Re: Sound waves in vacuum

Etienne,

Thanks for your questions.  
1.  The usual definition for sound presupposes some sort of medium for the sound 
to travel through.  This medium can be gaseous, liquid, or solid. In the usual 
sense, sound cannot exist in a vacuum.  It is like trying to observe waves on 
the surface of water when all you have is an empty pan.
2.  I can think of at least one counter example to sound not traveling from low 
to high pressure.  Jet aircraft that fly at high altitudes are flying in a 
region of very low pressure compared to the air pressure at ground level.  The 
sounds made by the jet engines are easily heard by people on the ground.  
For two region of air with differing densities/pressures, I think that the angle 
that the direction of the sound wave makes with the surface seperating the two 
regions is what determines how much sound is transfered from one region to the 
other.  This is related to the idea of an index of refraction for light.
3.  Such an experiment should not be possible, for the reason stated in #1.  

Regards,

Everett Rubel