If you live in a large city, you are quite familiar with some of the problems of noise, but, because of some of its harmful effects, you may not be aware of the extent of its influence on human behavior. Although everyone more or less knows what noise is, i. e., it is sounds that one would rather not hear, it is perhaps best to define it more precisely for scientific purposes. One such definition is that noise is sounds that are unrelated to the task at band. Thus stimuli (刺激物)that at one time might be considered relevant will at another time to be considered noise, depending on what one is doing at the moment. In recent years there has been a great deal of interest in the effects on human behavior, and concepts such as 'noise pollution' have arisen, together with movements to reduce noise. Exposure to loud noises can definitely produce a partial or complete loss of hearing, depending on the intensity, duration, and frequency composition of the noise. Many jobs present noise hazards (危险), such as working in factories and around jet aircraft, driving farm tractors, and working in music halls where rock bands are playing. In general, continuous exposure to sounds of over 80 decibels can be considered dangerous. Decibel values correspond to various sounds. Sounds above 85 decibels may, if exposure is for a sufficient period of time, produce significant hearing loss. Noise can have unexpected harmful effects on performance of certain kinds of tasks, for instance, if one is performing a watch keeping task that requires vigilance (注意), in which he is responsible for detecting weak signals of some kind (e. g. , watching a radar screen for the appearance of aircraft). Communicating with other people is unfavorably affected by noise. If you have ridden in the rear of a jet transport, you have noticed that it was difficult to carry on a conversation at first, and that, eventually you adjusted the loudness of your speech to compensate for the effect. The problem is noise. Noise differs from sound in that ______.