Coupling by Spiking Loudspeakers
Spikes are couplers and will couple the loudspeaker to the earth – or are they? Most of what you read will tell you that a spike is a coupler – and common sense will tell you that it is, until you examine it in much greater detail.
Considering the forces vertically, the spike is a coupler. Due to the tiny contact patch, it is a low impedance path to ground for all frequencies of vibration. Considerations include the size of the contact patch at the point of the spike (how sharp is the spike), the material the spike is made of (whether the spike deforms), and the material the spike sits on (whether it dents).
If the spike deforms, and the deformation is elastic, there may be a resonant frequency at which the spike will “bounce”. In general, we would consider the spike an excellent coupler in the vertical plane.
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However, what happens when we look at the spike with forces in the horizontal plane? Then, whether the spike is a coupler or a de-coupler takes a whole different complexion.
If, like skates on ice, the spike is very hard, the surface on which it sits is very hard, and there will be low friction, and the spike is a decoupler. Depending on the weight of the loudspeaker and the friction of the surface the spike sits on, a sufficiently large sideway force will cause the spike to slide around on the surface.
If the spike can deform the surface in any way, there will be a whole host of new things to consider. For example, if the surface is hard stone, and the spike is steel, a large enough horizontal force could cause the spike to just scrape around on the surface, and you might get a sound like fingernails on a chalkboard (blackboard).
If the surface is wood, and the spike makes a hole and nails the loudspeaker to the wood, then the wood couples to the loudspeaker, and you might get a warm, woody sound that some audiophiles will find attractive.
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Now that we understand that the spike is a coupler in the vertical plane and could be a de-coupler in the horizontal, we need to consider the forces that the spike will meet. The next consideration is simple vector mechanics that we all learned in High School.
Examining the dynamic forces generated by the movement of the driver cone (blue arrow) in a loudspeaker, there will be zero vertical forces since the cone is almost directly above the front spike, but the horizontal force will equal the force generated by the driver assuming always that the spike is tightly coupled to the loudspeaker cabinet by screwing it in or glueing.
The rear spike, on the other hand will have both a vertical element as well as a horizontal element. (The red vector.) We mustn’t forget that there are also static forces on the spikes due to the mass of the loudspeaker and gravity.
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Since the point of force and the two spikes are rigidly coupled by the loudspeaker cabinet, the forces on the two spikes will also be related. Looking at the rear spike, the combination of the horizontal and vertical forces, and friction will result in a third force (green vector below).
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To illustrate this third force, if you push the top of a spiked loudspeaker, and if there is sufficient static force (weight) on the spikes and if there is sufficient friction, the loudspeaker will start to tilt backwards on the rear spike. If you pull the top of the loudspeaker towards the front, the loudspeaker will tilt forwards on the front spike. On a hard, slippery low-friction surface, pulling or pushing the top of the loudspeaker, even when it is on spikes, will cause the loudspeaker to slide.
Thus, although we mentioned earlier that there is zero vertical force on the front spike generated by the horizontal driver forces, the front spike’s tightly coupled relationship to the rear spike plus friction at the rear spike creates some vertical forces. You will also find that due to the different vector forces, it is easier to pull the front of the speaker to tilt it forwards than it is to push the front of the speaker to tilt it backwards.
Hence, the spiked speaker will react asymmetrically to forces generated at the top of the speaker (usually the midrange – as the tweeter is usually too small and light to generate significant forces at the foot). With sufficient forces at a midrange/mid-woofer at the top of a tall loudspeaker, the front spike might chatter on a hard surface due to the up and down forces generated.
How about a woofer at the bottom of the cabinet? Looking at the forces created by a woofer at the bottom of the speaker, the forces on the spikes are primarily horizontal. However, as the woofer produces very much larger forces than the midrange and it is usually mounted at the bottom, most of the vibrational forces in a loudspeaker are horizontal – the plane on which a spike is least effective as a coupler. This is also true of the forces on a bookshelf loudspeaker on the top of a loudspeaker stand.
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A pair of spiked loudspeakers on a very low friction surface (steel spike on a diamond surface for example) might well be launched backwards with a single very loud drum kick. With sufficient friction on the other hand, the forces around the moment of the front and rear spike will generate vertical forces in the other spike – resulting in the subwoofer rocking on the spikes and hopping around the room.
treitz3Super Moderator
