longitudinal mode vibrations

[BDeTar@AOL.COM]

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!