The Magnetic Harp applies an input signal to a resonant system, thereby creating an acoustic signal processor. In this case, the resonant system consists of a stretched string; magnetic force applied to this string induce resonant vibrations, there by imparting unusual timbres to the signal being sent to this resonant system.
Figure 1 shows the basic idea behind the Magnetic Harp. A bronze wound steel string is stretched across two bridges, with magnets creating a fixed magnetic field. These two bridges are conducting, and have taps to which we can connect a power amp. In essence, the string itself becomes a very low resistance load (around 2 to 16 Ohms - a good match for many power amps, and since the load is essentially resistive, it places less strain on the amp than a heavily inductive load such as a speaker would). Applying a signal through the string produces a magnetic field around the string whose strength is proportional to the input signal.
The harmonic content of the applied signal will cause a magnetic interaction that will seek out resonances in the string. The overall effect depends on several factors, the most important being the natural harmonic resonance characteristics of the string. The fixed fields produced by the magnets, and their placement with respect to the string, are also very important. In order to create the proper physical vibrations, the lines of force should be perpendicular to the string and induced magnetic field. This will cause motion of the string that tends to be in a plane perpendicular to the polar axis of the fixed magnet's field. The placement of the fixed magnetic field determines the spectrum of resonant harmonics. For example, if the fixed magnet is placed so that the field lines are centered perpendicular to the midpoint of the string (as in figure 1), the resonant will theoretically contain no even harmonics. This is because all even harmonics have nodes in the center of the string, therefore excluding any even harmonic vibration since there would be no magnetic interaction at that point. Similarly, other placements of the fixed magnetic fields, including multiple fixed magnetic fields of various positions and polarities, will modify the resonant spectrum and planes of vibration. Another factor to consider is the ratio of the string length to the physical size of the fixed field(s), as this will determine the accuracy of the spectrum modification.
With the type of setup shown in figure 1 the acoustic sound produced is quite soft; this is due to limitations of how much current you can actually pump through the string, and the strength of the available fixed magnet(s). Thus, it may be necessary to have another transducer to convert the string into a signal that can be amplified and further processed. Unfortunately, this transducer cannot be any kind that works on a magnetic principle (dynamic microphones, guitar pickups, etc.), as the field created by the current flowing through the string will be directly induced into the transducer without including the overtones generated by the interaction of the string and magnet(s). It is possible to place the string between an optical transducer (such as a photo-transistor) and light source, since this will cause fluctuations in the light hitting the photo-transistor that are proportional to the string vibration. Although this works well in this application, there are some strange side effects: the signal will appear to be full wave rectified if the string is positioned exactly between the photo-transistor and the light source, and the system is also susceptible to ambient light (as well as the 60 Hz signal flowing through any AC powered light). A crystal microphone is a better way to amplify the Magnetic Harp effect, as it is more sensitive to some of the subtle resonances than an optic system would be. In order to accentuate these resonances, it is also important that the physical structure of the harp be designed with acoustic principles in mind. A hollow bodied structure with pleasing resonance qualities is desirable; a modified guitar, for example, works well.
For purposes of experimentation, I recommend various string gauges (steel only) and a set up that permits easy tension adjustment, such as a movable bridge. Also, new strings sound much better than old ones. To drive the signal, I have been using one channel of an audio stereo amplifier. As mentioned earlier, the resistance of the string is sufficient to give a reasonably good match. However, to be on the safe side you should probably use an amp that has current limiting and short circuit protection; a low voltage/high current amplifier design is recommended but not necessary.
It is also important to pay close attention to the amount of current being sent through the string, as it can get quite hot and burn up if you're not careful. Note too that as the temperature of the string increases, the fundamental gets lower.
Aside from these limitations, the Magnetic Harp is a relatively noncritical device that offers a new type of concept in signal processing. If you come up with any unusual applications, or have modifications to the system, please send them to Polyphony.
Feature by Richard Wolton
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