Noise in the environment
N
oise is not a necessary evil, and noise pollution problems can be solved. But like other pollution problems, the solution demands a blend of technology, public and private action, and healthy doses of economic realism. One of the reasons that noise pollution has assumed its present proportions is the widely held and mistaken belief that nothing can be done about noise. But noise can be controlled, and much of the technology required for noise control is presently available. By properly applying existing technology, advance planning, and appropriate considerations in designing vehicles, machines, and buildings, a substantial amount of relief from noise could be provided at relatively small cost. Noise p o l l u t i o n trends
Today, three related trends are focusing attention on noise pollution. First, noise is increasing and affecting more people. Second, the public is becoming more concerned about noise. And, third, acoustical specialists are stepping up their efforts to control the noise environment. Noise has penetrated virtually every aspect of modern life and, generally speaking, the problem is getting worse. An ever-increasing number of common noise sources-motor vehicles, aircraft, 124 Environmental Science & Technologj
power tools, household gadgets-is being put into use daily, and new noise sources such as snowmobiles and hovercraft are being added. New highways, new airports, and increased numbers of airplanes ensure that noise will be ever more widely distributed. Protection from this rising din is actually on the downswing due to increasing use of lightweight building construction and contemporary open plan designs. This trend toward increased noise has fostered another trend: growing public concern, which is just beginning to emerge as a force to be reckoned with. According to a survey conducted for the Automobile Manufacturers, Inc., two out of three people questioned in the greater metropolitan areas of Boston, Detroit, and Los Angeles regard their neighborhoods as noisy places. Chicago has recently enacted a comprehensive and quantitative regulation designed to reduce noise pollution, and other cities and states are considering similar or regulatory actions. Public concern about noise is beginning to be translated into action; for example, the noise argument against the supersonic transport and establishing the federal EPA Office of Noise Abatement and Control. The Environmental Protection Act requires that the environmental impact-including noise
effects-be assessed before proceeding on federally funded construction projects. In direct response to the expanding noise problem and the growing public awareness, a third trend is now developing. Acousticians are attempting to make noise control procedures more understandable, more easily applied, and, hopefully, more widely used. Some examples of this trend are given later in this article. M e a s u r i n g noise i m p a c t
One of the difficulties encountered by anyone attempting to understand the nature of sound is the multitude of units and rating scales that can be used to measure sound. The great variety of units and scales has grown in part from efforts t o achieve sophistication in measurement. For most noise pollution situations, the A-weighted sound level, expressed in units of dB(A) or decibels, is most useful (Figure 1). The A-weighted sound level results from a weighting of the sound signal that gives greater emphasis to components in the midfrequency region and less emphasis to components at lower and higher frequencies. The science of psychoacoustics studies how the physically measurable prop-
Peter A. Franken and Daniel G. Page
feature
Bolt Beranek andNewnian Inc. Cambridge, Mass. 02138
Available technology could effectively control many of the present noise levels and sources
erties of sound relate t o human judgments of “noisiness.” Much work has been done t o study human hearing responses so as t o quantify, in a statistical way, how people react t o different kinds and levels of noise. Psychoacoustics has assumed increasing importance with the growing concern for noise pollution. Particular attention has been focused on assessing the impact of jet aircraft noise o n communities near airports. A great deal of work has gone into the development of criteria for airport planning and of techniques for correlating human annoyance with such factors,as the sound level, the signal duration, how many flyovers occur, and what time of day they occur. One type of result is shown
in Figure 2. This describes projected 1975 operations a t O’Hare International Airport, Chicago, Ill. Outside contour 30, land is said to be normally acceptable for residential housing, but hospitals, schools, and churches may require special construction t o shield against aircraft noise. Effects of noise
When one thinks about the effects of noise on man, probably the first thing that comes t o mind is the loss of hearing that can result from exposures to very high noise levels. Brief exposure to levels of 140 to 150 dB(A) can rupture eardrums and cause permanent hearing loss. Longer exposure t o lower noise levels can also damage hearing. On the
