Physics of Bowed Instruments |  |
Basic Facts
The basic components of a violin are top plate, back plate, ribs, soundpost, bridge, tailpiece, fingerboard, neck, scroll, pegs, and strings. The soundpost enables the whole instrument to resonate and vibrate the sound. The bridge also aids in the delivery of the sound to the soundpost. A majority of bowed string instruments can be found to have the same basic structure.
The Physics of Bowed Strings
The core of the violin or any of its relatives (viola, cello, bass) is the bowed string. The string plays a major role in establishing the musical identity of this family of instruments. Conceptually the string is the simplest of components but in spite of this assumption of simplicity, the action of the string under the bow presents many unanswered questions.
One of the first attempts at understanding what actually happened when a string was bowed was made by Hermann von Helmholtz. He utilized an apparatus called "vibration microscope," invented by the French physicist Jules Antoine Lissajous, an elementary version of what we would now call and oscilloscope. With this instrument Helmholtz looked at a grain of starch fastened to a black string, which he set in vibration by bowing.  The objective lens of the microscope was mounted on a large tuning fork so as to vibrate slowly parallel to the length of the string. When he set the string in motion he saw a "figure," a form of oscillogram that displayed the position of the starch particle as it varied within the period of vibration of the fork. From this he acquired the basis for a mathematical description of the motion of the string as a whole.
Something similar to this can be observed by looking at the lowest string of an instrument while it is being vigorously bowed. In appearance it widens into a ribbon bounded end to end by two smooth curves.
Two simple physical facts underlie the action of the bowed string. The first is that "sliding" friction is less than "static" friction and that change from one to the other is almost discontinuously abrupt. The second is that the flexible string in tension has a succession of natural modes of vibration whose frequencies are almost exact whole-number multiples of the lowest frequency; as a result the duration of a single vibrations in the second mode, three in the third and so on. Without outside compulsion the string is therefore by its very nature given to supporting a "periodic" wave, that is, a repetitive series of similar vibrations with a wave form dictated by the "stick-slip" process.
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