When you add mass, you simply lower the resonant frequency. On top of that, you also make it harder to stop the resonance once excited. By lowering the center frequency of the resonance, you also increase the potential to excite this resonance in the listening room. Why is that? Simple.
Bass is both the most potent part of most any recording and it is also omni-directional in the frequency range where most "weighted" devices will resonate. The end result is that the low frequencies will "float" to wherever your equipment is located and resonate the rack, adding bloat and ringing bass. This makes music sound slow, thick and lifeless, just like "over-damping" the room acoustically does.
You need to find a way to damp / absorb vibration without adding mass. The higher in frequency that you can get the system to resonate, the less likely it is to be sonically noticeable. Not only do signals become more directional as frequency rises, they also lose intensity as distance increases at a faster rate due to their shorter wavelengths. As such, there's less potential to excite these resonances due to directionality. The energy that is able to excite them isn't as potent due to the wavelengths, reduced intensity and shorter duration of the signals involved.
As such, something that is rigid yet light in weight and has a high level of "self damping" i.e. is a "lossy material" is FAR superior to something that is rigid and a good conductor of vibration. Materials that tend to ring or oscillate quite easily, even if at a higher frequency, should be avoided. That's because you'll not only hear the sound coming from your speakers, but also the secondary oscillation as a source of sonic energy. Why in the world someone would want to put something that is "ringy" and easily excited in the same room as their audio system, i don't know. It just doesn't make any sense to me.
The one exception to this is when you use a "ringy" material ( like thin yet relatively stiff metal sheets ) in conjuction with another material that is low in mass and high in "self damping" traits. Constrained layer damping definitely works and can offer excellent rigidity with a high level of damping / absorption, but finding the right combination of materials can be tricky, time-consuming and you still have the potential for increased mass with a lower resonant frequency.
Quite honestly, i would not have believed that any of this could have affected the sonics of my system until i learned the hard way. That is, i changed racks in one of my systems and the sound of that system turned to crapola. After putting 2+2 together, i was on yet another search in terms of how i could solve this problem while moving forward.
It should be noted that different woods have varying "loss factors" i.e. rigidity to density ratios. Same goes for different types of "styrofoam". There are also different grades of Carbon Fiber, Fiberglass, plastics, metals, etc... Even the glues that one might use to bond specific materials together have different traits.
I know that some of you will think that this is crazy, but if you build a rack or shelf and keep it light yet rigid and moderatly damped rather than resonant, you'll instantly be able to hear the benefits. Sean
>
