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between the source and the receptor may result in additional sound absorption and/or reflection.Atmospheric conditions such as wind speed,temperature, and humidity may affect noise levels. Furthermore,the presence of a barrier between the source and the receptor may also attenuate noise levels. The actual amount of attenuation is dependent upon the size of the barrier and the frequency of the noise. A noise barrier may be any natural or human-made feature such as a hill,tree,building,wall, or berm(Caltrans 1998). All buildings provide some exterior-to-interior noise reduction. A building constructed with a wood frame and a stucco or wood sheathing exterior typically provides a minimum exterior-to-interior noise reduction of 25 dBA with its windows closed,whereas a building constructed of a steel or concrete frame, a curtain wall or masonry exterior wall, and fixed plate glass windows of one-quarter-inch thickness typically provides an exterior-to- interior noise reduction of 30-40 dBA with its windows closed(Paul S.Veneklasen&Associates 1973, cited in Caltrans 2002). Noise Descriptors The selection of a proper noise descriptor for a specific source is dependent upon the spatial and temporal distribution, duration, and fluctuation of the noise. The noise descriptors most often encountered when dealing with traffic, community, and environmental noise are defined below(Caltrans 1998, Lipscomb and Taylor 1978). ► L.,,x(Maximum Noise Level): The maximum instantaneous noise level during a specific period of time. The L.,,x may also be referred to as the"peak(noise)level." ► Li.(Minimum Noise Level): The minimum instantaneous noise level during a specific period of time. ► Lx(Statistical Descriptor): The noise level exceeded X%of a specific period of time. ► Leq(Equivalent Noise Level): The energy mean(average)noise level. The instantaneous noise levels during a specific period of time in dBA are converted to relative energy values. From the sum of the relative energy values, an average energy value is calculated,which is then converted back to dBA to determine the Leq. In noise environments determined by major noise events, such as aircraft overflights,the Leq value is heavily influenced by the magnitude and number of single events that produce the high noise levels. ► Ld.(Day-Night Noise Level): The 24-hour Leq with a 10-dBA"penalty"for noise events that occur during the noise-sensitive hours between 10:00 p.m. and 7:00 a.m. In other words, 10 dBA is"added"to noise events that occur in the nighttime hours, and this generates a higher reported noise level when determining compliance with noise standards. The Ldn attempts to account for the fact that noise during this specific period of time is a potential source of disturbance with respect to normal sleeping hours. ► CNEL (Community Noise Equivalent Level): The CNEL is similar to the Ld,,described above,but with an additional 5-dBA"penalty"added to noise events that occur during the noise-sensitive hours between 7 p.m. and 10 p.m.,which are typically reserved for relaxation,conversation,reading, and television. When the same 24-hour noise data are used,the reported CNEL is typically approximately 0.5 dBA higher than the Ld,,. ► SENL(Single-Event [Impulsive] Noise Level): The SENL describes a receiver's cumulative noise exposure from a single impulsive noise event,which is defined as an acoustical event of short duration and involves a change in sound pressure above some reference value. SENLs typically represent the noise events used to calculate the Leq,Ld,,, and CNEL. Community noise is commonly described in terms of the ambient noise level,which is defined as the all- encompassing noise level associated with a given noise environment.A common statistical tool to measure the ambient noise level is the average, or equivalent, sound level Leq,which corresponds to a steady-state A-weighted sound level containing the same total energy as a time-varying signal over a given time period(usually 1 hour). EDAW Manteca WQCF and Collection System Master Plans EIR Noise 4.4-4 City of Manteca