The distribution patterns for urban noise are quite complex and differ from city to city; yet, in general, common factors describe them.
A noise base exists twenty-four hours per day, consisting of household noises, heating and ventilating noises, ordinary atmospheric noises, and the like; this noise base is usually of low level, from 30 to 35 dB. Here and there are somewhat louder sources of noise: electrical substations, powerplants, shopping centers with roof-mounted equipment, hotels, and other buildings which do not change with the night hours.
During the day and evening hours this base level increases because of increased residential activity and also because of general widespread city traffic. A new pattern appears: in busy downtown areas traffic is heavy, on throughways and main streets extremely dense traffic occurs during rush periods with heavy traffic continually, some factories are at work, etc. Noise levels in the streets can rise to 85 to 95 dB locally. An intermittent pattern is added from emergency vehicles, aircraft, and the like. The general noise level for the entire city can increase by 10 to 20 dB. The highest noise levels remain local; after a few blocks, the noise is attenuated through scattering and reflections among buildings, and the many sources blend into the general noise pattern.
The intermittent noise pattern is usually more disturbing than the steady pattern, especially at night. Measurements near main highways and freeways show general traf fic noise levels at a distance of 30 meters to be in the 65 to 80 dB range with frequent excursions up to 100 dB or even higher almost always caused by trucks but sometimes by motorcycles.
Only at the edges of urban areas does the noise level drop appreciably; and even there main highways, airports, and such can prevent a reduction. In most cities, no place is further than a few blocks from some part of the grid of principal streets carrying heavy traffic.
Important contributions are made by entertainment installations. These noise sources include music on streets and in shopping centers, amusement parks and racetracks, paging and public address systems, schools, athletic fields, and even discotheques where performances indoors are often audible several blocks away. Other offenders include sound trucks, advertising devices, and kennels or animal shelters.
Because noise sources are distributed over an urban area, the sound-power output of a source can be more informative than the noise level produced at a specific distance. Figure 6.2.3 lists some urban sources with their approximate sound-power ratings.
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