How thick (acoustically) does a false wall need to

[mrel]

Hi,
I'm planning a HT room in a new house.
Planning for an IB sub behind a false, solid stud wall that encloses about 3" of one end of the room, providing the rearwave chamber.

With low bass wavelength being what it is, surely this wall (separating the front and rear waves) needs to be thick/absorbent enough to soak up serious LF? And if so I thought this took 8-12" of sound absorbing material? I haven't seen this in the builds I've looked at.

Or is it just that there is a null along the wall - the bass isn't contained by the false wall but the rear- and front- waves cancel out at this point?

If it is a null, does this work across all frequencies? I would've thought that each chamber (front or rear) would have some reflected waves that woud arrive at different times, preventing a perfect null?

So many questions!

To get around all this, what happens if you put serious (12") insulation on all the walls of the rearwave chamber? (Trying to attenuate that energy before it wakes the kids up!) Surely this is similar to an attic where all the roof insulation soaks it up? In other words, can you attenuate all the rearwave energy without it affecting the IB performance? I think absorbing all the energy with insulation must be the same as an infinite chamber size - like outside?

Thanks to anyone that can help, just trying to improve my understanding of IB principles.

[ThomasW]

This isn't a dipole so there's no null.

The wall completely isolates the front wave from the rear wave.

If you're only doing a 3' space it should be basically filled with insulation

[mrel]

OK, thanks Thomas. How does the wall isolate the front wave from the rear? Why don't the LF waves penetrate a single sheet of drywall?

The 3' deep rearwave chamber will be 9' high and 16' wide though - does that help? It's 12xVas of the 4xIXL18s I was planning on. Isn't that OK?

[ThomasW]

Sept 15, 2009 2:27:16 GMT -7 mrel said:

How does the wall isolate the front wave from the rear? Why don't the LF waves penetrate a single sheet of drywall?

The front and rear waves are out of phase, that equalizes the pressure on the wall

The 3' deep rearwave chamber will be 9' high and 16' wide though - does that help? It's 12xVas of the 4xIXL18s I was planning on. Isn't that OK?

That's fine but since the space is shallow you'll need to put insulation in the chamber

[mrel]

Sorry, just checking - the space is 3 feet deep, not 3 inches (which my first post might have indicated).

Still fill it completely with insulation?

[ThomasW]

I figured it was a typo. And yes either fill with standard insulation or line the entire space with 2" thick OC 705/703