Course:PHYS341/2018/Calendar/Lecture 24

Phys341 Lecture 24: Summary and web references
2018.03.09
Textbook Ch.17.1-17.2 again

1. Violin Family http://acoustics.phas.ubc.ca/musical-instruments/strings/bowed/violin/
2. Balsa violin (ignore the equations) http://acoustics.phas.ubc.ca/musical-instruments/strings/bowed/balsa-violin/
3. Why is wood so perfect for musical instruments? Mechanical Properties:
   • Wood is an orthotropic material.
     • Its stiffness and damping depend on the direction of bending.
     • (As opposed to an isotropic material, whose properties are the same in all directions, e.g. plastics, metals.)
     • This property is a natural consequence of the way a tree grows, the need to support its own weight and resist wind.
     • Wood is stiff in the L-direction, weaker in the R-direction, and weaker still in the T-direction.
   • There are three main ways of cutting up a log into planks
     • Plain (or flat or slab) sawn (P)
       • Common for shelves, and old qins
     • Rift sawn (R)
       • Optimum for Western soundboards, most wasteful of wood
4. Wood types
   • Softwoods
     • From gymnosperm trees (mostly conifers), e.g. spruce, pine, cedar, fir
     • Simple linear grain structure
     • Only strong in L-direction
     • Not necessarily “soft”, e.g. yew
   • Hardwoods
     • From angiosperm trees (fruit-bearing, flowering), e.g. maple, birch, poplar, beech, oak
     • More complex structure, with rays at 90 degrees to the grain
     • More isotropically strong
     • Not necessarily “hard”, e.g. paulownia, balsa
   • Monocots
     • Not technically true “wood”, e.g. bamboo. Used for wind instruments.
5. Acoustic woods
   • Tone woods
     • Sound boards
     • Light, stiff, resonant
     • Usually softwoods, e.g. spruce, fir or light hardwood, e.g. paulownia
   • Frame woods
     • Backs, ribs, necks
     • Heavier, stiff
     • Usually hardwoods, e.g. maple (also a tone wood), walnut, catalpa, or strong softwood, e.g. yew
   • Specialty woods
     • Pegs, fingerboards, woodwind
     • Dimensional stability, fine finish
     • Very hard woods, e.g. boxwood, ebony
6. Resonant quality: Q
   • Acousticians refer to the quality factor of a material, denoted by Q.
   • Roughly speaking Q indicates how many vibration periods does it take to damp out a vibration.
   • Example: spruce and hard maple have Qs of about 100.
   • Metals have a much larger Q-factor, e.g. aluminum (Q ~ 1000), the choice material for tuning forks, and xylophone bars (which sound very different than wooden marimba bars).
   • Q also affects the sharpness of the resonances: the peaks seen in a sonogram will be sharper (i.e. spikier) for high-Q resonances than for low-Q examples.
   • Soundboxes made of acoustic woods have Q ~ 30, which is an ideal compromise between too “ringing” (but only for certain notes) and too “dead”.