20 ATTACHMENT 3 - Laserfiche WebLink

3987 Stanley 4 March 2017 (revised 4 August2017) Environmental Noise Assessment Page 6 As it is often cumbersome to quantify the noise environment with a set of statistical descriptors, a single number called the average sound level or "Leq" is now widely used. The term "Leq" originated from the concept of a so-called equivalent sound level which contains the same acoustical energy as a varying sound level during the same time period. In simple but accurate technical language, the Leq is the average A -weighted sound level in a stated time period. The Leq is particularly useful in describing the subjective change in an environment where the source of noise remains the same but there is change in the level of activity. Widening roads and/or increasing traffic are examples of this kind of situation. In determining the daily measure of environmental noise, it is important to account for the different response of people to daytime and nighttime noise. During the nighttime, exterior background noise levels are generally lower than in the daytime; however, most household noise also decreases at night, thus exterior noise intrusions again become noticeable. Further, most people trying to sleep at night are more sensitive to noise. To account for human sensitivity to nighttime noise levels, a special descriptor was developed. The descriptor is called the Ldp (Day/Night Average Sound Level) which represents the 24-hour average sound level with a penalty for noise occurring at night. The L.d„ computation divides the 24-hour day into two periods: daytime (7:00 am to 10:00 pm); and nighttime (10:00 pm to 7:00 am). The nighttime sound levels are assigned a 10 dB penalty prior to averaging with daytime hourly sound levels. For highway noise environments, the average noise level during the peak hour traffic volume is approximately equal to the Lai. The effects of noise on people can be listed in three general categories: 1. Subjective effects of annoyance, nuisance, dissatisfaction; 2. Interference with activities such as speech, sleep, and learning; and 3. Physiological effects such as startle, hearing loss. The sound levels associated with environmental noise usually produce effects only in the first two categories. Unfortunately, there has never been a completely predictable measure for the subjective effects of noise nor of the corresponding reactions of annoyance and dissatisfaction. This is primarily because of the wide variation in individual thresholds of annoyance and habituation to noise over time. Thus, an important factor in assessing a person's subjective reaction is to compare the new noise environment to the existing noise environment. In general, the more a new noise exceeds the existing, the less acceptable the new noise will be judged. With regard to increases in noise level, knowledge of the following relationships will be helpful in understanding the quantitative sections of this report: 4. Except in carefully controlled laboratory experiments, a change of only 1 dB in sound level cannot be perceived. 5. Outside of the laboratory, a 3 dB change is considered ajust-noticeable difference. 6. A change in level of at least 5 dB is required before any noticeable change in community response would be expected. 7. A 10 dB change is subjectively heard as approximately a doubling in loudness, and would almost certainly cause an adverse community response. FNDA2LDN 3 October 1990 (reformatted 2012)