How can the choice of wood make a good violin? We always hear about some really fine and expensive wood that is used to make guitars, violins and other musical instruments.
What's the physics behind this? What parameters (e.g. bulk modulus etc.) do luthiers take into account in the choice of wood?
 A: Personally,  no offence intended, I think this is a music based question but if it is migrated this answer may be of some interest to that section of SE.
I have read of tests being carried out where a range of violins, either high quality modern instruments  or a Stradivarius, were given to a group of concert level players. Wearing blindfolds whilst they played,  in general,  they could not tell which instrument they were playing.
Subjectivity may  play a large part in assessing the sound quality produced by these instruments. So whether this is a physics question or a music question, I don't know. Possibly the only link is that Einstein played a violin, although this may be a subject for debate. 
Testing Violins, again, the debate continues....  

A modern instrument was the clear winner and a Stradivarius the loser in a double-blind test of old Italian and new violins, conducted at the Auditorium Coeur de Ville in Vincennes, Paris. In a follow-up to the controversial experiment conducted in Indianapolis in 2010, ten professional soloists compared the tonal qualities of twelve instruments – six by 18th-century Italian luthiers and six by contemporary makers. The results, published on 7 April in the Proceedings of the National Academy of Sciences, confirmed those of the 2010 study, which showed a general preference for new violins and that players were unable to reliably distinguish new violins from old.

Wood and Violins

Until now, however, no one has been able to explain why 300-year-old Stradivarius violins have never been matched in terms of musical expressiveness and projection.
  A study has found that the secret may be explained by the consistent density of the two wooden panels used to make its body, rather than anything to do with the instrument's overall contours, varnish, angle of the neck, fingerboard or strings.
  Scientists compared five antique violins made by the Cremonese masters Antonio Stradivari and Giuseppe Guarneri Del Gesu with seven modern-day instruments by placing them in a medical scanner that could accurately gauge the density of the two wooden plates that make up the top and the back of the body.
  They found that, overall, the density of the two groups of violins was the same, but what differed significantly was that the two plates of the older instruments had a more uniform density compared to the more inconsistent densities of the modern plates.
  The top plate of a violin is usually made of spruce and the back of maple. The scientists believe that the homogenous density of the Cremonese violins gives them the edge in terms of stiffness and sound-damping characteristics, which both help to produce superior musical notes.
  The classical violins made by the two Cremonese masters have become the benchmark against which the sound of all other violins are compared. Yet by general consensus no instrument maker since that time has been able to replicate the sound quality of those early violins, said Berend Stoel of Leiden University in the Netherlands.
  "The vibration and sound-radiation characteristics of a violin are determined by an instrument's geometry and the material properties of the wood. New test methods allow the non-destructive examination of one of the key material properties, the wood density, at the growth ring level of detail," Dr Stoel said.

A: This question is really broad so let me just point out some general notices and recommend further sources.
Bowed string instruments aren't actually made of one kind of wood. Plates, bridge etc. should posses a bit different features according to their functions. E.g. the bridge has to transfer the oscillations from the string to the body of the instrument. It shouldn't be then made out of too soft wood. 
Then there are radiation charcteristics of plates which are connected to the bulk modulus (radiation impedence is defined using bulk modulus and density).
Wood is highly anisothropic medium which could be (surprisingly) an advantage. Total symmetry in radiation is bad cause of interference (inhomogenous sound in near-field) and physiology of hearing. We like to hear a slightly unstable sounds - radiation from stable eigenmodes is a bit "boring and weary" for ears. We prefer kind of "shimmering".
I strongly recommend to read Fletcher's and Rossing's Physics of Musical Instruments for further discussion or pick something like Acoustics of Wood by Voichita Bucur.
