High frequency waves travel slower than low frequency waves.

高频波比低频波传播得慢。

They send out high frequency waves first followed by lower and lower frequency waves.

它们首先发出高频波，然后发出越来越低的频率波。

And because the high frequency waves travel slower, the lower frequency waves gradually catch up.

由于高频波传播速度较慢，低频波逐渐赶上。

Since longer, low frequency waves actually penetrate deeper than short, high frequency ones, multiple frequencies are often used together and composited into a life-like image.

由于较长的低频波实际上比短的高频波穿透得更深，因此经常将多个频率一起使用并合成为栩栩如生的图像。