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          Technology: Drainage and Isolation

The Cone

Everything you always wanted to know about cones- but were afraid to ask.

The first important invention of vibration control in audio was the spike or cone. No one is quite sure just who invented it; all we can be reasonably sure of is that the cone started becoming popular in the early to mid 80s. Today, almost everyone in high end audio or video understands the value of the cone; it's ubiquitous with most rack and stands, many speakers, and even built-in on some active components such as CD players and integrated amplifiers. There are many variations with regard to size, sharpness of point, and composition, but one thing remains more or less the same: the general shape. That is, it has a large area cross section at one end, and tapers to a small area cross section at the other.

How a Cone Works

Vibration can be thought of as mechanical waves. These waves are analogous to electrical waves, because there are similarities in their behavior. Mechanical waves propagate in materials much as electrical waves propagate through a conductor. The better the conductor, the more efficient the transmission, and the less "resistance." Mechanical waves propagate faster and with less "loss" through dense materials. This is why native Americans anticipated the approach of locomotive trains that were miles away by putting their ears directly to the iron track rails; the iron was dense and its metallic crystalline atomic structure conducted sound far better than the surrounding air, wooden ties, or earth did.

Most cones are made of a hard material. Thus, it conducts mechanical energy with good efficiency, far better than softer materials, such as the rubbery materials that most equipment feet are made of. When a cone is installed, it is placed directly against the chassis of the component. Doing this achieves an important goal which is easily overlooked: it has created a mechanical "short circuit" around the less efficient conductive path of the rubber feet.

The Ground Connection

Now let's back up a few steps, and imagine that there is mechanical energy present in the component. How do we get rid of it? The easiest way is, of course, not to let it get there in the first place, but for the sake of our argument, let's say it's being produced in the component, say, by a vibrating transformer. If we can't remove the transformer from inside the component, we need to try something else. That something else is to provide an escape path for the vibration. Enter the cone. Since the hardness of the cone is much more similar to the hardness of the equipment chassis than the rubber feet, there is a "mechanical impedance match," and waves can easily cross over the boundary between chassis and cone top, because as far as the waves can "see," there is no difference between the two. Mechanical energy which was previously "cut off" from escaping through the softer, less dense material of the existing feet, passes much more easily into the top of the cone. From there, it's all downhill. By connecting to the chassis and making a "link" out, and past the rubber feet, energy may be drained off because it now has a place to go, instead of rebounding inside the component. In a way, we have reduced the internal pressure or mechanical "voltage" present in the component, by grounding it to an external ground, where it will propagate and eventually be dissipated.

Skipping Stones and Index of Refraction

When many of us were young, we used to skip stones off a surface of water: lake, pond, etc. Two things were needed: a smooth, flat stone, and a throwing trajectory of an oblique angle. When these two requirements were met, magically, stones could be made to "bounce" off the surface of water! Magic? Well- not exactly.

Everyone knows about light reflecting off the surface of water. We learned in school that the angle at which it penetrates the water is depedent upon the water's index of refraction - an indication of how much the light path is bent as it crosses over from air to water. There is a critical angle which determines whether the light penetrates through the surface or reflects back, and in general, this angle is small. That is, if the light is coming in at a low enough angle, all of it will reflect off the surface and none will penetrate through. Now let's go back to stones.

When you want to skip a stone off water, you must throw it at a similarly low angle. Take the same stone that skips off the surface if thrown at a low angle, and throw it straight down into the water- and it goes right through, with no bounce or skip. This is because the 90° angle is the maximum angle for penetration- if you want something to go through, you send it at this angle into the object or surface of choice. Want to get a bullet through a wall? Your best chance is to shoot head on. Shoot at a low enough angle, and you might even have a richocheting bullet, because the wall's mechanical "index of refraction" may be too great for the bullet's entry angle, and so deflects it.

