Sounds hurt the ears when they register about 130 on the decibel scale. Decibels are units for measuring the loudness of sound. They show how strong the sound waves are.
The lightest sound - perhaps that of a butterfly landing - would register about zero on the decibel scale; the noise level of a house with the television turned on would be about 50; the din in a car factory would measure 95 or more; and some amplified pop music may be near the limit where the ears would be hurt.
Sound waves are formed by millions of molecules of air bumping and vibrating against one another. Weak sound waves move the eardrum only slightly, but strong ones cause it to respond violently and, in time, may damage hearing permanently. The waves get weaker the further they travel.
The word decibel was coined by scientists in honor of Alexander Graham Bell whose interest in sound waves led to the invention of the telephone.