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Discussion Starter · #1 ·
I've been considering building this trap. When I was buying the Owens-Corning 703 at a local insulation dealer, I noticed several of the pipe insulation sleeves were in stock. However, I'm thinking of trying to improve it by stuffing ti with Owens-Corning 705. Opinions and suggestions please. Here's the trap http://ic.net/~jtgale/diy2.htm
 

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Discussion Starter · #3 ·
Thanks Ethan. They have some 705 in stock as well, one package of 4 two inch thick panels. I guess I'll pick it up tommorrow. Why wouldn't the pipe insulation work as well? It's 1.75 inches thick and 17 inches on the outside diameter. I saw that you recommended 703 for absorption and "ran out" and bought some. When I checked further, I saw thay you felt 705 was even better. However it was too late as I had purchased 80 square feet of 703 and opened the bundle:).
 

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Do you recall what the 705 and pipe insulation prices were?
 

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Discussion Starter · #6 ·
Thanks again Ethan. Toxarch the 705 is 48.00 for 4 panels that are two inch thick. The Pipe insulation was about 28.00 a three foot section . They have quite a bit in stock but tell me that once it's gone they will not re-order any more as that type of insulation is out dated and newer forms are used for larger pipes. Let me know if you want a few tubes and can't find it locally. My local pack and wrap doesn't charge to pack only for the box and shipping. I figure for about 45.00 each with shipping included, I can ship some to you. Let me know. What's the catch? I want someone to be a guinea pig along side me :D. I inquired about larger pipe insulation but they said they would only order in large quantities. Translation, very very expensive. I think that I am going to take the design and re-enforce it so that I can stack two on top for two traps 6 feet tall and stuffed with as much 705 as I can fit. Hopefully the whole package of 705 will go into the two 6 foot high traps. I'll measure before and after and see what kind of results I get. That's a hell of a lot of fiberglass in the corners not to have some kind of bass absorption. The question is though, how much? I'lve already started to build the absorption panels. The "final straw" was when I open a plastic container with a plastic film on top in my theater. The was a sharp zinging reflection as soon as I did. That's ridiculous.
 

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Ethan, when straddling corners with rigid fiberglass, do they need to run from floor to ceiling? In otherwords, do the top and bottom need to be closed/sealed?
 

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Sorry, I don't mean seal the fiberglass. I meant, suppose I mounted one in the corner, but it does not reach the floor nor the ceiling. Do the ends need to be closed or capped? Or do you just leave the top and bottom open?
 

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Discussion Starter · #12 ·
Well I decide to experiment and build the trap. I constructed 4 with a panel of 705 folded inside. In my two front corners, I placed two traps each stacked on top of each other. The results were not very impressive. I only have a test CD called Sound Check from Mobile Fidelity and a radio shack sound meter. The only difference I could measure was a 2 db drop in bass at 63Hz and about the same at around 160 hz. The test CD doesn't have a lot of bass frequencies. Also it was evident that I have a null at 100Hz, with a drop of about 12db. 50Hz appears to be a peak with bass being up from my listening position also about 12db. I think I need to educate myself further before I start "playing around" with bass absorption. I've started to construct the panels for mid and high frequency absorption. Loews' has pre-cut 1/2 bu 48 pieces of pine that were less than 2.00. I'm waiting for my Guildford of Maine fabric to arrive so I can complete these.
 

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A porous absorber like fiberglass placed perpendicular to a wall or corner works by converting the particle velocity of sound waves into heat. At the wall surface, the particle velocity is zero. That's because the wall, being rigid, doesn't move with the air.


But 1/4 wavelength away from the wall, the particle velocity reaches a maximum. So this is the most effective place to put an absorber for a particular frequency. For example, for 100 Hz, that distance is (1130 feet/sec)/(100 cycles/sec)*4 = 2.825 feet.


That's not to say that an absorber of this type will have no affect if it is placed a different distance away. The absorption will simply be less.


For a corner absorber, there isn't just one distance to the wall, but a range of distances, so a range of frequencies will be affected. Suppose you are using a 2 foot wide absorber oriented vertically and placed flush in the corner, so its surface is perpendicular to the ceiling. Its distance to the corner where the walls intersect is exactly 1 foot, so it will best absorb at (1130 feet/sec)/(1 foot)*4 = 282.5 Hz and above. Not very impressive for a bass trap, I'm afraid!


CORRECTION: For the calculation immediately above, one really should use the perpendicular distance from the wall surface to the absorber, at least for axial modes. This gives a distance which is SQRT 2 times greater (1.414). Also, adding the thickness of the fiberglass (2 inches) for an optimistic estimate, you get:

(1130 feet/sec)/(1.167 feet)*4*1.414 = 171 Hz minimum frequency.


Regards,

Terry
 

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Discussion Starter · #15 ·
Ethan, I have a masters in Experimental Psyc, and I know what you mean about those expectation effects;). I do know that I was clearly able to detect a -2db drop in the midrange of my speakers without the grills on. I complained to Mcintosh about the midrange being recessed before I purchased them. Ron at Mcintosh immediately asked if the grills were on. It turns out that the grills horn load the midrange and without it the midrange is down 2db! My home theater has two windows in the front right corners that are 6 feet by 3 feet and start 1 foot off the floor. With drapes, that makes it very difficult to mount traps across that corner. Slap echos are not necessarily an increase in high frequencies are they? Can bass also be "reflected" in some way in a room to cause problems? Terry, thanks. I guess I need to be sure that the trap is at least effectively designed before I consider placement. Either I can purchase some that have "numbers" to back them up or try to build some DIY traps and test them myself which may be beyond what is practical.
 

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All I'll add is that my company's MiniTraps include an integral membrane that extends absorption to a much lower frequency than would be predicted by the depth of the air gap.
Excellent, Ethan! I looked at the corner-placed lab data for your MiniTraps (on your web site), and this was the only explanation I could come up with. :) There had to be both membrane (panel) resonance for the 100 Hz absorption, and porous absorption for the peak at around 500 Hz.


Regards,

Terry
 

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So bridging a corner with rigid fiberglass is not going to help much, but if you build an airtight panel absorber (with fiberglass inside), that across the corner is more likely to help. Am I correct?


Eric.
 

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but if you build an airtight panel absorber (with fiberglass inside), that across the corner is more likely to help. Am I correct?
I'm afraid it's not that simple, Eric. When you seal it airtight, you are making a pure panel absorber. The resonant frequency is hard to compute unless the front panel can be treated as a "limp acoustic mass." In the latter case, there's an easy formula.


Ethan's trap is a hybrid, which relies on the cavity being open. I don't think it's right for me to say anything more, or "reverse engineer" his intellectual property. I can see the principle on which the MiniTraps operate, and Ethan's got the lab data to back him up. While I haven't seen or tried them, I have reason to believe that they work, and he is charging a fair price.


Regards,

Terry
 

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I'm just going by Ethan's on line how to article. I'll have to read it more closely to find how it's not entirely closed.


Of course his newer, for sale versions are probably quite a bit more sophisticated than the older DIY version.


Eric.
 
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