Sound waves(The behavior of soundboards)

[Phillip L Ford]

On Tue, 15 Jan 2002 19:44:46  
 John Delacour wrote:


>If we consider our body, the bridge, as a single vertical "ray" or 
>line of particles rather than a three-dimensional mass of particles, 
>it will be easier to explain.  The particles of an elastic body have 
>a certain relationship to each other in space when they are in 
>equilibrium.  If a homogeneous elastic body is heated, for example, 
>the body, as we all know, will expand and the particles be more 
>distant from each other.  The bond, i.e. the atomic force, between 
>them becomes weaker.  At a certain temperature, the bond will become 
>so weak that the body liquefies, and at a higher temperature this 
>fluid will vaporize.  In the cases we are considering, we are forcing 
>the particles of the body to take up different positions in relation 
>to each other.  We are opposing a force or pressure to the forces 
>that holds them in equilibrium.  So long as we don't overdo it, much 
>of the energy we have expended in stretching the wire or compressing 
>the bridge, or whatever, will remain as strain energy in the body, 
>and this will be converted to kinetic energy, when our force is 
>removed, to restore the body to its original shape.  All this is to 
>emphasize that any elastic body, by definition, is composed of 
>particles that can to a degree (that being the elastic limit of the 
>material) be moved closer together or further apart by an imposed 
>force.
>
>To return to our bridge consisting of a vertical "ray" of particles 
>and let me return to an analogy I used earlier -- that of a 
>transparent tube containing a stack of little cylindrical magnets 
>disposed North<>South<>S<>N<>N<>S<>S<>N etc.  Let the magnets 
>represent particles of an elastic medium and the repulsive force 
>between them the atomic forces between them.  This ray of magnets is, 
>of course, compressible.  If we close the bottom of the tube so that 
>the bottom magnet can't move, and press down on the top magnet, we 
>oppose our force to the magnetic force and the magnets are pressed 
>closer together.  If we move the top magnet a certain distance and 
>maintain the pressure, the magnets will (very fast but 
>_not_instantaneously_) take up different positions in respect to each 
>other.  Each magnet, or particle will have been displaced a different 
>amount and the ray will be in a state of compression.
>
>I'll break off there and continue the analogy once you have accepted 
>that this is a clear description of a demonstrable experiment.  If 
>there are any problems with it, I'll try to make myself clearer in 
>response to your queries.  So far, of course, I have attempted only 
>to draw a picture of a one-dimensional bridge, which will take us 
>quite a long way and is easy to visualize.
>
>JD
>
John,
I don't have a problem with any of this.  Please proceed.

Phil F