basis of available data, standards set under the Walsh-Healey Public Contracts Act now establish 8 hr as the maximum time that a worker can be exposed t o noise levels of 90 dB(A) o r more. Noise levels high enough to damage hearing are usually not encountered in community situations. But even lowerlevel and shorter-duration noises that intrude into communities also have important effects. Speech interference is a n easily demonstrable effect of lower-level noises. For situations such as listening to the radio or having a conversation, speech interference can be a distinct source of annoyance. For situations in which verbal communication is criticalcommands shouted to the operator of a
Typical sources of community noise vary in intensity Representative situations
Sources
dB(A) (A.weighted sound level)
Volume 6, Number 2, February 1972 125
Figure 2
Noise intensity for O’Hare International Airport (projected for 1975)
wrecking crane, for example-speech interference can be a very real hazard. Noise can also disturb sleep and produce startle reactions in humans and animals. The immediate effects seem t o be relatively mild and short-lived. However, the longer-term effects of noiseinduced annoyance are still a subject of much speculation. Careful and controlled scientific research is needed to determine what the long-range effects of noise really are. Land-use planning
One of the most appealing approaches t o the community noise problem is land use planning-establishing land use patterns that separate the most objectionable noise sources from noise sensitive areas. The noise levels for proposed airport, highway, and building sites can be predicted. An urgent need a t present is to persuade planners, builders, and designers to use the available techniques for assessing the compatibility of proposed land use with the acoustic environment. One way to make acoustic planning tools more widely used is by developing highly simplified tests and screening procedures that enable persons with no background in acoustics to make “first-step” determinations of the acoustic suitability of proposed sites. For example, the computational details involved in generating Noise Exposure Forecasts (NEF), like those shown for O’Hare International Airport, are quite complicated; indeed, a computer is usually required. But a greatly simplified version of the NEF techniques can furnish preliminary information that is useful in determining the potential severity of a noise problem. The technique was developed for the U S . Department of Housing and Urban Development (HUD) and is essentially a n extrapolation of results previously computed for some 30 airports. It requires a knowledge only of the number of daytime and nighttime jet aircraft operations, the major runway locations and flight paths, and whether any supersonic aircraft will be using the airport. On the basis of this information, any airport can be assigned into one of four general categories for which the locations of the NEF contours are given. Of course, this simplified procedure 126 Environniental Science & Technolog>
a
Oft 4000 8000
provides approximate, though conservative, results; in cases where contours must be determined more precisely or where marginal errors can make a substantial difference, the complete NEF procedure would be required. As another example of simplified noise estimation procedures for planning, consider highway noise. Noise surveys along roadways have shown that noise levels depend on a great many parameters, including the distance from the roadway to the observation point, the total volume of traffic, the average speed of traffic, the percentage of trucks in the total traffic volume, the slope of the road, the type of road surface, the surrounding terrain, and the location of natural and artificial barriers. The ways in which these parameters combine t o make up the overall noise level at a given location are quite complicated, but programs are available to assess the resulting effects on noise levels from highways. A greatly simplified version of this forecasting technique can be used as a screening procedure for determining areas that represent potential noise problems along a highway. This simplified procedure requires no acoustical experience on the part of the user and a knowledge only of the distance from the
roadway to the receiver, the number of cars and trucks per hour, and their average speed. Another simple test uses speech interference criteria to rate the noisiness of proposed housing sites according t o HUD’S interim guidelines. The test requires no special equipment, just a tape measure and two people. One person reads aloud from a book, and the other person moves away until he can only catch a scattered word or two in a 10-sec period. If difficulty in understanding occurs at approximately 7 ft or less, the site is clearly unacceptable for housing. If understanding is difficult at 7 to 25 ft, the site should not be used for housing except with extensive acoustic treatment. The site is considered normally acceptable (noticeable noise but not a serious problem) if hearing does not become difficult until distances of 26 to 70 ft are reached. If the listener can understand the words at a distance of 70 ft or greater, the site is clearly acceptable for housing. Such simplified, rule-of-thumb tests are valuable planning tools that are likely to be widely used because they are easy, cheap, and d o not require the services of an expert. The obvious advantage of land use planning is that it prevents noise from
T a b k %,Uirbsn areas iist FwajQ!' nOiSt!
SOUf'CeS
Weighted % Source Aircraft Construction
Los
Boston Angeles Detroit
17 2
16 1
1
Industrial noise