In like manner, a cone "funnels" energy that's conducted from its body down further and further toward the point. At this place, there is only one angle possible into the next surface- a 90° angle. This is the optimal angle for penetration into a surface- whether it be a wood shelf, a floor, concrete, or what-have-you. Forcing the energy that is traveling through the cone to intersect the next surface at a right angle maximizes your chances of getting this energy to penetrate through that surface. That's one of the main "points" of a cone or a spike- to present the mechanical energy being led out of the component at a 90° angle to the supporting surface.

Too Much Pressure

And then there's the issue of all that weight being concentrated on one point, which exploits the relationship between mechanical pressure and contact efficiency. Anyone who's ever boiled water on a stove with electric heater elements knows that if you push the pot down onto the metal heating elements, the water will come to a boil faster. This is because you are making a better mechanical connection between the pot and the heating element. Heat energy is also a form of vibration; in like manner, if you apply more pressure between a component and its support, there will be a better energy transfer. So, reducing all the weight of a component down to two or three tiny points makes the transfer at those points much more effective, since the effective pressure may be hundreds of pounds per square inch, or more. This is analogous to reducing the resistance in a circuit from many hundreds of ohms to a fraction of an ohm, and energy has a commensurately greater chance of passing through.

Does a Cone Isolate?

Many people think of cones as isolation devices, but are they really? The answer here is a resounding yes and no, although in general, the answer is "no." The main function of a cone is to drain. "Isolation" requires cutting off the component from its support mechanically, which we have seen a cone does not do- quite the reverse, in fact: the main function of the cone is to establish a connection to its support which is superior to its normal built-in feet, which are usually of a soft, compliant material. In fact, these feet are actually "vestigial" remnants of the ancestors of modern components: their original and true purpose was not to scratch the furniture they were placed upon. Back in the 1940s and 50s, that's about the extent that "hi-fi" feet were good for.

But we said no and yes. So what's the yes part? Just this: a cone is a two way street- energy can pass out, and it can also pass in.

The difference is that the entry point is a tiny point, so that of all the energy that may be circulating in the support surface (and it is), only a fraction of the entire surface is being "sampled" for energy, and so, in general, less may be thought to be getting back in. However, one must bear in mind that energy is constantly moving- like waves- and sloshing back and forth across the surface. Eventually, this energy will "hit" one of the cone points and find an entry point. Because of this "time delay" aspect to "vibrational energy re-uptake," phase anomalies may be introduced to the noise being induced into the component; this may be partly responsible for what we hear as the differences in placement of cones or feet under a component.

True Isolation?

With regard to low frequency isolation from a cone (below about 50 Hz), there is none, since the connection of the cone to support surface is rigid. When the support shakes, so will the cone, and so will the component. However, something else is at work: if the supporting surface is softer than the cone (it usually is), then the point will only more or less be sitting exactly on the surface. Looking at it microscopically, the surface, since it is not really very hard, exhibits a sort of "trampoline" effect because all of the weight of the component is concentrated at 3 (or 4) tiny points, stressing these parts of the surface, and usually distorting it. It's like the difference between balancing a stone and a pool cue in the palm of your hand; with the stone, your hand stays flat; with the pool cue tip, there's some stretching, and the skin more easily "gives" way to abrupt changes in the pool cue's position. In similar fashion, the surface of our component's support is going to "give" a bit under the increased inertial mass of the vastly increased weight-per-square-inch of a cone point, unless that surface is very, very hard and rigid. As vertical waves move our less-than-perfectly-rigid support surface, they will tend to "flex" the surface around the intersection of cone point and surface- in other words, there is some decoupling- and a degree of low frequency isolation. But the amount of isolation of wave motion is extremely limited (due to the very small excursions possible), unpredictable, and certainly not a constant value. It's very dependent upon the support surface material, its hardness, and many other factors - cone points impinging upon granite or steel will behave differently than when on wood or plastics. But in general, it's safe to say that isolation with cones exists in a very limited form, if at all- and quite by accident.


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