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Glossary The following are definitions of terms either used in this report and its accompanying data printouts or are in common usage in environmental noise descriptions. They are presented in a conversational format in some instances to enhance the readers understanding. "A" Weighting - A mechanical means of adjusting the readout of a sound level measuring device to approximate the sound levels as the average, person would hear. This method decreases the lower frequencies contributions while maintaining the contri- butions of the middle frequencies. An "A" weighted level will be somewhat less than a "Linear" or true measurement. Ambient Noise Level - The noise level in an environment without the addition of intruding noise sources. Community Noise Equivalent Level (CNEL) - The average sound level over a twenty four hour period the evening and night hours weighted to simulate the more noticeable evening and night noise. This measure is similar to Ldn with an additional evening period from seven o'clock to ten o'clock weighted by a factor of three. This measure was initially adopted by the Military and the State of California as the noise standard for aviation in anticipation of it becoming a national standard. The Day Night Noise Level (Ldn) became the standard for every other state and for the Federal Aviation Administration leaving California and the Military as the only users of CNEL. Both have converted to Ldn as the standard measure. CNEL is sometimes called the California Noise Exposure Level. Day-Night Sound Level (Ldn) - The average sound level over a twenty four hour period with the night hours weighted to simulate the more noticeable night noise. To calculate Ldn, the HNL for " each hour from seven o'clock in the morning to ten o'clock in the evening is added to ten times the HNL from ten o'clock in the evening to seven o'clock in the morning. This total is divided by twenty four to determine the Ldn. since the measure of sound is logarithmic rather than linear, The addition and division of the noise levels must be done logarithmically. Decibel (dB) - The unit of measure for sound. This unit is peculiar in the sense that it is used both to indicate a level or loudness, and to indicate the amount of sound energy over a period of time. Both uses are stated as decibels (dB) . If the measure is weighted the written abbreviation is dBA or dBc. 8 '