vibes, soundboards, and compression waves

[Richard Brekne]

Hi guys and gals.

I was sitting this evening watching the Harry Potter movie on my nice little home theater outfit. You all know me and magic...grin.  Neat with 5.1 channel sound. I have this old (oh at least 130 years old) little rocking chair...wooden, that I like to sit in while watching the tube. Towards the end of the movie I turned up the sound a bit and after a few seconds I noticed this tingling in my hand. It was resting on the armrest... and hey... this old wooden chair was vibrating in sympathy with the sound from the movie. Stronger here, weaker there... but shore nuf following right along. So much for the argument that longitudinal waves cant excite a soundboard. Anyways... I got to thinking a bit about some of the discussions we had in the past months about how things vibrate or are made to vibrate... their vibrating modes... and the rest... I got this picture in my head of this small piece of wood... say a 1 by 2 by 6 long suspended in air and subjected to a sound field.... whats this thing going to do really ? Its going to get hit by a seires of sound waves... and how it vibrates could probably be looked at from the perspective of what happens to the wave structure as it passes through, and around the block. From that perspective its evident that there is a lot more going on then just one or another kind of vibratory movement. Sure the thing is going to vibrate transversely as it tries to absorb some of the energy, but its also going to vibrate longitudinally,,,,,, or rather the wave is going to (to some degree or another) pass directly through... reduced in amplitude yet largely unchanged in shape. Indeed it would seem to have to be... for if the block represents an absolute blockage to sound waves.... then sound waves would be absolutely reflected and the darned thing wouldnt move at all. So what were my fingers feeling as I let them rest on the arm rest of my aged rocking chair ? Was it the wood simply transversly moving ?  Was it sound waves passing directly through ? Or do you simply have to have one to have t
 opportunity to be inside of a submarine and listen and feel to a sonar beam pass through the hull.

Just a musing along as usual.

RicB