post #45 of 58 Old 08-01-2004, 10:20 AM
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Another kind of bass trap is the hanger. But I don't know how effective they are.

http://www.saecollege.de/reference_m...requencies.htm
http://www.saecollege.de/reference_material/
http://www.johnlsayers.com/Studio/Im...ions/Up4/8.jpg
http://www.johnlsayers.com/Studio/Im...ions/Up4/9.jpg

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post #46 of 58 Old 08-02-2004, 07:39 AM
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Hi all. I built some room treatments based on some research I did on the internet and now I'm wondering how effective they actually are at their true purpose. I use Mirage Omni-polar speakers, so my goal was NOT to kill all reflections in the room. The room is not large, but isnt overly live either. I basically wanted to build some bass traps. I searched and searched locally for rigid fiberglass and couldnt find any. So what I ended up with were 2' x 4' frames made from 2x4s so they are roughly 3.75" deep (right?). I packed each frame with two layers of R19 insulation so that is 13" of fiberglass packed into a 4" space. I am placing these in the corners.

From my research, I know the short answer is, rigid fiberglass 4" thick would have been better, but my question is, did I totally waste my money (considering my purposes) or not? To what frequency will these likely absorob?
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post #47 of 58 Old 08-02-2004, 07:53 AM
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Charles,

> that is 13" of fiberglass packed into a 4" space ... To what frequency will these likely absorb? <

It's certainly possible to pack fluffy fiberglass tightly enough to be equal to rigid fiberglass. I'm not sure what the weight of fluffy fiberglass is, but Bob can probably tell you the exact compression ratio/factor you need to equal 703 or 705 rigid fiberglass.

So the short answer is you did not waste your time or money. But the only way to know how effective they really are is to measure them in a lab.

Also, it's not like they absorb completely down to a certain cutoff frequency and then suddenly stop working. Rather, the absorption begins to fall off starting at some frequency.

Have you put them in the corners yet and listened?

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post #48 of 58 Old 08-02-2004, 08:03 AM
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Quote:
So the short answer is you did not waste your time or money. But the only way to know how effective they really are is to measure them in a lab.
Thank you, that's really all I wanted to hear.

Quote:
Have you put them in the corners yet and listened?
Yes, and they have tightened up bass considerable, and this was BEFORE I added the additional layer of fiber. I've only done that to one trap so far due to time constraints. So short of lab testing, that is the ultimate test, yes?

Thanks for the response Ethan, I REALLY appreciate it.
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post #49 of 58 Old 08-02-2004, 06:35 PM
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Quote:
Originally posted by Ethan Winer

> The air particles slow down slowly and smoothly as they approach the wall. <

Thanks for clarifying. Do you happen to know what sort of distances are involved? Feet? Nanometers? :D
For subwoofer frequencies, feet. The standing wave velocity in a room with rigid walls is nothing more complicated than the sine of the distance from the wall normalized by the wavelength. Zero velocity occurs at the wall, at 1/2 wavelength away, 1 wavelength away, etc.

So for a 50 Hz room mode and a rigid wall, the maximum velocity (zero pressure) is around 6 feet from the wall. The velocity falls off smoothly according to a sine function whose phase angle moves from 90 degrees down to 0 degrees as it approaches the wall.

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post #50 of 58 Old 08-02-2004, 08:14 PM
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Charles J P:

JM fluffy pink R19 I believe is around 0.5 to 0.8 pcf.
I measured some OC R31 once, but I don't have that handy.
OC 701 is 1.5 pcf
OC 703 is 3.0 pcf
If you have 13" of fluffy pink you'd have to mash it down to 2' 2" to get it to the same density as 703.
If you have 13" of fluffy pink you'd have to mash it down to 4' 4" to get it to the same density as 701.
As for what happens to the gas flow resistivity (and it's absorbtion) when you stomp on it like this I have no idea.

For corners, it's been recommended that 701 or lighter is likely better than 703 if the corner is filled (i.e. more than 4" thick).

