Have you ever noticed how，on a rainy night，all other sounds seem to disappear? The sound of the rain drowns （淹没） them out. And even though the sound of the rain goes on all night，it doesn't keep you awake. Rain is a kind of white noise﹣a steady and unchanging sound.
White noise is made up of sounds from all the frequencies （频率） a human ear can hear，with the sound at each frequency having the same power. It's called "white" because it is like that of white light，which is a mixture of all the light we can see.
You might think that a mixture of all possible sound frequencies would be terrible，but it's not ﹣ quite the opposite. The reason for this is that white noise covers other sounds. Think of it like the rain：the sound of one drop falling，like from a leaky tap （漏水的龙头），would be very different and hateful. Two drops would be the same. You could even tell three drops apart. But if there were 5 or 10 or 1，000，you couldn't pick out each drop. They'd all mix into a kind of quiet sound and probably lead you to sleep.
According to a scientific study，white noise can help treat different problems. It can help worried people sleep and help those who have headaches sleep through their pain. It can be very useful for people with attention problems，who have trouble telling background noise and can't focus on something well. White noise can help them pay attention to something. It can even cover the sound of conversations by doctors and others who want to keep privacy. Who would have thought that mixing lots of sounds together would actually turn out to develop peace and quiet?
