Spikes and cones for loudspeakers to avoid vibrations

Spikes and Cones – What’s the point?

In recent years it has become common for items of audio equipment to be mounted using “spikes” or “cones”. These come in all sorts of shapes, sizes, and materials, at all kinds of prices. The Hi-Fi magazines sometimes ‘review’ these accessories, and recommend their use. However, are they worth buying and using? On this page I’ll consider their use with loudspeakers, and discuss some alternatives.

The above diagram shows a typical arrangement. In this case there are a set of cones between the actual loudspeaker and its stand. (In some cases these cones are placed ‘point upwards’ rather than as shown here.) The base of the stand also has spikes fitted – i.e. long thin cones. These can penetrate the carpet, and if you have a wooden floor, they may then stick into the floorboards through the carpet.

In some cases, people use such spikes with some form of washer or coin so that the points sit on a small ‘puck’ rather than dig into the floor. Also, sometimes the spikes do not penetrate the carpet or flooring. To start with, I’ll ignore these possibilities, and return to them later on.

In general, Hi-Fi magazines claim that using cones and spikes like this will improve the sound. They also sometimes tend to describe the action of the cones or spikes as providing “vibration isolation”. Indeed, cones are sometimes also recommended for use under CD players, amplifiers, etc, and it is sometimes claimed that this can improve the sound by “isolating” the units from vibrations. This leads to two questions:

Do these cones or spikes actually do anything worthwhile?
If so, what do they do, and how do they work?

At first appearance, it seems plausible that cones under a loudspeaker should reduce the amount of sound energy transmitted between speaker and stand because they reduce the area of contact between the speaker and the stand. In the same way, we might expect spikes at the base of the stand to provide isolation. The basis of this idea seems to be that if the contact area was zero (i.e. no contact at all!) then no solid vibrations could pass. Hence if we make the contact area tiny, this approaches zero, so should only allow a small amount of vibration through. Unfortunately, although plausible, this idea may simply be incorrect.

We can understand how the above idea might be wrong by considering another example of the use of “spikes” – a running shoe. Here the purpose is to increase the grip between two objects (shoe and ground) and hence transmit forces more efficiently. This example instantly shows that a cone or spike might not reduce the amount of actual force transmission, but actually help ensure effective transmission. Hence spikes and cones – far from “isolating” – might sometimes give a more effective link for vibrations to pass though.

In some recent issues of the audio magazine, ‘Hi Fi News’ [ref 1], Keith Howard used an accelerometer to measure the levels of vibration in various objects. When he tried measuring the vibrations produced in a loudspeaker stand by playing the speaker he found a result that surprised him. With the speaker unit sitting on cones the level of vibration of the stand was over one hundred times greater than if the cones were replaced with small rubbery feet. This indicates that cones are of doubtful use if the intention is to stop vibrations passing from the speaker to the stand, or to any other solid objects against which the speaker may sit.

In fact, this result is also consistent with the predictions you can get by looking into the theory of vibrations in solid structures and objects. [ref 2] (Although such predictions sometimes need to be treated with care as the results depend upon the assumptions made when doing an analysis!) The theory and actual measurements seem to agree that hard cones and spikes are not necessarily a sensible choice if vibration isolation is what you require. An air gap or soft rubbery feet would probably be much better.

So do cones and spikes do anything useful, and are they worthwhile?

Spikes into the floor.

Despite the above, there may be cases where the use of cones or spikes do have an effect – although probably not for the reasons often given in audio magazines. In some cases the effects may be unwanted, though, so beware. To illustrate this we can use two examples.

The first example is shown in diagram 2. This shows a side-view of a speaker on a stand without cones or spikes. The stand rests on the carpet. When we play music the speaker cones vibrate backwards and forwards. This produces the sound we want. However by Newton’s Laws of motion it also shakes the speaker and the stand. The result, especially at low frequencies is a tendency for the speaker and stand to ‘rock’ back and forth. Compared with solid materials like metal or wood, the carpet is quite soft and spongy. Thus it can act like a spring and allow the base of the stand (or the legs) to move, and the whole system wobble.

This behaviour may have various effects. One is that vibrational energy now finds it more difficult to ‘escape’ from the speaker and stand into the floor. In this case, it is the carpet that is providing some “vibration isolation”. Another possible result is that, by allowing the speaker to ‘recoil’, we may be influencing how easily the speaker can produce low-frequency sounds. Hence we may find this alters the bass response. It may also be worth noting that when the floor does vibrate at low frequencies we may sense or hear these vibrations. Hence isolating the speaker from the floor might mean some other changes in the sound due to reducing the floor vibrations.

By having spikes on the base of the stand (as illustrated in diagram 1) we allow the speaker stand to ‘grip’ the floor under the carpet. As a result, we can expect the spikes to reduce how much the speaker and stand wobble around, particularly at low frequencies. Unwanted vibrations or shaking of the speaker box and stand at low frequencies could then be reduced. By holding the speaker more firmly we may alter or assist its ability to radiate low frequencies. In addition, the floor may now act as a secondary radiator or medium to convey sound vibrations to the listener. Hence spikes under a stand may well have an effect we can sense, not because they “isolate” vibrations, but for precisely the opposite reason! To do so, however, they need to effectively penetrate through the carpet so the stand can grip the floor underneath.

