# For a sound wave how/why does the compression and rarefaction occur?

1. Aug 24, 2014

### hongiddong

I cannot seem to visualize how this compression and rarefaction occurs for example when I clap my hands, I know I move air molecules away from me, but it's so weird that such compression(high air pressure) and rarefaction(low air pressure) occurs. I also do not quite understand why this phenomena occurs.

Maybe somehow the clap creates a region where there is a high air pressure due to more air molecules leaving the area adjacent to it causing a cascade effect in which the air molecules move in this direction: high<---low, but we also get this direction of air movement low---> high, how does a sound wave continue to cascade the air molecules through space in this high<---low---> manner?

I am really confused??? Please, I need help understanding this?

2. Aug 24, 2014

### davenn

3. Aug 25, 2014

### hongiddong

Wow that helped a lot.

Can I ask you if I now have the right understanding. A high pressure(amplitude of the wave)' moves across the medium, in which as it moves, causes areas of low pressure, but as it continues to move,the area that had high and low pressure comes back to normal. And this is all happening at the speed of sound, 340m/s ? The medium only moves side to side, so the first high pressure simultaneously causes a low pressure to form, and as the air goes back to status quo to fill the low pressure again, we create another area of low pressure and high pressure?

Thanks so much Daven.

Last edited: Aug 25, 2014
4. Aug 25, 2014

### davenn

Yes

Yes, give or take a bit, different mediums, depending on their density, have different velocities
here's one small list ... am sure you could google others say for wood etc

a side to side movement is known as a traverse wave. where the particles of the material are moving sideways ( at 90 deg to the direction of travel of the wave)

a sound wave = a compressional wave and it moves back and forwards along the direction of travel of the wave as in the animation

cheers
Dave

5. Aug 25, 2014

### hongiddong

I am just confused with one last part to this. I see that the high pressure moves, however, I don't understand how that high pressure and low pressure keeps propagating. The medium itself does not propagate, but it vibrates in a parallel manner.

I at first thought that the initial high pressure or that pocket of the initial air kept moving across space.

The medium moves parallel to the propagation, so the first high pressure simultaneously causes a low pressure to form, and as the air goes back to status quo to fill the low pressure again, we create another area of low pressure and high pressure? I am still a bit confused.

Thanks Dave for all your help. I am definitely starting to get it.