Tweakers' Asylum

Posted by Jon Risch (B) on October 22, 2009 at 16:43:43

In Reply to: RE: How does the bass get into a sealed trap? posted by crossram2x4@aol.com on October 21, 2009 at 21:00:08

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