Scientists believe that one of our most important means of knowing what is going on around us is the sense of sound. We are warned of danger by sounds. Sounds serve to please us in music. Sound has a waste product, too, in the form of noise. Noise has been called unwanted sound. Noise is growing and it may get much worse before it gets any better. In order to know how noise affects people and animals, scientists have been studying for several years. They are surprised by what they have learned. Peace and quiet are becoming harder to find. Noise pollution is a threat that should be looked at carefully. Sounds is measured in units called decibels  . At a level of 140 decibels people feel pain in their ears. Trucks , buses , motorcycles, airplanes, boats, factories---all these things make noise. They trouble not only our ears, but minds and bodies as well. There is a saying that it is so noisy that you can't hear yourself think. Doctors who study noise believe that we must sometimes hear ourselves think. If we don't, we may have headaches, other aches and pains, or even worse mental problems. Noise adds more tension   to a society that has already faced enough stress. The sense of sound   _  .
makes us feel excited helps us hear more clearly is something harmful to us helps to know what is happening around us