Many planetaries (as we call them for short) , are round and look like soap bubbles, pretty much what you expect when you look at an expanding shell of gas.

许多行星(我们就这么简称了)是圆的，看起来像肥皂泡，和你所预料的膨胀的气体外壳差不多。

Those particles hit the next set of adjacent particles, and so on , thereby creating an expanding shell of air particles just bouncing into one another This is called a pressure wave or sound wave.

这些粒子撞击下一组相邻粒子，依此类推，从而产生一个膨胀的空气粒子壳， 只是相互反弹。 这称为压力波或声波。