# Rarefaction? How is it formed. Sound (Simple question)

1. Aug 30, 2008

### sameeralord

Rarefaction? How is it formed. Sound!!(Simple question)

Hello everyone,

Simple question. If a object vibrates forward causing a compression and then vibrate back wouldn't it regain the normal air pressure (end up at the same place). How does it lower the air pressure and cause rarefaction by moving back. Thank you!!

2. Aug 31, 2008

### billiards

Re: Rarefaction? How is it formed. Sound!!(Simple question)

Simple question, I guess you want a simple answer then!!

Consider a parcel of air, in fact, consider two adjacent parcels of air. Now put them in a perfectly rigid box (so its volume can't change) and let the box be perfectly sealed (so no mass can escape/get in). Now compress one of the parcels of air; what must happen to the other parcel of air?

Remember "nature abhors a vacuum" [one of the reasons you can hold liquid in a straw when you cover the top with your finger, even though the bottom of the straw is open, is because if water dropped out of the bottom there would be nothing to fill the vacated space at top of the straw]. So the other parcel of air will expand to fill the gap - we have rarefaction.

I wouldn't think too much in terms of particles, particles themselves don't change size, they just get bunched closer together or farther apart. If we looked at the motion of particles we would see that they oscillate, a bit like a loaded spring really. And that just like a loaded spring that has been disturbed, particles will oscillate about their rest position.

3. Aug 31, 2008

### HallsofIvy

Re: Rarefaction? How is it formed. Sound!!(Simple question)

Also the air doesn't move as a single entity. The molecules closest to the object move back more quickly than those farther way. That increases the distance between them and you have "rarefaction".