What kind of sound is noise?
　 　 　
The sound of cars or airplanes flying overhead can be quite noisy when you start to notice it. So are all noisy sounds noise? People normally think of noise to be sounds you wish went away, like sounds so loud that you can’t hold a conversation or sleep. Of course, each person has different ideas of what noise is. You might think even a tiny sound is noise if you dislike it. Noise levels are measured in a unit called decibels. Noise that you can’t hear unless you listen very carefully is 0 decibels, while noise in a subway, for example, may reach 100 decibels. There isn’t a rule that says noise is sounds louder than some number of decibels. Still, various clever ideas are used to make trains, airplanes, and other things less noisy. If clever ideas were used everywhere, noise just might disappear.