Noise can actually be a form of pollution that characterizes urban and industrial areas. It affects not only the human hearing, but all our functions. The unit for measuring how loud a sound is, is called decibel. One decibel describes the minimum difference between two sounds, so that they are audible by humans. Whispering has an intensity of 20 decibel, while the sound produced by an aeroplane taking off, intensity of 150 decibel.

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