In Wall Speaker Back Box Design

Frank_Oz

For the front speakers in my new 5.1 setup, I have I have two 265-LS and one 255c LS. I would like to add back boxes as it seems the universal opinion is that they add to sound quality. Can anyone give advice in design, i.e. dimensions, materials and acoustic fillers?

Here in Australia the standard framing timber is 90mm (3.5"), plus 10mm plasterboard (dry wall). The wall in question backs on to a walk-in cupboard under the stairs, so I plan to pull off the plasterboard on the back side, to give easy access to make any adjustments to the framing timber, before re-sheeting. So, I could add some depth to the frame thickness if it helps, e.g. another 1" or 2" wouldn't compromise the cupboard space.

I've tried e-mailing Polk customer support but no reply after a couple of weeks.

Thanks in advance.

SeleniumFalcon

Hello,
The recommended back box enclosure should be 1.2 cubic foot sealed, using a moderate amount of an absorbing material inside, such as Dacron.

Frank_Oz

I'm now clear on volume, thanks SeleniumFalcon.

What about dimensions, do they not make any difference providing you get the right volume?

To clarify, the 265 LS cut-out is around 20" x 7", so if I kept the same width and height I would need to go around 15" deep to get 1.2 cubic feet. Which would be impractical. So, I'll have to increase the width and height to enable a smaller depth. But does it matter what once I increase the most, i.e. go tall and thin or short and wide, or maintain the same proportions as the original speaker, or does it not matter at all?

Also just out of curiosity, what happens when there is no back box fitted, is the wall cavity essentially the back box?

Thanks,

SeleniumFalcon

I don't believe the dimensions (except of course to be able to fit the back of the speaker into the cavity) make any difference. It's the internal volume that's tuning the speaker.
If there's no back box there's no internal air pressure acting on the motion of the bass driver. When the woofer moves inward the internal air pressure damps the cone's motion. When the cone begins moving outward the now lower internal air pressure begins braking the cone's outer movement causing the cone to move inward to a rest position. The result is more controlled woofer movement and improved bass transient response.
The second effect is to correctly tune the speaker system's resonant frequency response, lowering the frequency point where the bass performance begins to roll off or diminish. So, tighter more controlled bass and lower bass performance capability.
I hope this is helpful information.

Frank_Oz

That's great info, thanks for clarifying.

j_gibbs

I actually just heard back from Polk about an enclosure for my vanishing in walls, this is what they responded with.

They are designed to be installed free air or infinite baffle. If you must enclose them, then build a sealed enclosure with a net volume of .25 to .50 cubic feet and install a 1/3 of a pound of Dacron to dampen it.

SeleniumFalcon

I checked again with Polk and confirmed that the recommended internal volume is 1.2 cubic feet with 1/3 pound of Dacron inside. This information came from Scott Orth the head of engineering.

j_gibbs

I found it odd that everywhere I'll read Polk has recommended 1.2 Cubic Feet and then Polk e-mailed me back with 0.25 to 0.50 cubic feet.

I don't think the enclosure is to be that different between the LS and RT models.