Bad Vibes! Page 5
Very few structural materials deal efficiently with the requirements of both rigidity and damping. Some materials address one aspect while degrading the other. Therefore, better performance is usually obtained by a composite approach to platform or shelf construction. Mass can be a desirable quality in a material used for equipment supports as long as it contributes to dynamic rigidity. But excess mass that does not aid the cause of stiffness may actually be detrimental, as it can cause a reduction in the platform's resonant frequency, requiring extra damping material to attenuate the increased displacement. Since most materials used for damping tend to be compliant, the amount needed to even partially reduce lower-frequency amplitudes can cause an unwanted reduction in the stiffness of the structure. Also, if a platform's ratio of mass to stiffness is excessive, mass may contribute to a subtle sagging of the platform, degrading static and dynamic rigidity.
Most materials that are desirably stiff do have a good deal of mass, so successful plinths, platforms, or shelves used for audio combine rigid mass with the right amount of uniform damping, often by constraining one or more layers of visco-elastic material (such as E.A.R. "Isodamp") between two or more much thicker layers of stiff material (such as granite, steel, or 6061-T6 aluminum). Another successful technique sandwiches lighter but damped materials, such as MDF or acrylic, between two skins of steel or granite. Steel has a better stiffness:weight ratio than granite, though both can be used to good effect either singly or combined, as long as their tendency to ring in the lower midrange/upper bass is controlled with damping. High-quality aluminum is approximately one third as stiff as steel but is nonmagnetic, which can be useful with some components. Certain carbon-fiber composites show particular promise as well, as do several new designs offered with well-built stands using multiple layers of various hardwoods alternating with thin damping layers. The ubiquitous shelves made from medium-density fiberboard (MDF) benefit from uniformity and are fairly well damped, but are not particularly stiff.
We've now defined our near-ideal audio support platform. It will be uniform in structure as well as rigid for its size, weight, and shape. If we map out the composite sum of its modal shapes, we will find relatively few areas of significant displacement. As a result, the motion of the platform will be limited to the six basic degrees of freedom defined earlier. It will have a relatively high natural frequency and correspondingly low amplitude of resonance that will be further reduced by use of sufficient damping. This applied damping will also provide a sink for a broad range of component-generated vibrations. Sounds pretty good, doesn't it?
Several available platforms are headed in the right direction. One example, offered as a separate item to audiophiles by D.J. Casser Enterprise's Black Diamond Racing label, is a carbon-fiber composite platform simply called "The Shelf." (Footnote 2) It's reasonably stiff, has a fairly simple modal signature, and contains a good degree of self-damping.
The Rigid Coupling Surprise
Unfortunately, once we've built or purchased our dream platform, we then have to connect it to a stand or floor and place a component on top. This is the kicker: When you couple the most ideal practical platform to the floor with cones, spikes, or any other rigid footing, even at the ideal locations with respect to each, the best vibration performance you can achieve is nearly 100% transmission of floor-borne vibrations through the platform, without amplifying them or generating any new resonances in floor or platform! The same applies to component-generated vibration. At the very best, the combined structures will roughly approximate the "ideal rigid body" we mentioned earlier, moving through space in synchrony relative to each other so that the motion of the floor is matched by the motion of the shelf, with nothing added.
Any technique that does not provide isolation of external vibrations will only vary the amount of resonant stimulation added to the components concerned. It cannot reduce at all the level of baseline vibrations in the floor or those coupled from the air!
This principle is illustrated by both the "ideal rigid body" line in the compliance curves shown in sidebar 1, and the horizontal unity-gain line (labeled "1.0" in the various transmissibility graphs of sidebar 2). A perfectly rigid structure would not diverge from this unity-gain baseline in either direction, indicating nearly complete transmission of all vibrations between both the floor and the coupled elements.
At first glance, transmitting nearly all of the floor vibrations to a component might seem to be of no benefit at all. On the contrary, this would be a significant accomplishment compared to most real-world coupling schemes, due to an appreciable reduction in random levels of resonance affecting key components, as described above.
Indeed, it is the degree of deviation from this ideal that defines the wide variety of subjective sonic changes experienced by audiophiles using various non-ideal rigid coupling devices, stands, shelves, and components in actual audio systems. Also, when you consider all the ramifications of this scenario, it appropriately undermines the claim by certain purveyors of cones and spikes that these devices have a directional "diode-like effect," forcing discrete vibrations to flow like water from a dam: out of a component, through a coupled shelf, and then into the floor, where they are finally dissipated.
Footnote 2: For information about "The Shelf," contact Black Diamond Racing, 301 North Water St., Milwaukee, WI 53202. Tel: (414) 224-5300.