Cabinet building materials

[fredk]

Having engaged in a discussion on another forum over the properties of cabinet materials and having had some time to think, I want to test my understanding of different materials, properties and their uses.

Stiffness. affects the resonant frequency of the material (point where the air and the material couple most effectively??)

Density. Affects the damping ability of the material (the ability to convert mechanical movement into heat)

In a sub, you would want to use the stiffest material possible for an inner layer to push the resonant frequency outside the drivers band.

The viscoelastic layer would be to decouple the inner and outer layer of material. Essentially to contain as much of the resonance as possible within the box.

The outer layer would be high density (mdf) to absorb what little resonant energy was transferred through the viscoelastic layer.

So, do I have it right or do I just have an overactive imagination?

 

[jliedeka]

You've got it right. Another thing to consider when you work out your bracing scheme is that smaller areas resonate with higher frequencies. Think of a guitar string. The high notes are where you shorten the string by pressing on a higher fret. That's part of why lots of bracing is better. The other part is more stiffness.

OTOH, more mass means lower frequency vibration. The thicker strings give lower notes. Look at a bass compared to a guitar or violin.

You are on the right track with using two layers of wood coupled with a visco-elastic layer like Peel 'n' Seal or similar. The transitions from one density layer to another will cause a lot of the mechanical energy to turn to heat.

I bet WmAx could explain this better. He's done a bunch of projects making nearly dead cabinets.

Jim

 

[fredk]

Another thing to consider when you work out your bracing scheme is that smaller areas resonate with higher frequencies

I'm not so sure about that. When I was looking into helmholtz resonators, the size of the resonating sheet made no difference in the frequency of resonation. Dang, my resonator calculator link is down. I can't confirm that.

The transitions from one density layer to another will cause a lot of the mechanical energy to turn to heat

Is that the case? The amount of energy converted to heat is dependant on mass and I suspect that the stuff you talk about has a lower mass than the other two layers. When you have an elastic layer, it is able to stretch and compress without absorbing or passing much energy through transmission.

It would be the same as having two sets of stud walls with a layer of air between. The result would be a large reduction in the coupling between the inner and outer layer allowing the inner layer to resonate more freely... I think...

 

[WmAx]

Stiffness. affects the resonant frequency of the material

Yes, assuming that density and mass are held constant, increased stiffness would yield a higher resonant frequency.

Density. Affects the damping ability of the material (the ability to convert mechanical movement into heat)

Density alone is not really a dampener. Density reduces vibration only by the physical reaction of increased mass requiring more energy to excite it. But take steel for example. At almost 500 lb/ft^3, it's very dense. Compare to 45 lb/ft^3 of MDF. If you have solid steel piece with no mechanism to dampen it's oscillation, it will ring(resonate) for a considerable time and at a high amplitude within a narrow frequency band. On the other hand, this higher amplitude, but much narrower band of resonance will be less audible than the lower amplitude, but wider band resonance of MDF/wood, for example.

In a sub, you would want to use the stiffest material possible for an inner layer to push the resonant frequency outside the drivers band.

True. But a wide band sub is trivial to design to be non resonant using a singe 3/4" layer of high grade ply with bracing every 8" - 10" on every axis.

The viscoelastic layer would be to decouple the inner and outer layer of material. Essentially to contain as much of the resonance as possible within the boxh.

I am assuming you mean a constrained layer construction, using two stiff walls with the visco-elastic layer in a channel between them. In this case, it is not a good de-coupling mechanism. The method of action here is that the two stiff walls act as a shearing mechanism. The visco-elastic material that is glued to both layers, is not excited maximally on a molecular level, thus resulting in maximum conversion of molecular movement energy to a thermal energy. If you were to instead glue the two stiff walls directly to each other, and then apply the visco-elastic layer on a side(internal or external), it's molecular level movement would be far less. In this case, there is no effective mechanical impedance match to transfer energy into it on a molecular level. The only opposing force is the air on one of it's sides. Not a good mechanical impedance match. But when you trap it between two stiff plates, the one (inner) plate vibrates, then the visco layer vibrates, and the other opposing plate acts as an opposing anchor force, causing the molecules in the visco layer to move against each other with high friction. Without this opposing plate, the visco material is left vibrating mostly as a single mass, without much internal molecular friction occurring.

The outer layer would be high density (mdf) to absorb what little resonant energy was transferred through the viscoelastic layer.

I usually only recommend MDF externally if you want to paint the cabinet. Ply is superior - but it is not easy to paint and get perfect finish results.

-Chris