What the above can’t tell us, of course, is whether any changes produced by such spikes will be judged to make the resulting sound “better” or “worse” – or even if the changes will be noticeable. That will be a matter of individual circumstances and the personal preferences of the listener. There are various reasons for this. One example is that, in some cases, the movement on the springy carpet might help damp out higher-frequency resonances in the stand more effectively than being gripped by the floor. Matter of circumstances which might be judged ‘best’ for changing the performance in the way the individual user might prefer.

Not making holes in the carpet!

An obvious problem with using spikes as considered above is that we end up with holes in the carpet and floorboards! The good news is that with narrow spikes, the holes in carpet tend to close up and become invisible when the stand is moved. However, wooden floors are less forgiving, and there is an obvious risk of damage that might sometimes be unacceptable. Hence we may allow the spikes to press onto the carpet but avoid pushing them hard enough so they drive through onto or into the floor. When the weight of the speaker and stand is high people can choose to place the spikes on coins, or washers that act as a supporting ‘puck’ and prevent damage to carpet or floor.

By placing the spike on a small puck we make it impossible for the spike to contact the carpet or floor. This means we can’t expect the combination to dig in as effectively as the spike would do by itself. The combined spike and puck then will tend to act as a narrow hard foot. For much the same theoretical reasons [ref 2] as before, the result is unlikely to effectively isolate or damp any vibrations, although the stand may be held less firmly than if the spikes can grip the floor. Having said the above, using narrow feet might be useful in some cases. The weight and vibrational forces of the speaker will then be pressed onto a much smaller area of carpet (assuming the pucks are above the carpet) than if no feet or spikes are used. This may alter the effective stiffness of the carpet, and alter the ease with which the speaker may wobble, or lose vibrational energy. The resulting changes may be less when using a puck than when the spike is allowed to ‘dig in’, but may be useful in some cases.

When sitting on a concrete or stone floor it is unclear if a puck should be expected to do much in terms of vibrations. It may just prevent the spikes from blunting themselves digging small holes in the floor. However, either way, this arrangement will lack the grip we might get with spikes driven into a wooden floor. In general, therefore, we should perhaps view pucks or washers with caution as their benefits may be cosmetic rather than acoustic. Indeed, they may tend to defeat the purpose of fitting spikes at all. As usual, though, the results may depend upon the detailed circumstances!

The effect of using spikes which do not penetrate the carpet even without a puck can, again, be expected to be much like using narrow feet. It seems doubtful that they will isolate the stand from the floor. The results will depend upon how well (if at all) the spikes can grip the carpet without making holes. They may, however, act like the cones under a speaker and provide a more stable seating when the surfaces are not flat.

Softly softly…

For what it is worth, my own experience is that spiking the loudspeaker/stand through the carpet to ‘grip’ a wooden floor seems to have a noticeable effect when I have tried it, and I think I prefer it in the main audio system I use. However there is no guarantee this impression would be the same in every case, or that you would agree with my preference, or indeed that I am not imagining the change it made! I can’t say that I have ever had the feeling that spikes which do not drive through the carpet had any audible effect. Where genuine vibration isolation or vibration absorption are required my experience (plus the analysis and measurements I have mentioned) make me doubt that cones/spikes of hard material are a good bet. My own experience is that something like soft rubber feet (bumper buttons), or felt feet are more effective for isolation purposes.

Some people have experimented with cutting squash-balls in half, then using the resulting hemispheres of soft rubbery material as ‘feet’ under loudspeakers or other units. I don’t know of any measurements upon the effects of their use, however it seems quite possible that this arrangement would be better at isolating vibrations that hard cones or spikes. An alternative that was once popular was using a layer – or blobs – of ‘Blu-tack’. This is quite soft and squidgy when you apply a steady pressure. However it is more elastic and springy when vibrated. A potential problem with Blu-tack is that it contains a solvent which may discolour or mark other materials if held in contact with them for a long time. The material may also ‘creep’ or dry out, and its behaviour change with time. Hence it may not be suitable for long term use.

Another alternative I have not seen suggested or recommended anywhere is the use of cut-up pieces of carpet tile. This tends to be a composite of a layer of a soft rubbery material with a fibrous layer. An advantage of these tiles is that they are cheap to buy, and easy to cut up into shapes. You can also stack them into piles of a few layers if you wish. Hence they are very convenient to experiment with if you wish explore any effects they may have. Another advantage is that they are available in a variety of thicknesses, colours, and patterns.

I don’t know of any reliable measurements of the possible effects of Blu-tack or bits of carpet tiles used as ‘feet’. Hence if you try them you may decide that do nothing much, or you may find you dislike them. However if you are interested in damping or isolating, they seem worth considering as an alternative to the now-conventional hard spikes and cones.

The real difficulty with trying to deal with vibration and acoustics to ‘improve’ a domestic audio system is that the circumstances vary a great deal from one system and room to another, and the preferences of the user also varies. My own experience is that the choice of loudspeakers, their location, and the listening position generally have a larger effect upon the sound quality than the use of spikes, etc. In some cases cones or spikes may be desired to give a better ‘grip’. In other cases some damping or isolation may be desired, so soft inelastic materials may be preferred. Hence when it comes to loudspeakers and stands, any specific recommendation may be fine in some cases, and poor in others. The best bet is therefore simply to experiment. I hope the above gives some food for thought if you are interested in doing so!

[1] Articles on vibration by Keith Howard in the July and August 2002 issues of Hi Fi News

[2] Textbook: Structure-borne sound, by Cremer, Heckl, and Ungar, published by Springer-Verlag.

By AUDIOMISC © – http://www.audiomisc.co.uk/cones/speak.html