Bad Vibes! Page 8

Since most chassis have anything but uniform resonant signatures, the problems encountered in connecting a platform to the floor are compounded further when spiking a component to the top of the platform mounted on a stand. It's virtually certain that the irregular modal profiles of most component's thin enclosures will not only encourage significant amplification of related external frequencies, but the creation of new resonances as well.

Limited vibration attenuation provided by composite cones that possess a degree of damping may provide some latitude with respect to their placement between components and platforms, falling somewhere between solid cones and compliant pucks in performance. However, for the reasons described above, these devices, and rigid cones as well, can actually exacerbate a resonance problem when misapplied. Also, I've yet to find any devices from either category that produce repeatable, predictable results, regardless of what component or system they are used with. I'm therefore concerned that audiophiles be aware of those few products that do address the issue of vibration control at a more fundamental level.

Sand Damping
We've noted the benefit of using sand to fill equipment stands. Another important damping application is in products like the Bright Star sand bases. (Footnote 3) These boxes, ranging from 2" to 5" deep, are filled with sand on which a plinth is placed to support a component. The ability of sand to conform to the entire surface of the plinth material efficiently constrains and partially damps the platform's vibrational modes. While the volume of sand typically used in these bases will not result in true isolation---particularly for the most damaging low frequencies---it is sufficient to reduce the amplitude of resonances across a broad range of frequencies arising from the modal activity of the floor, stand, and supported component. This damping effect can be surprisingly beneficial.

Speaker Coupling
Perhaps nowhere are cones and spikes more widely used and accepted than when coupling speakers to a floor. Their sonic effects in this application are generally more pronounced in scale and more uniformly positive than when coupling linestage components to various shelves and stands. While many of the foregoing principles also apply to spiking speakers, there are important differences. An argument could be made that by placing speakers on cones rather than setting the entire speaker flush with the floor, the contact area between the various modal waves of the floor and those excited in the speaker's cabinet by the driving force of the woofers will be somewhat reduced, lowering the overall resonance of both structures.

This theory may provide some explanation for the effect of spiking a speaker, but the biggest impact results from simply stabilizing its motion relative to the air in the room. If a speaker is placed on a carpet or uneven floor, it will subtly rock back and forth due to the large excursions from the bass drivers. For example, when a signal comes down the cable, telling the woofer to move 1/16", its movement may be slightly reduced relative to the air due to the cabinet's pivoting action. In this example, spiking will tend to fix the pivot and result in a tighter, better-defined bass response and a cleaner presentation in the midrange. Very heavy speakers make this less of an issue, but since we're dealing with a transducing element, even subtle variations can have an audible effect.

These hypotheses are more straightforward when applied to a concrete floor. Spiking a speaker to a suspended wooden floor is fraught with the same difficulties as is a stand, yet the consequences for the whole system can be even worse.

Fundamentals Of Isolating Suspensions
Now that some of the benefits and limitations of rigid coupling are better understood, we should maximize its usefulness by employing rigidity where it counts: through supporting structures and the selective application of cones as an integral part of a complete isolation/damping/tuning system. In so doing we can move beyond simply reducing additive resonant effects, transmitted by the floor/stand/component interface, through genuinely de-coupling key components from the main sources of vibration while providing an efficient mechanism for attenuating residual disturbances.

Before I explore these issues of suspensions and isolation in more depth, I'd like to address another misconception prevalent in our hobby: that of "over-damping" non-transducing electronic components.

Damping is always present to some degree in any real system. Without it, there would be no way to limit the amplitude of a resonance. The materials employed to provide damping, the ratio of these materials to those elements that need damping, and the method in which they are applied, all determine how effective any scheme is at reducing broad-band vibrations. However, you cannot mechanically "over-damp" a structure or component that is not designed to be a transducer. You can over-damp some circuits electrically; you can over-damp the "Q" of a speaker as well. You can also over-damp structures associated with a phono cartridge: a tonearm, plinth, etc. But you cannot over-damp a preamp's chassis or an equipment rack. What you can do is "mis-tune" the chassis or structure by shifting resonances around and attenuating them in a frequency-selective manner that results in a dulling of the sound, which is then characterized as over-damped. You can also misapply certain damping materials in the construction of a platform so that its overall rigidity is compromised.



Footnote 3: Bright Star Audio, 2363 Teller Road #115, Newbury Park, CA 91320. Tel: (805) 375-2629.
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