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4.13 NOISE This section includes an evaluation of noise impacts of the proposed project in terms of short-term construction noise; stationary source noise;traffic noise;and compatibility of proposed land uses with predicted noise levels. 4.13.1 PHYSICAL SETTING _ ACOUSTICAL FUNDAMENTALS Noise is defined as unwanted sound. Sound is a mechanical form of radiant energy transmitted by pressure waves in the air. Sound is characterized by two parameters,amplitude(loudness)and frequency(tone),and attenuates (softens)with distance from the noise source. Noise Amplitude, Frequency, and Attenuation Amplitude is the difference between ambient air pressure and the peak pressure of the sound wave.Amplitude is measured in decibels(dB)on a logarithmic scale.For example,a 65 dB source of sound, such as a truck,when joined by another 65 dB source results in a sound amplitude of 68 dB,not 130 dB(i.e.,doubling the source strength increases the sound pressure by 3 dB).Amplitude is interpreted by the ear as corresponding to different degrees of loudness. Laboratory measurements correlate a 10 dB increase in amplitude with a perceived doubling of loudness and establish a 3 dB change in amplitude as the minimum audible difference perceptible to the average person(California Department of Transportation[Caltrans] 1998;Federal Highway Administration [FHWA] 1980). Because of the ability of the human ear to detect a range of sound pressure fluctuations, sound pressure levels are expressed in logarithmic units called decibels(dB).The sound pressure level in decibels is calculated by taking the log of the ratio between the actual sound pressure and the reference sound pressure squared.The reference sound pressure is considered the absolute hearing threshold(Caltrans 1998).Because the human ear is not equally sensitive to all sound frequencies,a specific frequency-dependent rating scale was devised to relate noise to human sensitivity.An A-weighted dB (dBA)scale performs this compensation by discriminating against frequencies in a manner approximating the sensitivity of the human ear.The basis for compensation is the faintest sound audible to the average ear at the frequency of maximum sensitivity.This dBA scale has been chosen by most authorities for purposes of environmental noise regulation.The sound levels of different typical daily uses and activities are presented in Figure 4.13-1.As indicated,typical sounds range from 40 dBA(very quiet)to 100 dBA(very loud).Conversation occurs at roughly 60 dBA at 3-5 feet.As background noise levels exceed 60 dBA, speech becomes increasingly difficult.Noise becomes physically discomforting at 110 dBA. Stationary point sources of noise,including stationary mobile sources such as idling vehicles,attenuate at a rate between 6 dBA over hard surfaces and 7.5 dBA over soft surfaces for each doubling of distance from the reference measurement.No excess ground attenuation is assumed for hard surfaces and the changes in noise levels with distance(drop-off rate)is simply the geometric spreading of the noise from the source. Soft surfaces have an absorptive effect: sound attenuation over soft surfaces is normally assumed at 1.5 dBA per doubling distance. Line sources(such at traffic noise from vehicles)attenuate at a rate between 3 dBA for each doubling of .. distance. In addition,atmospheric conditions,such as wind speed,temperature,and humidity,may affect noise levels,as does the presence of a barrier between the source and receptor that may attenuate noise levels.The actual amount of attenuation is dependent on the barrier size and noise frequency.A noise barrier may be any natural or human-made feature, such as a hill,tree,building,wall,or berm(Caltrans 1998). Noise Descriptors The selection of a proper noise descriptor for a specific source is dependent on the spatial and temporal distribution,duration, and fluctuation of the noise.An individual's noise exposure is a measure of noise over a College Park at Mountain House Specific Plan III Draft EIR EDAW San Joaquin County 4.13-1 Noise