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Re: Yes, I think that concept's full of holes ... :-))

Posted by lne937s (A) on April 26, 2007 at 15:04:14

In Reply to: Yes, I think that concept's full of holes ... :-)) posted by andyr on April 25, 2007 at 20:55:36:

I think there are a couple of things at play here.

One is rigidity. The more rigid a structure is, the less it will vibrate. When it does vibrate, more rigid structures will vibrate at a higher frequency. If this frequency is within the audible spectrum and pronounced, a more rigid structure may not necessarily sound better. There are two main ways to increase the rigidity of a solid flat structure: use a more rigid material or increase thickness- getting exponentially more rigid the thicker you get. For example, steal is stronger than aluminum, but aluminum's lower density allows it be used at much thicker gauges per unit of weight, allowing the structure to be lighter and more rigid when made from aluminum. If you plied on an additional layer of MDF, it would become more rigid. However, on a flat structure, the majority of the strength is on the outside surfaces. By reinforcing the outside surface, you can make something much stronger than the underlying core (e.g., carbon fiber with an aluminum honeycomb core). By applying a fiberglass and resin skin (or, if you have the $$$ Kevlar/carbon fiber) - the strength and rigidity of the panel will increase greatly with minimal impact on thickness or weight.

The next is mass. The more mass that needs to be moved by the vibration, the more energy is required. The heavier a rigid panel is, the less it will noticeably vibrate. You could add mass to the panel (lead, etc).

In terms of dampening, vibrations tend to travel along directional structures (strings of a guitar, grain of wood, etc). Typically, materials with a random structure (felt, asphalt dampening pads, etc.) dampen vibrations better- (the sound travel a short distance along any directional fibers and then hits a dead end, transfer of energy has to occur to reach the next directional segment of a different length, which converts a portion of that energy into heat, repeating itself over and over again within an inch of material). MDF, having a fairly random fiber structure, is actually fairly well damped. Is MDF the most dense or rigid material... no. Do the resonances in MDF that do exist sound "natural" or pleasing... maybe not- probably depends on the application. Beyond the material itself, by adding dampening material to the surface (felt, foam, cork, etc.) the vibrations of the structure will be absorbed, along with vibrations transmitted from the surrounding air.

Overall, a mineral filled resin frame (like a thick Corian countertop or preferably one of the other brands that offer more mineral content) wrapped with a structural skin and then a dampening material may offer the stiffest, heaviest, and most damped frame. However...

That does nothing about resonance in the driver- that may also need to be stiffened and dampened to eliminate any resonance...

And then you get into personal theories. If the frame is too rigid, some people believe that no vibrations will be transferred and, therefore, no vibrations to be absorbed by the frame and there will be more audible resonance overall. Some people also believe that a rigidly coupled driver will sound better, some a decoupled driver. Some people also take the "musical instrument" perspective and believe that resonant frames/panels are actually a good thing- adding pleasing coloration, like the warmth of a tube amp....

Making a more rigid/damped/heavier frame is relatively easy- the more subjective "make it sound better" question is the difficult one.

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