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Slink Technical Info

General Background - Strobing and Resonance

Certain objects move faster than the naked eye can perceive. An example is a guitar string. From the tone it produces we know it vibrates hundreds of times every second; however, our eye can only see up to roughly 30 vibrations of anything per second [this is why tvs and movies operate in this range; any slower and we notice flickering, any faster and we are wasting information]. However, if we flash a strobe light once every time the guitar string hits the end of one vibration [where it is always in the same position], the strobe will occur fast enough that all we will see is an apparent frozen motion of a guitar string, in it's bent/waving state. This sculpture is an exploration of that idea, combined with resonance phenomena.

A parent pushing a child on a swing set is an example of a resonant system. With one push, it is almost impossible to swing the person into a full, large swing. However, with carefully timed pushes, applied in the same part of the child's cycle, a parent can easily, after four or five swings, have the child move back and forth very large distances. This is because the parent can add energy to the system coherently, and thus continue to add energy to the system cycle after cycle, at a rate faster than the energy is dissipated [by air friction, mostly].

Slink Resonant Behavior

The resonant behavior in slink is caused by a spring. All springs have resonant modes [a swingset is approximately a gravitational spring, after all] - rates at which they can be vibrated, such that adding energy at the same point in their cycle will be a natural frequency of the system, a frequency that the system would have on its own were an external force not being applied. In the case of child on swing, this is the natural speed at which they swing back and forth. A spring compressed and then released will shake back and forth at one of its resonant modes. Therefore, if we apply coherent energy to a spring, we can excite it like the swingset, and get it to move much more than it would normally under the same external force.

A 4' long, 1/8" diameter extension spring attached to two posts at either end of a strobe light box. One post is a custom made resonant linear voice coil motor. This motor is specifically designed to hit resonance behavior in the range of 50hz. The spring itself has a resonance frequency [approx sqrt(T/mu) with tension T and material density mu], at which it is excitable into it's fifth resonant mode of vibration [observable by it's 5 half-wavelengths, or 2 and a half full 'waves']. By changing the tension of the spring T, we can tune this resonant frequency to precisely match the resonant frequency of the motor. Thus the motor motion is amplified by its internal resonances [also caused by springs], and the spring motion is further amplified by its own matched resonance to the motor drive.

When the strobe lights behind the spring are off, it is impossible to see a clear image of a moving object. All one sees is a hazy image anywhere the spring is during its cycle. So, if we light the object extremely briefly [approximately 1/100,000 of a second], then our eyes will only really notice the spring at that time, in that position. If we continue to do this every 1/50 second, when the spring is exactly in the same place, then our eyes will not only perceive the spring in the same location, but the strobing effect will occur faster than the eyes notice is, so all we will see is a spring in an apparent state of suspended animation.

When slink begins, all the strobes behind the spring do are slightly and slowly change their timing of when they flash, so that the spring has a chance to move one full cycle plus a tiny bit between every flash. This gives the spring an apparent very small motion between flashes, but at this speed, it appears as if the spring is moving very slowly. In reality it is moving approximately 1000 times as fast as it appears! The actual spring's motion never changes throughout the duration of the progression.

With a variety of timing effects, the spring is made to move at different speeds, and also to suddenly jump to new positions, something typically impossible with normal motion. Then an additional effect is employed. The strobe timing can be thought of as a position selector - by flashing at a certain time, the spring appears in a certain place. So, instead of just using one strobe, we use twelve columnar strobes, each 3" in width, to cover the 3' wide backlight. Normally these all blink as one, so the spring appears to move as one. However, halfway through the progression, the strobes begin to slowly diverge in their timing. This causes parts of the spring to suddenly appear to be located in different places - effectively, this splits the spring into twelve completely separate parts, each floating in mid-air.

A variety of effects are explored in the progression. The video on the main page, shows the slink progression, as well as some different views of the experience.

© Jeff Lieberman 2oo5