Loudspeaker Design Overview Page 3
How then do we accept that we must build enclosures out of rigid, inert materials that are incapable of absorbing or dissipating energy? This is exactly the opposite of the enclosure’s function. “Stuff it with foam, that will solve the problem.” Anechoic chambers use 12 foot wedges of foam. Enclosures that accept 12 foot wedges of foam are not acceptable. Especially when we realize that the enclosure would still need to be the size of a gymnasium. Perhaps sometime in the future a foam will be developed that is capable of absorbing this much energy in a small space, but until that day, I know that current enclosure design does not function to properly serve its purpose. Common excuses include the mantra that “if we allow the cabinet to vibrate, it will make sound of its own.” This will only occur if the cabinet is not properly designed to only dissipate energy on the inside of the enclosure. These two concepts are treated as parallel issues, when they are in fact separate and distinct engineering criteria. You can design a high loss enclosure that effectively “kills” sound projected into it, while not letting any of the internal energy be re-radiated by the outside cabinet walls. (I think I have just re-stated the definition of an enclosure...)

If we examine what was just discussed, it becomes clear we must, in effect, have two cabinets. One that is an inner high loss enclosure, and one that is an acoustic barrier (sorry for the engineering term). Here we must realize that single materials that exist in nature are either good energy absorbers or good acoustic barriers. This is the reason for the foam/rigid cabinet argument. The foam is simply too inefficient to accomplish the task. Therefore, I know that no single material can fill this role. Multiple materials must be used each with a specific purpose. They cannot all be rigid (barrier) or all absorbing. The only proper answer to our enclosure is a composite of dissimilar materials or a new family of materials that do not behave like traditional materials. Either that or Magic Foam!

Composites open an entire new world of possibilities, not specific to enclosures only. We have the trade off of rigid versus damped (paper) cones. Single material metal or ceramic cones are great pistons but they ring. Some creative applications are used to force these resonances out of the listening spectrum but they will never be inherently stable. I know this is how materials behave but some attempts at working around them are successful (please no angry letters). Paper cones are very stable and well controlled but lack the stiffness to be successful pistons and loose detail. Composite cones are capable of combining elements of both worlds resulting in true pistonic behavior with tremendous energy damping characteristics. These are drivers that are inherently stable and detailed needing no super human means of correcting for flaws. Composite drivers are the coming revolution in audio (because they make sense at an engineering level).

Forming a Design Philosophy

Keep it Simple (Stupid). Choose designs that are stable with the fewest negative side effects. Have a clear understanding of what you are actually trying to achieve and let that guide you to the solution. Never start with a “preliminary” solution based on a preliminary assessment of tasks to be accomplished. Once you head down the wrong road it gets harder to find “reverse.” If you are going to design something based on current design ideas or “common knowledge” chances are, it has been designed and marketed already. Don’t fear new thinking, no matter how scary it can be at first examination.

Finally, don’t take the Red pill or the Blue pill. Take the Aspirin...
Robert L. Grost
Director of Engineering
Cerious Technologies
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