Controlling Vibrations Part I
- Why do better speakers come with spikes? Did they always?
- Well, no. And those machined cones you see under speakers, amplifiers and CD players weren’t always around either. Indeed, their existence was long ridiculed by engineers who were willing to consider everything but the evidence. And it still is, as you’ll know if you occasionally wander over to the high end audio discussion groups on UseNet.
- Linn’s Ivor Tiefenbrun likes to take credit for launching the first loudspeaker with spikes…though we don’t know whether he’s right. And it is usually considered that Steve MacCormack, who owned a company called The Mod Squad, first popularized cones with the now defunct Tiptoes. Or perhaps not, because Steve himself told us in an interview that he isn’t sure why they work.
- Of course intuition seems to run against spikes. If you want to prevent vibrations from traveling to or from components, wouldn’t rubber be a better solution? But then intuition, as someone once said, is just obsolete science. Spikes and cones do work, sometimes better than materials that look as though they can prevent transmission of vibration.
- In this installment, let’s look at the spike, a device that is supplied with nearly all modern floorstanding speakers with high end pretensions. They look something like this.
- Since there isn’t anything “soft” in a spike, which is rigidly screwed into the speaker enclosure, you would think that energy would flow right through it like electricity along a wire. In fact it won’t, because although the spike does provide a path for the electricity to flow, it isn’t an efficient path. And the lower the efficiency, the less energy will be transferred. To figure out the reason, look at the very tip of the spike, where it meets the floor. The energy must travel along a device whose mass decreases constantly until it becomes very small, and then must leap into the huge mass of the floor. The discontinuity is sudden, and it is huge. In engineering terms, there is a large impedance mismatch.
- And what happens when the energy reaches this change of impedance is that most of it is actually reflected back. It does not enter the floor.
- The cone (Tenderfeet, the original Tiptoes, etc.) carries this a step further. It looks something like this.
- The principle is the same, of course, except that the taper is much more radical, going rapidly from the quite large base to the tiny point, which contacts the large mass of the floor. The mismatch is even more pronounced, and the efficiency of energy transfer will be even poorer…which is what we want.
- Why should we care about energy going into the floor? Well, your neighbors could quote you a reason or two: once vibrations enter the structure of a building, no soundproofing measures can keep it out. If you don’t have neighbors within earshot, you probably won’t want acoustic energy traveling along the floor to where the rest of your equipment is (that’s true even if you don’t own a turntable, by the way). The final reason is that all of the energy fed to the speaker should be going into the production of sound, not in shaking up the structure of your home.
- Can resilient materials, such as rubber, also prevent the transfer of vibrations? The answer is a resounding maybe. More on this in the next installment of this online feature.
- However resilient materials should not be used under loudspeakers. To figure out why, look to Newtons Third Law: “to each action, there is an equal and opposite reaction.” That wasn’t entirely intuitive when Newton first proposed it three centuries ago, because it seems to mean that if you pushed against a wall, the wall would push back. Absurd? Not really. If the wall didn’t push back, it would move. If it doesn’t, it must push back sufficiently to counter the force of your hand.
- What does this mean for a speaker? Let’s say that your woofer’s coil, which is rigidly attached to the woofer frame and therefore to the speaker cabinet, pushes the cone forward. The Third Law says the cone will push back on the rest of the speaker with an equal and opposite force. As the cone moves forward, the rest of the speaker will move backward.
- Oh, it won’t move very far, not far enough that you can see it move. It’s too massive, and it has too much inertia. But the energy that moves it will be lost to the speaker. Instead of producing sound, that part of the energy will be wasted. The speaker will be less dynamic, and will have less punch.
- Is this audible? It certainly is!
- The spike or the cone actually does two things at the same time. As we established, it offers an inefficient path to ground for stray energy, and it also anchors the speaker to the floor mechanically, so that it doesn’t move.
Naturally, the spike has other applications too: under equipment racks, for instance, to prevent energy from traveling up from the floor. - Carpeting
- Some audiophiles hesitate to use spikes, because they fear damaging their carpets. Actually, spikes are much better for carpets than the feet usually found on tables and sofas, which crush the pile. A spike or cone will make its way among the tufts of the carpet and bite through the backing to the floor below. Placing an unspiked speaker on a rug would leave a rectangular mark that would never come out. And of course, the speaker wouldn’t sound right.
- Materials for spikes
- The spikes that come with most floor-standing speakers and racks are steel, simply because softer or more ductile metals could bend under the mass of a large speaker or a rackful of equipment. Steel may not be best for larger cones, however, and aluminum is commonly used instead. Manufacturers of some very large speakers or heavy racks prefer brass.
- Other materials are sometimes used too. We still have a set of cones made of a ceramic material (not as good as aluminum or brass, as it turns out). At least one manufacturer makes cones of carbon fiber. These cones work very well, but the tips are rounded rather than sharp, which makes them unsuitable for carpets.
- Under the spike
- Now that we’ve reassured you about carpets, perhaps we should do the same thing about your hardwood floors. Can a spike or cone make a hole in your oak floor? Sure it can.
- That’s why some audiophiles place something under the point. Indeed, some cone makers make little cups that can be placed underneath. In the case of the Tenderfoot, for instance, you can add a ‘Tendercup” underneath.
- Does a cup weaken the effect of the spike? Yes…slightly. Unless you are placing your spikes on a wood floor, don’t use these devices. And avoid any cup-like devices which incorporate resilient (rubber, etc.) materials. They’ll wreck the dynamics.
- Do resilient anti vibration devices work? Are there circumstances when they might be preferable? Yes indeed. We’ll go into that on Part II of this series.
