1. REFERENCES. References used in this Appendix are listed at the end.
2. INTRODUCTION. A characteristic of environmental noise is that it is not steady, but varies in amplitude from one moment to the next. To account for these variations in the sound pressure level with time, and to assess environmental noise in a consistent and practical manner, a statistical approach has been used to reduce the time-varying levels to single numbers. The currently accepted single-number evaluators are the equivalent sound level (LEQ) and the day-night level (DNL).
3. BACKGROUND.
a. Noise is defined as unwanted sound. Sound is the variation of air pressure about a mean (atmospheric) pressure. These changes in the atmospheric pressure [100,000 Pascals (14.7 pounds per square inch) (psi)] vary from approximately 0.0006 Pascals for a whisper at 2 meters to 1,000 Pascals for firing an M-16 rifle near the firer's ear. Because of this large range of sound pressure and the fact that the human ear responds more closely to a logarithmic scale rather than a linear scale, sound pressure level is defined as 20 times the common logarithm of the ratio of the sound pressure to the reference pressure (0.00002 Pascal). The sound pressure level is measured in decibels (dB). For example, if the sound pressure doubles from 0.2 to 0.4 Pascals, the level increases by 6 dB from 80 to 86 dB.
b. In environmental noise, the sound pressure level is usually measured using one of the frequency networks of the sound level meter. Since the human ear is more sensitive to sounds of 1,000 Hertz (Hz) and above than sounds of 125 Hz and below, it is appropriate to apply a weighting function to the noise spectrum that approximates the response of the human ear. The A-weighting frequency network of the sound level meter deemphasizes the lower frequency portion of the noise spectrum to approximate the human ear's response to the noise. This A-weighting frequency response is specified by American National Standards Institute (ANSI) standard S1.4-1983 (ANSI 1983). Thus, the A-weighting of the frequency content of the noise signal has been found to have an excellent correlation with the human subjective judgment of annoyance of the noise. The sound pressure levels measured using the A-weighting network are expressed as dBA.
c. To assess the additional annoyance caused by low frequency vibration of structures, the C-weighting network is used to evaluate the impulsive noise from all weapons larger than small arms. This weighting is also specified by the standard. The sound pressure levels measured using the C-weighting network are expressed as dBC.
d. For small arms ranges, the linear weighting network is currently being used. The peak decibels sound level (dBP) is used to evaluate this noise. This weighting is also commonly used to measure the dBP from impulsive events. The linear weighting network weights the sound energy contained in all frequencies equally.
4. HISTORY.
a. Before the mid 1970's, every organization had its own set of preferred environmental noise evaluators. This resulted in a wide variety of evaluators. Since each evaluator was developed for a specific purpose, a noise environment measured with one evaluator could not be compared with an environment measured using another evaluator.
b. In carrying out its responsibilities under the Noise Control Act of 1972 (PL 92-574 1972), the EPA recommended the adoption of a single environmental noise evaluator, the LEQ and its 24-hour version, DNL. The Department of Defense, along with most other U.S. Government Agencies followed the EPA recommendation. The DNL is the most widely accepted descriptor for environmental noise (FAA 1990) because of the following characteristics:
(1) The DNL is a measurable quantity.
(2) The DNL is simple to understand and use by planners and the public who are not familiar with acoustics or acoustical theory.
(3) The DNL provides a simple method to compare the effectiveness of alternative scenarios.
(4) The DNL is a "figure of merit" for noise impacts which is based on communities' reactions to environmental noise.
(5) The DNL is the best measure of noise exposure to identify significant impacts on the quality of the human environment.
(6) By Federal interagency agreement, the DNL is the best descriptor of all noise sources for land use compatibility planning.
(7) The DNL is the only metric with a substantial body of scientific survey data on the reactions of people to noise.
c. In recommending the DNL, the EPA noted that most noise environments are characterized by repetitive behavior from day to day, with some variation imposed by differences between weekday and weekend activity, as well as seasonal variation. To account for these variations, an annual average is used.
d. Since annoyance is caused by long-term dissatisfaction with the noise environment, the annual average is an excellent predictor of the average community annoyance when there is not a large variation in the day to day or season to season DNL. The annual DNL is not a good predictor of noise complaints, since complaints represent the person's immediate dissatisfaction with the noise environment.
