The measure should be closely related to existing methods currently in use.
The single measure of noise at a given location should be predictable, within an acceptable tolerance, from knowledge of the physical events producing the noise.
The measure should lend itself to small, simple monitors that can be left unattended in public areas for long periods of time.
EPA also published its rationale for choosing A-weighting and for leaving open the possibility of using a different metric in the future (EPA, 1974; von Gierke, 1975):
With respect to both simplicity and adequacy for characterizing human response, a frequency-weighted sound level should be used for the evaluation of environmental noise. Several frequency weightings have been proposed for generalusein the assessment of response to noise, differing primarily in the way sounds at frequencies between 1000 and 4000 Hz are evaluated. The A-weighting, standardized in current sound level meter specifications, has been widely used for transportation and community noise description. For many noises, the A-weighted sound level has been found to correlate as well with human response as more complex measures, such as the calculated perceived noise level or the loudness level derived from spectral analysis. However, psychoacoustic research indicates that, at least for some noise signals, a different frequency weighting which increases the sensitivity to the 1000–4000 Hz region is more reliable. Various forms of this alternative weighting function have been proposed; they will be referred to here as the type “D-weightings.” None of these alternative weightings [have] progressed in acceptance to the point where a standard has been approved for commercially available instrumentation.
It is concluded that a frequency-weighted sound pressure level is the most reasonable choice for describing the magnitude of environmental noise. In order to use available standardized instrumentation for direct measurement, the A-frequency weighting is the only suitable choice at this time. The indication that a type D-weighting might ultimately be more suitable than the A-weighting for evaluating the integrated effects of noise on people suggests that at such time as a type D-weighting becomes standardized and available in commercial instrumentation, its value as the weighting for environmental noise should be considered to determine if a change from the A-weighting is warranted.
The decision to add 10 dB4 in measuring nighttime levels and the selection of a two-period (day-night) metric rather than a three-period metric (day-evening-night) was based on community reaction studies at the time and tests that showed little difference between a two-period and a three-period metric. Thus, the DNL (A-frequency weighting for both day-time and nighttime levels and a 10-dB increase in measuring system gain at night) came into being for the evaluation of community noise.
In the United States, DNL and the percentage of persons highly annoyed (discussed in the next section) are widely used, especially by the Federal Aviation Administration (FAA). The Federal Highway Administration uses A-weighting and the average sound pressure level during the busiest traffic hour as a measure of community impact. The difference between C-weighted and A-weighted levels is used as an indication of the low-frequency content of the sound, and the sound exposure level (see Appendix A) is used to evaluate sounds of finite duration—for example, an aircraft flyover.
Day-evening-night sound level is widely used in Europe. In some countries, Lday and Lnight, (average A-weighted sound pressure levels) are used in addition to or instead of a DNL-type metric. None of these metrics takes into account the time of night when the noise occurs, even though noise appears to cause greater sleep disturbance at the beginning and end of the night.
Several issues have arisen from the use of DNL and the percentage of persons highly annoyed: no one actually “hears” a DNL; there is a high variability from study to study around a nominal Schultz curve; and in many situations “highly annoyed” is not an appropriate measure of human response. Although the percent highly annoyed and DNL approach has been widely endorsed, variability around a nominal Schultz curve is troubling, and there are reports that this approach is not sufficient to predict community response (Fidell, 2002). Attitudinal and personal variables impact people’s responses and are, to some extent, the reason for scatter (Fields, 1993; Flindell and Stallen, 1999; Miedema and Vos, 1999).
As shown in Figure 3-1, some researchers (Miedema and Oudshoorn, 2001) have found in their analyses of survey results that the nominal Schultz curve appears to depend on the noise source (e.g., aircraft, road traffic, rail traffic). In addition, DNL is a relatively insensitive measure of sleep disturbance and thus is not an appropriate metric for predicting awakenings in sleep disturbance studies. Finally, A-weighting is not the best weighting for measuring noises with unusual spectra (e.g., excessive high- or low-frequency noise or noise that has unusual peaks in its spectrum). For sounds with levels that evolve over time, the most appropriate
A number of metrics have been developed to take into account day-time versus nighttime operations around airports. These include Noise Exposure Forecast, Community Noise Equivalent Level, and Noise and Number Index. The EPA rationale for selecting a 10-dB nighttime penalty (EPA, 1974) is as follows: “Methods for accounting for the differences in interference or annoyance between daytime/nighttime exposures have been employed in a number of different noise assessment methods around the world. The weightings applied to the nondaytime periods differ slightly among the different countries but most of them weight night activities on the order of 10 dB; the evening weighting if used is 5 dB. The choice of 10 dB for the nighttime weighting made in Section 2 was predicated on its extensive prior usage, together with an examination of the diurnal variation in environmental noise.”