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San Joaquin County Environmental Checklist ■ Sound. A vibratory disturbance created by a vibrating object that,when transmitted by pressure waves through a medium such as air, is capable of being detected by a receiving mechanism, such as the human ear or a microphone. ■ Noise. Sound that is loud,unpleasant,unexpected,or otherwise undesirable. ■ Decibel(dB). A unitless measure of sound on a logarithmic scale that indicates the squared ratio of sound pressure amplitude to a reference sound pressure amplitude. The reference pressure is 20 micro-pascals. ■ A-Weighted Decibel(dBA). An overall frequency-weighted sound level in decibels that approximates the frequency response of the human ear. ■ Equivalent Sound Level(Leq). The average of sound energy occurring over a specified period. In effect,Leq is the steady-state sound level that in a stated period would contain the same acoustical energy as the time-varying sound that actually occurs during the same period. ■ Exceedance Sound Level(LXX). The sound level exceeded XX percent of the time during a sound level measurement period. For example,1,90 is the sound level exceeded 90 percent of the time,and Llo is the sound level exceeded 10 percent of the time. ■ Maximum and Minimum Sound Levels(L..,,and L.i„). The maximum or minimum sound level measured during a measurement period. ■ Day-Night Level(Ld„). The energy average of the A-weighted sound levels occurring during a 24-hour period,with 10 dB added to the A-weighted sound levels occurring during the period from 10:00 p.m.to 7:00 a.m. ■ Community Noise Equivalent Level(CNEL). The energy average of the A-weighted sound levels occurring during a 24-hour period,with 5 dB added to the A-weighted sound levels occurring during the period from 7:00 p.m.to 10:00 p.m. and 10 dB added to the A-weighted sound levels occurring during the period from 10:00 p.m.to 7:00 a.m. Ldn and CNEL values rarely differ by more than I dB. As a matter of practice, La„and CNEL values are considered to be equivalent and are treated as such in this assessment. In general,human sound perception is such that a change in sound level of 3 dB is just noticeable,a change of 5 dB is clearly noticeable,and a change of 10 dB is perceived as doubling or halving sound level. Existing Conditions Land uses in the vicinity of the NCRC&SL are primarily agricultural. The existing noise environment in the immediate area is dominated by noise from various stationary and mobile sources. These include agricultural activities on the adjacent agricultural lands,vehicle travel over public and private roads, occasional aircraft overflights, and the operation of stationary and mobile noise sources associated with landfill operations. Based on the rural nature of the project area,noise levels in the vicinity of the project area are expected to be between 40 and 50 dBA La,,. Notice of Preparation for the June 2006 North County Recycling Center and Sanitary Landfill 2-60 Permit Revision AS 06307.06