Bad Vibes! Page 7

For those fortunate enough to have concrete floors, the modal waves are spread out, and the peaks of the waves are far apart compared to those in the platform or stand. From the point of view of the platform, it would be similar to driving over a broad speed bump. You'll notice it as a gentle rolling, but nothing like hitting a sharp, narrow bump. In other words, most vibrations transmitted to the floor, from whatever source, will couple directly through the cones without much amplification of the modal activity from the floor. In any event, there isn't a whole lot we can do about floor vibrations, whatever their origin, by using rigid coupling alone. Therefore, in order to get the best performance from this method, we must direct our attention toward placing the cones at the minimum nodes on the bottom of the platform.

While many people have noticed positive changes in system tonal balance with various cones, others have at times experienced degradation, particularly if they did not make an effort to find the quietest locations for their cones. As we inferred, the shape of a cone and its composition may contribute somewhat to its sonic effect, particularly if it contains some inherent damping qualities. However, it is their placement relative to modes and nodes on the coupled structures that has the largest impact on overall performance. Regardless, it is clear that this method, which falls in the domain of system tuning, is best used as an element in a more comprehensive vibe-reduction plan incorporating isolation and damping. (The benefits and limitations of compliant Navcom-like pucks are discussed under "Suspension Fundamentals.")

Even though rigid coupling has limitations, it does play a necessary and important role in nearly every stereo system. Therefore, if you've had bad luck with certain cones, try placing them in a variety of locations before you give up or rush out to buy new ones touted as sounding inherently better.

A good place to start your experimentation for that quietest location is 22% in from the two ends of any homogeneous rectangular platform. Simply measure the width of the platform and multiply that figure by 56%; the result will give you the spacing between the two cones, keeping them the same distance from the two ends of the platform. This corresponds to the minimum node of the first, most prominent bending mode on a typical shelf, and should put you in the ballpark. (I'm having a an independent lab test a well-made platform for transmissibility when it's supported by a number of different cones and elastomer pucks. If we gain further insights with respect to cone placement, I'll be happy to share them with you in a future article.)

Stand Design
So far, I've primarily dealt with the problems faced by uniform supporting structures such as shelves and floors. The modal shapes of most equipment stands are far more complicated, and add a second challenging element to the vibration equation. Since space limitations generally require the use of shelved stands, knowing what to look for is critical.

Again, the most important quality is rigidity. Though a number of stands on the market are very rigid in the vertical plane, most bend like a house of cards in the horizontal plane when bearing a heavy payload; eg, a turntable on a granite slab. This happens because stands are usually made of three or four vertical legs connected with horizontal tiebars at the top and/or bottom. If diagonal tiebars or turnbuckles were placed across one side and/or the rear, these stands would be far more stable in both planes of motion. Fortunately, many heavy-duty steel or aluminum stands are filled with sand and lead shot, which increases mass and partially damps the tendency for the metal shelves to ring when stimulated by both floor- and acoustically coupled vibrations. A few stands are made with inherently well-damped materials.

It's as simple as this: Keep your stands as short as possible. The shorter the stand, the more rigid it will be.

Use all of these tips when shopping for a stand, and by all means don't skimp on quality and rigidity for the sake of looks alone. Armed with these guidelines and a willingness to investigate the available options, you're likely to find some creative and unique variations on these techniques that work very well.

Component Level Control
The first elements in the vibration equation affecting audio systems, and the final constituent in our look at rigid coupling, are, of course, the actual components. Since vibrations are best dealt with closest to their source, equipment designers bear the brunt of responsibility for minimizing internally generated resonances and the damaging effects of external vibrations on their chassis. As we've seen, building an inert component that is immune to outside disturbances is completely impractical; informed designers attempt to minimize the strength of offending internal vibrations while shifting inevitable resonances to a less harmful region through careful layout, strong chassis construction, and local damping.

Those manufacturers who fine-tune the sounds of their electronic devices with minor circuit adjustments or component changes are, in part, adapting the sound to the resonant signature inherent in their components' designs. The Jeff Rowland Design Group products---as well as the MFA MC Reference, CAT Signature, Accuphase gear, and the new electronics from Balanced Audio Technologies---are just a few of a growing list of examples in which attention to vibration at the design level has paid off in improved low-level detail and subtle refinements in tonal balance.

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