longitudinal mode vibrations

[Jos]

BDeTar@AOL.COM wrote:
> 
> List,
> 
> If you think about string vibration, it's easy to see that the string
> vibrates up and down.  It also vibrates side to side AND back and forth.
> It's this back and forth vibration that is the longitudinal wave.  To
> demonstrate this, try clamping a long (4-6 foot or so) metal bar to a table.
> Place a marble at one end of the rod, making sure it is touching the end of
> the rod.  Next tap the other end of the rod with a small mallet.  Watch as
> the marble is propelled away from the end of the rod.  You have just
> witnessed a LONGITUDINAL WAVE in action!
> 
> Here's a quick way to test a bass note to discern whether that high "squeal"
> or harmonic is coming from a longitudinal wave or a hard spot in the hammer.
> Drop the pitch about 1/2 or 1 whole step.  Because the AMPLITUDE (volume) of
> the longitudinal wave is a product of tension, it will disappear when the
> tension of that string is reduced.  In fact, as you drop the tension of the
> string, you can listen for when the longitudinal wave disappears!  If,
> however, when you drop the tension that high harmonic is still present, your
> culprit is typically a hard spot very near the crown of the hammer.
> 
> Hope this answers your question!
> 
> As a PS, if anyone is interested, I believe I still have the formula for
> predicting the frequency of a longitudinal wave.  It's a nice formula, but
> frankly, I think a formula to predict the AMPLITUDE of the wave is of more
> value.
> 
> Ain't information wunnerfull?!
> 
> Brian De Tar, RPT
> Portland, OR
> BDeTar@aol.com
>                         NO OBSTACLES... ONLY OPPORTUNITIES!

Oh, so there is real longitudinal motion in a string? 
I'd be interested at which frequencies this takes place!

Jos.