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<br /> <br /> <br />Memorandum <br />February 26, 2025 <br />Page 2 <br />21202-23 North County Landfill_Noise DRAFT <br />7:00 am to allow the acceptance of commercial waste during this time period. The project <br />would not change the North County Landfill’s capacity and would not involve new construction. <br />NOISE AND VIBRATION CONCEPTS <br />Noise Concepts and Terminology <br />Noise is commonly defined as unwanted sound that annoys or disturbs people and can have an <br />adverse psychological or physiological effect on human health. Sound is measured in decibels <br />(dB), which is a logarithmic scale. Decibels describe the purely physical intensity of sound based <br />on changes in air pressure, but they cannot accurately describe sound as perceived by the <br />human ear since the human ear is only capable of hearing sound within a limited frequency <br />range. For this reason, a frequency-dependent weighting system is used, and monitoring results <br />are reported in A-weighted decibels (dBA). Decibels and other acoustical terms are defined in <br />Table 1. <br />Table 1. Definition of Acoustical Terms <br />Term Definition <br />Frequency (Hz) The number of complete pressure fluctuations per second above and below <br />atmospheric pressure. <br />Decibel (dB) <br />A unit describing the amplitude of sound on a logarithmic scale. Sound <br />described in decibels is usually referred to as sound or noise “level.” This unit <br />is not used in this analysis because it includes frequencies that the human <br />ear cannot detect. <br />A-Weighted Sound <br />Level (dBA) <br />The sound pressure level in decibels as measured on a sound level meter <br />using the A-weighting filter network. The A-weighting filter de-emphasizes <br />the very low and very high frequency components of the sound, in a manner <br />similar to the frequency response of the human ear, and correlates well with <br />subjective reactions to noise. All sound levels in this report are A-weighted. <br />Maximum Sound Levels <br />(Lmax) The maximum sound level measured during a given measurement period. <br />Exceedance Level (Ln) The A-weighted noise levels that are exceeded 10%, 50%, and 90% (L10, L50, <br />L90, respectively) of the time during the measurement. <br />Equivalent Noise <br />Level (Leq) <br />The average A-weighted noise level during the measurement period. For this <br />evaluation, Leq refers to a 1-hour period unless otherwise stated. <br />Day/Night Noise <br />Level (Ldn) <br />The average A-weighted noise level during a 24-hour day, obtained after <br />addition of 10 decibels to sound levels during the night between 10:00 p.m. <br />and 7:00 a.m.