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Administrative Draft Environmental Impact Report Gill Medical Center Project Table 4.15-1.Common Acoustical Descriptors Descriptor Definition Decibel,dB A unit describing the amplitude of sound,equal to 20 times the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure.The reference pressure for air is 20. Sound Pressure Level Sound pressure is the sound force per unit area,usually expressed in micropascals(or 20 micronewtons per square meter),where 1 pascal is the pressure resulting from a force of 1 newton exerted over an area of 1 square meter.The sound pressure level is expressed in decibels as 20 times the logarithm to the base 10 of the ratio between the pressures exerted by the sound to a reference sound pressure(e.g.,20 micropascals).Sound pressure level is the quantity that is directly measured by a sound level meter. Frequency,Hz The number of complete pressure fluctuations per second above and below atmospheric pressure. Normal human hearing is between 20 Hz and 20,000 Hz. Infrasonic sound are below 20 Hz and ultrasonic sounds are above 20,000 Hz. A-Weighted Sound Level,dBA The sound pressure level in decibels as measured on a sound level meter using the A weighting filter network.The A-weighting filter de-emphasizes the very low and very high frequency components of the sound in a manner similar to the frequency response of the human ear and correlates well with subjective reactions to noise. Equivalent Noise Level,Leq The average acoustic energy content of noise for a stated period of time.Thus,the Leq of a time- varying noise and that of a steady noise are the same if they deliver the same acoustic energy to the ear during exposure. For evaluating community impacts,this rating scale does not vary, regardless of whether the noise occurs during the day or the night. Lmax,Lmin The maximum and minimum A-weighted noise level during the measurement period. Lo,,L,o,L5o,Lso The A-weighted noise levels that are exceeded 1%, 10%,50%,and 90%of the time during the measurement period. Day/Night Noise Level,Lan or A 24-hour average Leq with a 10 dBA"weighting"added to noise during the hours of 10:00 p.m.to DNL 7:00 a.m.to account for noise sensitivity in the nighttime.The logarithmic effect of these additions is that a 60 dBA 24-hour Leq would result in a measurement of 66.4 dBA Ldn. Community Noise Equivalent A 24-hour average Leq with a 5 dBA"weighting"during the hours of 7:00 p.m.to 10:00 p.m.and a Level,CNEL 10 dBA"weighting"added to noise during the hours of 10:00 p.m.to 7:00 a.m.to account for noise sensitivity in the evening and nighttime,respectively.The logarithmic effect of these additions is that a 60 dBA 24-hour Leq would result in a measurement of 66.7 dBA CNEL. Ambient Noise Level The composite of noise from all sources near and far.The normal or existing level of environmental noise at a given location. Intrusive That noise which intrudes over and above the existing ambient noise at a given location.The relative intrusiveness of a sound depends on its amplitude,duration,frequency,and time of occurrence and tonal or informational content as well as the prevailing ambient noise level. Decibel,dB A unit describing the amplitude of sound,equal to 20 times the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure.The reference pressure for air is 20. Noise 4.15-4 October 2021