[Phys-L] Violin string excitation, spectrum, and Q Was: Re: A beautiful (short) demo for sound and waves...

[Bernard Cleyet <bernard@cleyet.org>]

On 2016, Aug 25, , at 18:11, Derek McKenzie <derek@physicsfootnotes.com> wrote:

When my 9 year old daughter didn't believe that the musical instruments
she practices so diligently all work by vibrations, I showed her this
superb slow motion footage of a vibrating violin string...

http://physicsfootnotes.com/physl-vibrating-string  


> On 2016, Aug 26, , at 09:17, Dan Beeker <debeeker@comcast.net> wrote:
>
> Slip - stick - Definitely not a sine wave! Nice video.
>
> Dan


The excitation (drive) is, of course, an impulse , but the result is quite different.  

Furthermore, the appearance of the string is a sine wave. (Wrong;  see below, Helmholtz.)  However, when the bowing is stopped (drive removed) the appearance gradually changes showing the harmonics are dying.  I suspect the vid. clip is of an open string, not usual in classical music.  

As opposed to some wind instruments, all the harmonics are more then ten dB “down” in intensity.  The graph is, obviously, I pray,  for the stopped A string in which the finger damps, preferentially, the overtones.    

http://www.ccp14.ac.uk/ccp/web-mirrors/isotropy/~stokesh/vspect.jpg  

Here’s an animation of the drive:  

http://www.physicscentral.com/explore/action/fiddle.cfm


 With the trumpet the fundamental does not, even, have the maximum intensity! 

View figs.. 1a and b.

http://www.cco.caltech.edu/~boyk/spectra/spectra.htm


bc  Notes the approximate Q may be determined from the clip (McKenzie’s), the D string is weakly excited, and after bowing the G is exhibiting 3-D motion.  

Here’s Helmholtz motion.  I think a stopped string drives the bridge, as in B of the first graph.   

http://knutsacoustics.com/files/The-Helmholtz-motion.pdf  


bc, again,  another reason he’s sad from T.K.’s death.