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.