Bad Vibes! Page 10
A rudimentary version of the traditional damped suspension is formed when elastomer materials such as Navcom or Sorbothane are used to support a heavy preamp or amplifier, either directly or with an intervening platform. These elastomer pucks can be quite effective at isolating moderate amplitudes of vibration ranging from the upper bass and above, and will generally have a fairly predictable performance throughout this range of frequencies when used with a wide range of gear. Also, a broad band of vibrations generated within the component is partially damped by these compliant materials. Unfortunately, their damping and isolation ability is not only ineffective at very low-level vibrations of any frequency, but is essentially transparent to all amplitudes of very low frequencies, acting basically like rigid coupling rather than an isolator in response to vibrations lower than the natural resonance of the suspension.
For many systems using rubberlike pucks, the resonant frequency ranges from approximately 10Hz to 20Hz or higher depending on the actual compliant material, how it is shaped, and the load it bears. So even though the peak displacement at resonance will be reduced, vibrations below resonance will either pass right on through or be amplified. In practice, many such suspensions have relatively high resonant points, so this amplification will often extend into the lower audio band. For example, a system formed by typical rubber pads or pucks supporting a moderately heavy steel plate will have a vertical resonance of around 15Hz or so. Its related resonant displacement is fairly well controlled, yet the zone of amplification actually extends from approximately 3Hz up to around 25Hz---above which isolation finally begins. This scenario can contribute to the subjective impression of a "mushy," "soft," or "boomy" bass response, even as the suspension reduces the amount of transmitted vibrations from the midbass on up, and partially damps the component-generated vibrations.
Unfortunately, this limitation of certain elastomer supports is often misconstrued as "over-damping," even when describing its effect with amps and preamps, and has led to the unfortunate condemnation by some of any sort of damping at all. Actually, this negative subjective effect, reported when elastomer supports are used in some systems, stems from the amplification of the suspension's relatively high resonant frequency intruding into the lower audio band (the opposite of damping).
Paradoxically, systems that emphasize the bass can sometimes sound rolled-off in the treble as well, although this is usually a psychoacoustic effect rather than a genuine rolloff. In any event, this example highlights the danger in drawing cause-and-effect conclusions about subjective experiences in audio without trying to tie them back to real physical principles. The positive sonic effects of elastomers are almost entirely due to their damping and isolating qualities; when properly applied, elastomers can result in a significant reduction of vibrations from the upper bass on up.
Incidentally, several equipment supports or footers now on the market combine a degree of rigidity with a measured amount of damping, without being overly compliant. These devices seem particularly well suited for connecting components to a platform already isolated by a suspension. (See my Townshend/Vibraplane review elsewhere in this issue for some examples.)
The Phono Challenge: The Pneumatic Edge
Far more elaborate and effective examples of damped suspensions can be found in certain high-end turntable designs employing very compliant, viscous-damped springs, generally with lower, less intrusive resonant frequencies. However, the tonearm mass and cartridge compliance of most LP players results in a natural resonance with a frequency between 10Hz and 15Hz. Therefore, with a typical turntable, we need to achieve excellent isolation by just 10Hz! Unfortunately, the vertical resonant frequency of many turntable suspensions overlaps this region of tonearm resonance. To complicate matters, the suspension's horizontal frequency will tend to be even higher than the vertical. In such a system, amplification of the suspension's natural resonance directly exacerbates that of the tonearm/cartridge, resulting in the addition of sonic colorations.
Among the notable exceptions to this quandary are the unusual and reportedly successful suspensions found in the SME 30 (around 4Hz), the Versa Dynamics 'tables (2.5Hz in both planes), the Kuzma Reference (at 2.2Hz), and a number of other high-end designs. Better turntables employ suspensions that damp both the suspension and tonearm to minimize the amplitude of these resonances, particularly with regard to vibrations in the audible bandwidth, and often achieve good results. However, even these well-designed spring-based suspensions have a tough job handling the isolation requirements of a turntable. These additional factors partially explain why:
Earlier, I alluded to the need for vibration attenuation in both the vertical and horizontal planes. Almost every spring- or elastomer-based design has significantly better vertical than horizontal isolation. As a result, "flanking paths" can be created, allowing horizontal resonances to impair the overall isolation effectiveness of a suspension. Without nearly equal isolation in both planes, serious compromise is inevitable for any turntable system. The flanking mechanism can also be exacerbated by the reactive damping inherent in most spring and elastomer systems.
Modern low-output cartridges, the vagaries of the stylus/groove interface, and ever-more-refined amplifying electronics place tremendous burdens on not only the suspension, but every aspect of turntable design. These systems can amplify a signal over 30,000 times to portray the subtlest hall ambience or textural nuance---those tiny details that give recorded music so much life, vitality, and spatial resolution. Since a turntable's arm/cartridge system requires maximum isolation to be achieved in both planes by 10Hz, we need a suspension with a bidirectional natural frequency well below 5Hz, when possible.