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String Vibration

String vibration measurement has been studied and used extensively in the music industry in the form of amplifying guitars, resulting in the electric guitar. In the traditional single-coil magnetic pickup, a strong magnet is wound with copper wire. This acts as a sensor as the motion of the string causes a change in the magnetic field and as a result a change in the current through the copper wire. This signal, proportional to the movement of the string, is then amplified. For some guitars, such as Gibson's Les Paul, the guitar is close to being purely electric, in that energy is maintained within the vibration of the string to drive the pickups that amplify its sound. The guitar body is literally a block of solid wood meant to not resonate or color the timbre of the guitar's sound, thereby minimizing energy tranferred to the guitar body to create longer sustain in the string.

Another method for measuring a string's displacement is with use of a light-emitting-diode (LED) and a photo-transistor. The photo-transistor measures the amount of light received from the LED as light is obstructed by the movement of the string. This sensor is an inexpensive method for measuring the motion of the string in two-orthogonal planes.

Vibrometry is another method used for measuring the motion of a string as it measures the frequency shift of the back-scattered light reflected from the measured surface to compute the changing distance between the laser and the vibrating string at high sampling rates.

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Download phys_mod_overview.pdf

``Virtual Stringed Instruments'', by Nelson Lee and Julius O. Smith III,
REALSIMPLE Project — work supported by the Wallenberg Global Learning Network .
Released 2008-02-20 under the Creative Commons License (Attribution 2.5), by Nelson Lee and Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University