Stop and listen! What do you hear? You may hear many different sounds. Some of those sounds may be noise. Noise is a loud or unwanted sound.
Many kinds of machines, such as motorcycles, jet planes（喷气式飞机）, farm tractors can cause noise, and power motors can cause noise. And, to some people, rock music is also a noise.
What happens to people who live near noisy machines or use them over a period of time? Doctors have found that these people have trouble in sleeping. And most importantly, constant（连续的）loud noise can cause loss of hearing.
Scientists use a unit of measure called a decibel（分贝）to measure the loudness of a sound. The sound of a quiet room, for example, measures 35 decibels. Talking measures between 40 and 65 decibels. Sounds from traffic and from some rock bands measure over 120 decibels. Noise at such high level causes the greatest hearing loss.
