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4 <br />TABLE 1 Definition of Acoustical Terms Used in this Report <br />Term Definition <br />Decibel, dB A unit describing, the amplitude of sound, equal to 20 times the logarithm to the <br />base 10 of the ratio of the pressure of the sound measured to the reference <br />pressure. The reference pressure for air is 20 micro Pascals. <br />Sound Pressure Level Sound pressure is the sound force per unit area, usually expressed in micro <br />Pascals (or 20 micro Newtons per square meter), where 1 Pascal is the pressure <br />resulting from a force of 1 Newton exerted over an area of 1 square meter. The <br />sound pressure level is expressed in decibels as 20 times the logarithm to the base <br />10 of the ratio between the pressures exerted by the sound to a reference sound <br />pressure (e. g., 20 micro Pascals). Sound pressure level is the quantity that is <br />directly measured by a sound level meter. <br />Frequency, Hz The number of complete pressure fluctuations per second above and below <br />atmospheric pressure. Normal human hearing is between 20 Hz and 20,000 Hz. <br />Infrasonic sound are below 20 Hz and Ultrasonic sounds are above 20,000 Hz. <br />A-Weighted Sound <br />Level, dBA <br />The sound pressure level in decibels as measured on a sound level meter using the <br />A-weighting filter network. The A-weighting filter de-emphasizes the very low <br />and very high frequency components of the sound in a manner similar to the <br />frequency response of the human ear and correlates well with subjective reactions <br />to noise. <br />Equivalent Noise Level, <br />Leq <br />The average A-weighted noise level during the measurement period. <br />Lmax, Lmin The maximum and minimum A-weighted noise level during the measurement <br />period. <br />L01, L10, L50, L90 The A-weighted noise levels that are exceeded 1%, 10%, 50%, and 90% of the <br />time during the measurement period. <br />Day/Night Noise Level, <br />Ldn or DNL <br />The average A-weighted noise level during a 24-hour day, obtained after addition <br />of 10 decibels to levels measured in the night between 10:00 p.m. and 7:00 a.m. <br />Community Noise <br />Equivalent Level, <br />CNEL <br />The average A-weighted noise level during a 24-hour day, obtained after addition <br />of 5 decibels in the evening from 7:00 p.m.to 10:00 p.m. and after addition of 10 <br />decibels to sound levels measured in the night between 10:00 p.m. and 7:00 a.m. <br />Ambient Noise Level The composite of noise from all sources near and far. The normal or existing <br />level of environmental noise at a given location. <br />Intrusive That noise which intrudes over and above the existing ambient noise at a given <br />location. The relative intrusiveness of a sound depends upon its amplitude, <br />duration, frequency, and time of occurrence and tonal or informational content as <br />well as the prevailing ambient noise level. <br />Source: Handbook of Acoustical Measurements and Noise Control, Harris, 1998. <br />8.A. - Page 120