I’ve just read this entire thread and agree with some points and disagree with others. I also understand some of the arguments but do not understand others. Firstly, a disclaimer …. I claim nothing in what I am about to say as “fact” or as being provable or supported by science. I have simply read a great deal of information and misinformation by many well intentioned forum members (on several forums) and am trying to filter it all in order to come up with a resonance control strategy that seems to make sense and is worth going to the effort and expense to try. Currently I use Aurios and Symposium RollerBlocks (sitting on DIY constrained layer platforms) under my gear and found them to be an improvement over the Polycrystal cones I used before. I have no intent, or the required knowledge, to get into arguments with anyone on this subject. Like many here, I’m just trying to understand the concepts so that I can take a step toward a sensible solution. I totally accept that I may be totally wrong in all I am about to say. If so, then hopefully some replies and discussion by readers will help me to understand why and to get onto the right track. It is also possible (likely?) that I will use the wrong terms to try to describe the concepts I discuss, so please bear with me ….
It seems to me that we are trying to deal with three things:
1) structure borne resonance
2) air borne resonance
3) self generated resonance
For the moment, I don’t really care which of these is highest in amplitude but am interested in attending to them all.
If I understand correctly (big assumption
), Hantra is suggesting that the sorbethane feet do not provide a conduit to ground and the vibrational energy is reflected back into the component. Gary, suggests that they are a conduit to ground but some of the energy is dissipated as heat. It would seem to me that both are right, to a degree.
Surely, the more rigid the material linking two objects is, the more effective it will be as a conduit and the more effectively it couples the two objects. Think of the vibrations of a distant train that can be felt by touching the rails. Surely the vibrations could not be felt from anywhere near as far away if every second piece of rail was made of sorbethane.
So, spikes and cones will provide a more rapid and effective conduit than soft footers, but it is two way and leaves our components exposed to the structure borne resonances from traffic, trains, seismic activity, speakers and subwoofers, etc.
Using soft footers between a component and the shelf it sits on will be less effective at allowing rapid transmission of a component’s self-generated resonance to flow to ground, but will also be less effective at allowing structure borne resonance to pass upward to the component. Again, if I understand correctly, the less tightly coupled two objects are, the greater the isolation between them. The ultimate example (short of a vacuum) would be where the two objects are only connected by air. In this case there would be very little coupling and a high degree of isolation from structure borne resonance.
So, if we apply visco elastic damping products to the outside and/or inside of the component’s chassis (eg. Dynamat Extreme, Cromolin, 3M constrained layer dampers), then we are damping the component generated resonance and also the effect of air borne resonance on the component, and reducing the need for the component to “drain” to the rack and ground. At the same time, the soft feet underneath the component are reducing the amount of structure borne resonance entering the component.
If energy can only be lost in the forms of “heat” or “work”, and if the sorbethane visco elastic footers are compliant in all directions, then it seems to me that these footers will reduce the amplitude of resonances via a combination of heat and work. The compliance of the soft footers seems somewhat akin to the movement of the balls in the various rollerball devices (Aurios, Darumas, Symposiums, etc) where energy is dissipated mainly as “work”. In addition, some small amount of energy may be lost as “heat”, such as in the case of the visco elastic damping materials mentioned above. By the way, although there is sometimes dispute over how much of these chassis damping compounds to apply, everyone seems to agree that they do effectively damp chassis resonance. Of course, more resonant components (usually cheaper ones with thinner chassis and poorer design etc) will usually be more sensitive to such treatment.
Taking this a step further, there is probably no such thing as a perfect isolation device or a perfect damping device. And, the higher the resonance frequency, the easier it is to damp. This seems to be the theory behind the movement toward “light-and-rigid” rack designs and platforms (eg. Ikea Lack tables, Systrums, Neuances, etc). So, if we change from a massive rack to one that is light but rigid, the natural resonant frequency of the rack will be higher and any isolation devices used between the rack and the component will be more effective as they don’t need to isolate down to as low a frequency. If the rack is suitably light and rigid (eg. made of the Lack tables) then it has insufficient mass to support the resonance amplitude of a heavier rack and may need little or no damping. As the natural resonant frequency is also much higher, then soft feet can more easily damp the structure borne resonances and provide more effective isolation to the component. And, this is without having to worry about additional “floating” platforms between the rack and the component, or the additional footers they would need to rest on. Typically, the more massive the rack or platform, the greater the amount of damping material is required. With sheet metals, typically the thickness of the damping material (eg. Constrained layer damping tape) should be the same as the thickness of the material it is trying to damp. If you flick a crystal wine glass to make it ring, then put your finger on it, it will stop ringing immediately as your body is now damping it. However, if you strike a large gong, then put your finger on it, it won’t be nearly as effective.
It seems to me that many people try to make things too complex with multiple levels of isolation and platforms etc. Adding a heavy floating platform (eg. marble slab on inner tube) between the rack and the component is only going to lower the resonant frequency and make the isolation products above them less effective. Even though the inner tube will isolate the marble slab from the structure borne resonances to a degree, the airborne resonance will still cause the marble to resonate and it will do so at a lower frequency than the Lack table it sits on. As there is no damping compound usually applied to the marble, it will “ring”. If hard footers are used between the marble and component then the ringing will readily excite the component chassis. If soft footers are used, they will provide some isolation but will not be as effective as they otherwise would have been if used directly on the Lack table.
OK, so lets have some constructive criticism. Where do my theories fall down, where can they be improved, where are they just plain wrong?
Thanks for reading and thanks in advance for any constructive feedback.