There's absorbtion coefficients for both R19 and 701 here
http://www.bobgolds.com/AbsorptionCoefficients.htm

Unfaced 12" R42(?) on wall 1.14 1.09 1.09 0.99 1.00 1.21
Unfaced 6.25" R19 on wall 0.64 1.14 1.09 0.99 1.00 1.21 1.05
701, plain 4" (102mm) on wall 1.5 pcf (24 kg/m3) 0.73 1.29 1.22 1.06 1.00 0.97 1.15

Mounted 16" out from the wall, uncompressed:
Unfaced 6.25" R19, 16" air gap, 0.96 1.03 1.13 1.02 1.04 1.13 1.05

Terry wrote
Quote:
In designing porous absorbers, you can do some simple linear scaling of frequency and thickness, but only if you scale the flow resistivity.
If you build an absorber 12" thick instead of 1", it will absorb the same at 1/12th the frequency, but you have to change the material to one whose flow resistivity is proportionally smaller. So lets say you scale up a 1" thick absorber made from Fibertex 650 Rockwool, which has an acoustic resistivity of about 50000 mks Rayls/m. If you divide this by 12, you get about 6000 mks Rays/m. That's roughly the resistivity of fiberglass batt.
For corner absorbers, since dipole surrounds are aimed at the corners, and often bass absorbtion is the flat RT60 problem, lately I've been considering trying slatted corner traps, at least for the top half of the corner.

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post #51 of 58 Old 08-03-2004, 11:05 AM
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Charles,

> short of lab testing, that is the ultimate test, yes? <

Yes, mostly. The only reason I qualify that is because people sometimes think they hear things change even when they don't (speaker wires, etc). But the improvement from having a sufficient number of bass traps should be quite apparent to anyone.

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post #52 of 58 Old 08-03-2004, 11:07 AM
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Terry,

> velocity in a room with rigid walls is nothing more complicated than the sine of the distance from the wall ... <

That makes sense. Thanks.

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post #53 of 58 Old 08-03-2004, 08:40 PM - Thread Starter
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So I put 4, 6" thick 703 in 3 corners and 1 on the back but the bass is good only in the front of the room where the sub is located
LL
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post #54 of 58 Old 08-04-2004, 11:55 AM
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OK, I've been fiddling with the traps I built, and I think I have figured out what might work best, but I have three questions that I'd like to get some opinions on before I shell out any more money :)

1) I think I am going to use the panels I already built as regular room treatments (first reflection point etc.) and try to build something else out of the dense rigid fiberglass being discussed here. I have (4) 2'x4'x4" panels. I already have leaned two up against the walls at the first reflection points of my mains with startlingly good results. I will likely put the other two in the front of the room, but my question is should I make them span the corners or just mount them flush with the front wall?

Here is a picture of the front of my room.
http://www.hometheaterfanatic.com/images/rt1.jpg

2) I was unable to locate any OC 700 series, but I did find some JM rigid fiberglass locally. It is 1" thick but totally unfaced so I could sandwitch more layers. Is the JM stuff going to be as good as the OC 700 series?

3) What is the best shape and size for making these bass traps then? I thought about making pedistal style ones where they are like 1'x'1' 3' tall or something. Or should I leave them more panel shaped (I'm running out of wall/corner space for large panels) Thoughts?
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post #55 of 58 Old 08-04-2004, 04:24 PM - Thread Starter
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http://www.asc-home-theater.com/products-soffit2.htm
The above link gives me an idea. Why don't I put absorber wherever the wall meets the ceiling? I should be able to use 1'x 4'x 6" 703 fiberglass. It will be mounted against the wall. If that does not solve the bass problem then my problem might be something else and has nothing to do with bass traps.

I wonder what kind of bass trapping a stretched Vinyl projector screen on a fixed frame gives if any. I wonder if it will act like a membrane.
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post #56 of 58 Old 08-04-2004, 04:51 PM - Thread Starter
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BasementBob, Thanks for the link to absorbtion coefficients. That is very helpful.
I wonder how the 703 performs under 125Hz.
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post #57 of 58 Old 08-05-2004, 08:51 AM - Thread Starter
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At what freq does Drywall let LFE through without reflecting?
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post #58 of 58 Old 08-16-2004, 09:26 AM - Thread Starter
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I found a nice cloth to wrap the bass trap. It is call Dana foam also known as headliner. It is foam on one side and cloth made of 100% nylon on the other. The cloth is probably not hifreq transparent but should work well for bass trap.
I found the material at Joann fabric.

{ How do you insert pictures ? }
LL
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