These are longitudinal waves that have to travel through a medium.

这些是必须通过介质传播的纵波。

Sound, you'll recall, is a wave: a 'longitudinal' wave.

如前所述，声音是一种波： 纵波。

In the case of a longitudinal wave, the back-and-forth motion is more of a compression-and-expansion.

在纵波的情况下，来回运动更像是压缩和膨胀。

But there are also longitudinal waves, where the oscillations happen in the same direction as the wave is moving.

但是也有纵波，其中振荡发生在与波移动的方向相同的方向上。

But sound is the other kind of wave: it’s a longitudinal wave, meaning that the wave’s back-and-forth motion happens in the same direction in which the wave travels.

但声音是另一种波：它是一种纵波，这意味着波的来回运动发生在与波传播相同的方向上。