I have quite a few questions. First, I understand that sound waves are caused by disturbances in the mean positions of the molecules in the air, and this variation of density causes pressure to vary sinusoidally as well. In my textbook, I saw this diagram, which is quite standard and expected:
What I am confused about, is that why do some molecules move completely opposite in the direction of the wave that is travelling? I don't see any need for them to, for it appears that the molecules circled are going towards a region of HIGHER density, not lower. I am aware that there is some simple harmonic motion phenomenon that goes on, but could any one give a thorough explanation as to what happens?
Also, I was wondering if it is appropriate to think of each molecule being attached by a spring? This would be where SHM would fit into the picture. If it is not appropriate, why, and if it is, is there any justification? I think it is to do with statistical phenomenon as the molecules are in brownian motion anyway.