ARCHIVED REPORTS_1993_1 - Laserfiche WebLink

Home Browse Search

Appendix H NOISE TECHNICAL DATA This appendix presents background information on noise to aid the EIR reader in understanding noise assessment, a description of the applicable noise standards, and a description of the existing noise environment in the study area. Noise is defined as unwanted sound. Airborne sound is a rapid fluctuation of air pressure above and below atmospheric pressure. Sound levels are usually measured and expressed in decibels (dB) with 0 dB corresponding roughly to the threshold of hearing. Decibels and other technical terms are defined in Table H-1. Most of the sounds which we hear in the environment do not consist of a single frequency, but rather a broad band of frequencies, with each frequency differing in sound level. The intensities of each frequency add together to generate a sound. The method commonlyused to quantify environmental sounds consists of evaluating all o the frequencies of a sound in accordance with a weighting that reflects the facts that human hearing is less sensitive at low frequencies and extreme high frequencies than in the frequency mid-range. This is called "'A" weigghting , and the decibel level so measured is called the A-weighted sound level (dBA). In practice, the level of sound source is conveniently measured using a sound level meter that includes an electrical filter corresponding to the A-weighting curve. Typical A-levels measure in the environment and in industry are shown in Table H-2 for different types of noise. Although the A-weighted noise level may adequately indicate the level of environmental noise at any instant in time, community noise levels varcontinuously. Most environmental noise includes a conglomeration of noise from istnt sources which create a relatively steady background noise i which o particular source is identifiable. To describe the time-varying character of environmental noise, the statistical noise descriptors, Llo, L50, and Lo, are commonly used. They are the A- weighted noise levels equaled or exceeded during 10%,.50%, and 90%of a stated time period. A single number descriptor called the L is now also widely used. The Leq is the average A-weighted noise level during a stateaPperiod of time. 1 In determining the daily level of environmental nose, it is important to account for the difference in response of people to daytime and nighttime noises. During the nighttime, exterior background noises are generally lower than the daytime levels. However, most household noise also decreases at night and exterior noise becomes very noticeable. Further, most people sleep at night and are very sensitive to noise intrusion. To account for human sensitivity to nighttime nose levels, a descriptor,Ldn (day/night average sound level),was developed. The L divides the 24-hour a into the daytime of 7:00 AM to 10:00 PM and the nighttime t7odT0:00 PM to 7:00 AM. The nighttime noise level is weighted 10 dB higher than the daytime noise level. The Community Noise Equivalent Level (CNEL) is another 24-hour average which includes both an evening and nighttime weighting. The effects of noise on people can be listed in three general categories: • subjective effects of annoyance,nuisance, dissatisfaction • interference with activities such as speech,sleep,learning physiological effects such as startling,hearing loss H-1