# Clarifying molecular motion in air due to sound waves

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.

• What is that diagram about? I couldn't get a head or tail out of it. Aug 29, 2017 at 13:47
• Sorry i should have made it clerer. The top white dots are the molecules before being set into oscillation, and the the black after. I am very surprised by the black dots which i have circled move to the left. Aug 29, 2017 at 18:21

There are three factors in the oscillation, the speed, acceleration, and pressure. First the pressure accelerates the molecules toward the area of the lower pressure. In the process the molecules gain a certain speed. Once they start moving against the pressure, the pressure decelerates them, turns their movement around, and accelerates them in the opposite direction until the process repeats. Imagine a massive ball on a spring, the same thing. The ball also at times moves on inertia against the spring pressure. The point to clarify the confusion here is that the pressure causes the acceleration, but not the speed per se. The speed can be along or against the pressure, but the acceleration is always in the direction of the pressure.

• +1 The last statement ought to be modified to "...acceleration is always in the direction of decreasing pressure".
– Deep
Aug 30, 2017 at 6:04

In a sound wave, the medium is pressurized in a quasi-periodic way. These low pressure and high pressure zones travel forward, and become sound when processed by our ears.

The wave motion can be shown in the page on longitudinal waves.

There are clumps where the wave is high pressure, and "close", and points where they are low pressure, and "spread out". The particles you circled are moving towards a high pressure zone. Afterwords they will spread back out.

The particles themselves do not move in the wave. The high/low pressure zones travel through the medium, while leaving the particles in their approximately original position. It's the clumping and spreading motion that moves, not the particles themselves.

The SHM analogy isn't a bad one. They get pushed close together, and naturally they want to spread back out afterwords. It might not be a perfect analogy, but it's not bad.