Outside and Inside Noise Exposure in Urban and Suburban Areas

-In urban and suburban areas of the United States (away from major airports), the outdoor noise environment usually depends strongly on local vehicular traffic. By relating traffic flow to population density, a model of outdoor noise exposure has been developed for estimating the cumulative 24-hour noise exposure based upon the population density of the area. This noise model has been used to provide first-order estimates of noise exposure in urban areas of this country. The model also can be used to estimate the variation in noise levels in communities and to plan community noise surveys. Field measurements at 100 locations throughout the country have shown good agreement with this model. THE OUTDOOR NOISE environment in urban and suburban areas in this country is due to many different noise sources of varying frequency content and temporal characteristics. Actual noise levels and noise exposures vary over a considerable dynamic range. However, because of the prevalence of similar major sources of noise, one can expect many similarities in noise exposure and in daily patterns of noise exposure throughout the country, These similarities suggest that there should be good likelihood of developing useful models for predicting the noise environment for different neighborhoods throughout the country. Because of the well-identified and relatively well-studied impact of jet-aircraft noise and high density freeway traffic noise, rather detailed analytic models have been developed for predicting aircraft and highway noise. These models are quite accurate for locations relatively close to the noise sources. Although aircraft operations and freeway traffic produce some of the noisiest outdoor environments found in this country, only a small proportion of the total population is exposed to these rather intense noise sources. The majority of the population is affected by noise from other sources. A means for defining the noise exposure for this large segment of the population that does not depend upon detailed knowledge of source levels, operating conditions, or identification of specific noise sources is particularly useful for community and land-use planning. This paper discusses a simple model for estimating the noise exposure in urban and suburban areas not located near airports or major freeways. The paper also discusses measurements made to confirm and verify this model and also provide additional information about typical urban outdoor and indoor noise environments. An emphasis on the outdoor noise environment sometimes obscures the fact that most of us spend much of our lives indoors, and that actually, our noise exposure is not necessarily predicted well from knowledge of the outdoor noise levels alone. Thus, to provide some perspective, this paper also presents a brief comparison of simultaneous indoor and outdoor measurements made in residential areas. Before describing the noise model and measurements, we should first define the kind of noise measure that we will use. All measurements will be reported in terms of the A-weighted noise level in decibels. This level incorporates a frequency weighting of the noise signal similar to that of the human ear ; i t thus provides a reasonably good assessment of the noise in terms of people's subjective response to noise. Because of the time-varying nature of the actual noise environment, the temporal aspects of noise must also be considered in selecting a useful scale for measuring the noise environment. Given modern acoustic instrumentation and digital computers, i t is now easily possible to develop detailed and exhaustive statistical descriptions of the noise measured over any desired time period, ranging from a few minutes to a few years. However, for many planning purposes, such detail is not needed nor desired. Thus, in this paper the noise environment will be described in terms of the A-weighted equivnlent level measured over hourly periods or measured during the 24-hour day. For those not familar with the concept, the equivalent level is obtained by "energy" averaging the noise levels occurring over a stated period of time, an hour or day, for example. The equivalent level is equal to the level of a continuous noise source that would produce the same energy as contained in the actual fluctuating noise levels occurring over the same time period. Figure 1 shows a graphic level recording of the actual noise levels measured over a short (14-minute) time. The equivalent level noted on the figure may be compared to the maximum level observed during the period and the "arithmetic" average level existing over the period. The equivalent level is much more influenced by the presence of short duration noise intrusions than is the "mean" or "median" noise level. To describe the noise environment over a 24-hour day, the day-night average level (L,,) measure will be used. The day-night average level extends the equivalent level concept to the entire 24-hour period, but includes a weighting of 10 dB applied to the noise levels measured a t night (taken as the period from 10 P.M. to 7 A.M.). This weighting is applied to reflect the greater sensitivity to noise in residential areas during the night. Other measures of the noise environment are in more or less common use and are described in the literature. For most con~munity noise environments, the correlation of one noise measure with another will generally be quite high. Thus, with the exception of neighborhoods exposed to discrete high-level noise intrusions (such as neighborhoods near jet airports), general conclusions based on day-night levels or hourly equivalent levels will hold for most other noise measures in common use. Figure I .-An example of noise-level fluctuations over a short time period.