Noise, unwanted sound. Whether or not a certain sound is referred to as "noise" depends on the viewpoint of the listener. A jazz or rock composition might be regarded as noise by some listeners, but as pleasing sound by others. It is generally agreed, however, that the sounds created by such things as jet aircraft, automobiles and trucks, and heavy machinery constitute noise.

Noise can produce harmful effects on human health. Over extended periods, loud noise can cause permanent damage to the hearing of persons exposed to it. Such hearing loss usually occurs slowly and painlessly. Studies have indicated that exposure to excessive noise can also produce stress accompanied by high blood pressure, hamper work efficiency and learning, and disrupt sleep habits. The hazard presented by noise is especially great for people, such as miners and industrial workers, exposed on their jobs to very high levels of noise. However, even motor vehicles, power tools, air conditioners, hair dryers, toys, and stereo equipment can expose people to harmful levels of noise.

Noise and its control began receiving increased attention in the 1950's with the advent of commercial jet aircraft. Steadily increasing general noise levels gave rise to a widespread concern over "noise pollution." In 1972 Congress passed the Noise Control Act, giving the Environmental Protection Agency the responsibility of coordinating programs for studying and controlling noise. Numerous cities have passed laws and set up programs for noise control, limiting the sound level of many sources of noise.

Airborne noise is usually measured with a sound-level meter. This is an instrument that contains a microphone to detect the sound waves and convert them to electric signals, an amplifier to increase the levels of these signals, and a meter to indicate the final levels. Sound levels are measured in units called decibels. The average sound level of a whisper is about 20 decibels and that of heavy traffic, about 80 decibels.

Underwater noise is detected with a hydrophone, a device similar to a microphone but designed to be used in water. Most ships carry underwater detection apparatus known as sonar, and hydrophones form an essential part of this apparatus. The U.S. Navy devotes considerable effort to the measurement of the noise created by its surface ships and submarines, and attempts to reduce this noise by various techniques. The distance at which the noise of a naval vessel is detectable can be crucial in wartime. Noise made by a submarine's propellers can be detected as far as 100 miles (160 km) away by sonar.

Electrical Noise

The type of noise described in the preceeding paragraphs is sometimes called acoustic noise to distinguish it from electrical noise. The term "electrical noise" refers to unwanted electric currents always present in an electronic device or system.

Static, interference that frequently occurs in radio receivers during electric storms, is a common example of electrical noise. Many of the interference patterns observed on a television screen are caused by electrical noise generated by such things as appliancees, automobile ignition systems, and neon signs.