compression waves

[John Delacour]

At 12:32 AM -0500 11/24/01, Stephen Birkett wrote:

>  We
>know that the longitudinal wave velocity of the wire acting as a 
>thin rod is a function only of the material properties, viz. the 
>Young's modulus (E) and volume density (rho): v = (E/rho)^0.5. We 
>also know that the Young's modulus for high carbon steel music wire 
>is around 215 GPa and density about 7830 kg/m^3. Now there is a 
>little variation in reported values of these parameters but they 
>certainly ought to be constant for a particular piece of wire and 
>close to the values given here.

Stephen, as a cgs trained physics failure who can adapt painfully 
when required to MKS, I must put up my hand and ask what is GPa?, 
which is too recent a term apparently for my (not new) Websters and 
not used in any of my physics books.

As to the specific gravity of the wire, I would trust no published 
figures, since I have come across so many varying values (eg. 
Wolfenden's 7.60).  Besides, the specific gravity will fall as the 
string is stretched since its volume does actually increase and the 
thinning of the wire does not counterbalance the lengthening as one 
might imagine.  We are interested in the actual specific gravity of 
the wire after the molecules have thoroughly realigned themselves 
several days after chipping up and tuning and the pitch has been 
brought back up.  I think it quite probable that there are 
significant differences between different makes of wire and that it's 
quite possible Wolfenden's figure of 7.60 came from accurate 
measurement of the best wire ever produced, namely Poehlmann's.

JD