e. Currently, there are no guidelines for judging the land use compatibility for single noise events. Although much of the early work on annoyance was done on single events, each study was designed differently, and the results cannot be combined in a systematic fashion to form a statistically valid sample. Most of these studies were either done inside a laboratory or, if done outdoors, in controlled settings. Only recently has equipment become available that allows subjects to register their annoyance if single events are experienced during their routine activities. There is not enough of this information available to support setting standards on single events.
f. For impulsive noise, the Department of the Army uses the CDNL. The use of C-weighting is based on the findings of the National Academy of Sciences Committee on Hearing, Bioacoustics and Biomechanics Association (CHABA 1981). Studies have been performed by USACERL (Schomer and Neathammer 1984) to define the average annoyance as a function of the CDNL. The ANSI has endorsed this methodology for predicting the annoyance caused by impulsive noise (ANSI S12.4-1986).
g. Until recently the dBP was used to predict the annoyance caused by small arms range noise. In late 1996 the U.S. Army Small Arms Range Noise Assessment Model (SARNAM) was introduced. Developed by the USACERL with the assistance of the U.S. Army Center for Health Promotion and Preventive Medicine (USACHPPM), this model produces noise contours using the ADNL. In conformance with ANSI Standard S12.9, a 12-dB adjustment for impulsiveness is added to the ASEL values used to calculate the ADNL.
5. LEQ/DNL.
a. The LEQ is defined as the equivalent steady state sound level that, in a stated period of time, would contain the same acoustic energy as the time-varying sound during the same period. The LEQ is an energy average. The energy average puts more emphasis on the higher sound pressure levels than the arithmetic average. The LEQ is usually computed for a 1-minute, 10-minute, 30-minute, 1-hour, 8-hour, or 24-hour segment of environmental noise.
b. To assess the added annoyance of the environmental noise during the nighttime hours (2200 - 0700 hours), the DNL is used. The DNL is the 24-hour LEQ, with a 10-dB penalty added to the nighttime levels.
c. By using the LEQ and DNL, the three important determinants of noise annoyance can be described by a single number. The three determinants are the intensity of the noise event, the duration of the noise event, and the number of times the noise event takes place.
d. The noise from jet aircraft operations on a military training route is unique in several respects. The combination of low altitudes and high air speeds results in noise signatures with high levels and short durations. This results in a very rapid onset that may produce a startle response. Also, the noise events are highly sporadic. To account for the rapid onset and sporadic events, the onset rate-adjusted monthly day-night level (DNRML) is used. The DNRML is a monthly DNL that is adjusted for the added annoyance caused by the rapid onset of the noise.
6. NOISE CONTOURS.
a. Noise contours are generated using the A- or CDNL. The contours are computed by averaging over the time period of interest, the acoustical energy from the operations of the set of noise sources of interest. The averaging period is usually a busy day, a training cycle, or a year. The contours, representing the boundaries between the noise zones, are constructed by connecting points of equal acoustical energy.
b. The noise contours for small arms ranges are generated using the ADNL with a 12-dB impulsive adjustment added or the A-weighted sound exposure level (SEL).
c. For example, the contours for an airfield are computed by averaging at many points the acoustical energy arriving at these points from aircraft operations. A 10-dB penalty is added to all nighttime operations. The contours for the airfield are constructed by connecting all points having a total acoustical energy equal to 65 dBA and connecting all points equal to 75 dBA.
REFERENCES
American National Standards Institute (ANSI), 1983, S1.4-1983, "American National Standard Specification for Sound Level Meters."
ANSI, 1986, S12.4-1986, "American National Standard Method for Assessment of High-Energy Impulsive Sounds with Respect to Residential Communities."
Committee on Hearing, Bioacoustics, and Biomechanics Association (CHABA), 1981, Working Group 84 Report, "Assessment of Community Response to High-Energy Impulsive Sounds."
FAA, 1990, "Day Night Average Sound Level (DNL), The Descriptor of Choice for Airport Noise Assessment."
Luz, 1982, "An improved procedure for evaluating the annoyance of small arms ranges," presented at the 104th Meeting of the Acoustical Society of America, Orlando, Florida.
PL 92-574, 1972, Noise Control Act of 1972.
Schomer, Paul D. and Robert D. Neathammer, Community Reaction to Impulse Noise: A 10-Year Research Summary, USACERL Technical Report N-167, ADA141762 (USACERL 1984).