Home Theater Forum and Systems banner

1 - 4 of 4 Posts

·
Registered
Joined
·
57 Posts
Discussion Starter #1
I've been doing dome research on cabinet dumping materials, but can't find any reasonable explanation on what makes one product better than another. For starters, what frequencies are we trying to dump? I calculated the frequency for one-quarter wavelength of the longest dimension of my current build to be around 200Hz. Should I look for materials that mainly absorb in that frequency?

Secondly, why are we using dumping material? Are we trying to stop the cabinet from vibrating (dense is good) or absorb the air pressure (porous is good)?

In other words, are there any (scientific, not marketing) reasons why I should use this instead of this ?
 

·
Elite Shackster , HTS Moderator Emeritus
Joined
·
2,216 Posts
Well IIRC, cabinet damping is about increasing relative internal volume and preventing standing waves from forming in the box.

The speed of sound changes in the foam (just like stuffing as well) and makes the cabinet seem larger (just a bit). But the real benefit is reducing standing waves.

Any standing wave or panel resonance in the cabinet will translate to a muddy sound or a peaky frequency response. Neither or those is desirable.

You can stop panel resonances by bracing heavily, using heavy materials, using composited materials (MDF/roofing felt/MDF sandwich), or other design trades. However, standing waves are a function of distances and can exist regardless of how well you brace the structure. If the cabinet is fully stuffed, then foam on the edges is not necessary as the foam will inhibit standing waves. But if you are not stuffing it, then the foam will prevent the wave from forming.

Keep in mind, we're talking shorter distances, so higher frequencies. It's not just the longest distance you need to worry about. Some of the cabinet dimensions may ring at 1kHz, which is probably in the foam's absorption band.

Now as to velocity versus pressure absorber, I'm not sure. Putting foam on the walls is a matter of convenience, but foam is a better velocity absorber, so it should be out from the wall. Again, though, the foam is probably only helping at the higher frequencies, so it may even be thick enough to stick out to where it is useful. That would be an interesting study.

As for the two foams, I would bet the triangle foams work better more on geometry (jutting out to where they could be better velocity absorbers) than density. You may be able to mix and match. Put the thicker foam on the top and bottom (long dimension) and use thinner foam on the sides for higher frequencies.

Hopefully this helps some. I haven't thought about this stuff in a long time, so I may be a bit rusty :)
 

·
Registered
Joined
·
302 Posts
In general, there are two kinds of materials used for damping, surface treatments like those you reference and "acoustic" fiber used to modify properties in a volume rather than on a surface. The latter are most common in sealed and some vented enclosures to tune the sound by changing the apparent volume of the enclosure (it's actually a bit more complex). This is not what you ask about.

Surface treatments, in turn, come in two flavors, do the same sort of thing, but in different places.

The foams you reference in the first link are primarily used in ported enclosures for attenuating higher frequencies within the box while allowing low frequencies to resonate, based on port tuning. The second link is soundproofing material, intended more for room treatment than enclosure lining. The key thing here is attenuating the reflected energy within the box so it isn't heard through the drivers or port. You're damping reflected energy, but not affecting transmitted and absorbed energy.

A second type of surface treatment addresses absorbed and transmitted sonic energy. (Cross-bracing has many of the same effects.) Constraint Layer Damping (CLD) is the technical name for surface treatments comprised of an absorbing material sandwiched between the enclosure walls and a stiff inner layer, which is frequently cross-braced for additional damping. The key difference is that they do not address the reflected energy-based resonances that Sonic Barrier will damp. If you use CLD, the odds are you need Sonic Barrier, too.

Have fun,
Frank
 
1 - 4 of 4 Posts
Top