Lining sealed boxes?

[Lindahl]

From what I've read, it appears that lining a sealed enclosure (both speakers and subwoofers) with felt, sonic barrier, whispermat, or similar materials is a waste of money, compared to stuffing with fiberglass or polyfill. This is because you don't have to worry about standing waves escaping the box (no vents), and stuffing does an adequate job doing thermal absorption. The only problem left is panel resonances, which in a properly braced box, are not a problem. The panel resonances are even less of a problem in a subwoofer, since they are generally outside the passband, if considering a typical 80hz crossover. However, you still need to have good bracing, not because of panel resonances, but because you want to avoid efficiency loss.

Is there any misinformation in the above, or am I missing any critical information or gotchas?

Thanks!

[ThomasW]

How one places the damping in a sealed box isn't all that critical. If the amount added is such that it lines the box, that's fundamentally no different than placing the same amount in the middle of the box.

This is because you don't have to worry about standing waves escaping the box (no vents),

Standing waves are a function of the geometry of the box, not whether it is ported or sealed.

you still need to have good bracing, not because of panel resonances, but because you want to avoid efficiency loss

The idea is to avoid having the walls flex, because flexing panels are a bad thing Martha, regardless of the drivers passband or the frequency of the panel resonances.

[Lindahl]

How one places the damping in a sealed box isn't all that critical. If the amount added is such that it lines the box, that's fundamentally no different than placing the same amount in the middle of the box.

I guess I'm just looking for guidelines on which type of damping material to use. I'm building three different types of sealed enclosures. Subwoofers, bass bins (~80-250hz) and MTMs. From what I've read, a lot of people just use fiberglass for all three. However, I've also seen some people talk about using felt lining and other more serious damping materials, but only in the context of ported enclosures. And, I've read that you don't need to hassle with those materials in the context of sealed subwoofers - fiberglass is as good as it gets. But what about other types of sealed enclosures? The few sealed projects to draw ideas from don't touch on the damping materials used.

I searched these forums for the last two hours and couldn't come up with any reasonable guidelines. Perhaps you could give me some guidelines on when and why you'd use more serious damping materials (not fiberglass fill)? These materials are quite a bit more expensive than fiberglass. I wouldn't worry about overkill if I was making a true reference design, but I'm only putting around $1500 into my front three speakers, so the ~$300 premium is healthy chunk. I could try it and return it if it didn't make a difference, but the shipping is still $50, and my reference room won't be ready until long after the return period has expired. Decisions... decisions...

[Dennis H]

If your walls are stiff enough, fluffy fiberglass works a treat for any sealed box, sub or main.

Ported boxes need some open air space to work as resonators so people get into more exotic damping systems for them. Some foam or fiberglass on the walls is good enough for a ported sub. A ported MTM main might require a bit more for best midrange results -- Whispermat or PE damping material on the walls plus a bit of poly or fiberglass right behind the drivers, being careful not to block the air path between the drivers and the port.

That's my story and I'm sticking to it.....