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In Reply to: RE: How does the bass get into a sealed trap? posted by Jon Risch on October 21, 2009 at 20:30:58
I first built "mock-ups" with no FRK or plastic on the front and cheap fabric over the 705 panels and wood frame. I conducted numberous experiments with facings (none, kraft paper, plastic, polyester batting, and combinations). At that point, I preferred the kraft paper facing with no plastic or batting.
Today, I added FRK to the test and also my selected finish fabric. I again tested each facing with the FRK on the front and ended up preferring the FRK and the plastic with my finish fabric.
Tonight, I put the finish fabric on one panel (over only FRK and the plastic) and am pondering how to best attach the soft-rubber (not foam) garage door weatherstripping seal to the rear sides/back of the wood frame. Hope to finish up tomorrow and try them out.
I am still concerned about the effect of sealing THE SIDES of the 705 panels and the frame with plastic along with the air-tight seal around the edges of the panel. The panels should be completely air tight to the walls/ceiling/floor and also across the entire panel. How will air move in and out of the panel? Is this going to screw everything up? If so, I can leave the plastic off of the edges of the 705 panels on the second one and then perhaps "poke" hundreds of holes in the sides of the one I already covered to "vent" it.
Follow Ups:
You wrote:
"I am still concerned about the effect of sealing THE SIDES of the 705 panels and the frame with plastic along with the air-tight seal around the edges of the panel. The panels should be completely air tight to the walls/ceiling/floor and also across the entire panel. How will air move in and out of the panel? Is this going to screw everything up? If so, I can leave the plastic off of the edges of the 705 panels on the second one and then perhaps "poke" hundreds of holes in the sides of the one I already covered to "vent" it."
The pressure waves in a corner have minimum velocity at that point, just as where the velocity is greatest out towards the middle of the room, the pressure is at a minimum.
Think of the air velocity in the vent of a subwoofer, this amount of air motion would seem to indicate that the trap surface must be able to actually pass a stream of air, i.e., allow a breeze through.
Now think of how much actual air motion that same amount of vent air motion represents over in the other corner of the room; while you might be able to feel the breeze from the vent right in front of it, I doubt that you detect it at a in the other corner. The vent air velocity represents a special case of bass air pressure, where the motion of a given woofer cone is pushed through a much smaller vent area, speeding up the air velocity by the amount that the cone area and vent area differ.
If the vent is only 1/4 the area of the cone, then the air velocity has been increased by a factor of 4.
Now, think about how the total air pressure from that subwoofer spreads out into the WHOLE room. With the subwoofer playing at maximum excursion at some low frequency, the actual air velocity at any point in the room is much lower than even the velocity of the air directly in front of the cone, much less the vent.
As I said earlier, in the corners, the pressure is at a maximum, and the velocity at a minimum, so the plastic diaphragm you have created does not have to move very much to let the LF air PRESSURE through.
Realize that old-style pressure-based bass traps used 1/4", 1/8" and other sizes of plywood to create a diaphragmatic bass absorber, the 0.7 mil plastic is nothing compared to that.
Again, I never recommended to actually place a sheet of plastic over the whole of any bass trap, other than the SQ&D version, and that was a special case. You seem very concerned about sealing the edges, but in point of fact, the main concern should be the sealing of the frontal or rear surface of the panels.
In my opinion, the edges of the panels should be sealed against the walls, floor and ceiling, and the front and backs of the panels should not be "sealed". Thus an air cavity would be created behind the panels, where the only way for the air pressure to get there would be through the panels acoustic resistance.
Jon Risch
Ahhhh, we are talking PRESSURE. Now I get it. Thank you Jon.
I completed the second panel, only covering the front (over the already reflective FRK facing) and not the sides on this one. Did some listening on work breaks with only the first trap in place and was very impressed with the sound. When I finished the second one and placed it the the other corner of the front wall, all of the life in the music evaporated. Very dull and closed-in sounding' although the mid-bass was better. So I took it out, and I quite please with the sound using only the one panel.
I plan to try the other panel in a corner on the rear wall and leave the first on the front wall. Would it would work better with the second panel opposing (diagonally across the room from) the first panel or on the same side? I will end up trying both, but they are hard to move by myself (10' tall and heavy) and I wondered if there is any established science/principle or your experience on what happens acoustically when the panels face each other diagonally vs not?
Normally, good stereo imaging requires that the room be symmetrical with respect to absorption and reflections.
From a standpoint of just controlling the room modes, a diagonal corner layout should not be a significant penalty compared to both in one end or the other.
Jon Risch
Thanks Jon.
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