Course:PHYS341/2018/Calendar/Lecture 23

Phys341 Lecture 23: Summary and web references

2018.03.07

Textbook Ch.17.1-17.2

1. Violin Family CEW's violin pages
2. Bowed strings
   • Big advantage over plucking/striking: allows continuous energy input into the strings.
   • How do bows excite strings at precisely the right frequency?
   • (Plucking/striking works because the pluck/strike contains a broad spectrum of frequencies, and the string picks out the ones at which it resonates).
   • Bows work by the stick-slip interaction between horse hair and string:
   • High-speed video of bowing action
   • Horse hair can be coated with rosin.
   • See electron micrographs from IWK Vienna: http://iwk.mdw.ac.at/?page_id=96&sprache=2
   • Rosin is a glassy solid made from the resinous sap of pine trees.
   • Bows for musical use probably originated with nomads of Central Asia and had spread to all parts of Eurasia by 1000 CE.
3. Bow design
   • The earliest bows were probably similar in shape to a Central Asian archer’s bow (that’s probably also where the rosin idea came from). Violin bows were literally “bowed” in shape until c. 1800.
   • François Xavier Tourte (1747-1835) designed a bow with the opposite curvature. Big advantage over earlier bows:
   • Increasing downward force slackens the bow hairs (opposite to earlier bows).
   • Allows full length of bow to be used with a much more even sound production than hitherto. (Baroque players only used central part of bow.)
   • It is hard to maintain full force at tip end, but that is counteracted by tightening bow hairs.
4. Effect on sound box
   • Unlike plucking and striking, bowing imposes a sideways force.
   • On a symmetrical instrument like a guitar, this would impose a twisting force on the soundboard and the radiation from the left and right sides would tend to nullify each other.
   • (Of course the guitar bridge is not designed for bowing individual strings.)
   • Make the sound box asymmetric
5. Scaling issues
   • The viola plays a 5th lower than a violin, but it is not 1.5 times the length (it is 15% longer).
   • Viola lacks power, especially on the lower strings.
   • The ‘cello plays an octave lower than the viola, but it is not twice the length. It has to have a much deeper body to keep the air mode low, and relatively thin plates to keep the wood modes low.
   • ‘Cello has very flexible body – “Wolf tone” problem.