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*:/:I1-3Ill WA <br />FUNDAMENTALS OF NOISE <br />Day -Night Sound Level (Ld.. or DNL). The energy average of the A -weighted sound levels occurring <br />during a 24-hour period, with 10 dB added to the sound levels occurring during the period from 10:00 <br />PM to 7:00 AM. <br />Community Noise Equivalent Level (CNEL). The energy average of the A -weighted sound levels <br />occurring during a 24-hour period, with 5 dB added from 7:00 PM to 10:00 PM and 10 dB from 10:00 <br />PM to 7:00 AM. NOTE: For general community/environmental noise, CNEL and Ldn values rarely differ <br />by more than 1 dB (with the CNEL being only slightly more restrictive — that is, higher than the Ld„ <br />value). As a matter of practice, Ld� and CNEL values are interchangeable and are treated as equivalent in <br />this assessment. <br />■ Sensitive Receptor. Noise- and vibration -sensitive receptors include land uses where quiet environments <br />are necessary for enjoyment and public health and safety. Residences, schools, motels and hotels, libraries, <br />religious institutions, hospitals, and nursing homes are examples. <br />Characteristics of Sound <br />When an object vibrates, it radiates part of its energy in the form of a pressure wave. Sound is that pressure <br />wave transmitted through the air. Technically, airborne sound is a rapid fluctuation or oscillation of air <br />pressure above and below atmospheric pressure that creates sound waves. <br />Sound can be described in terms of amplitude (loudness), frequency (pitch), or duration (time). Loudness or <br />amplitude is measured in dB, frequency or pitch is measured in Hertz [Hz] or cycles per second, and duration <br />or time variations is measured in seconds or minutes. <br />AVNude <br />Unlike linear units such as inches or pounds, decibels are measured on a logarithmic scale. Because of the <br />physical characteristics of noise transmission and perception, the relative loudness of sound does not closely <br />match the actual amounts of sound energy. Table 1 presents the subjective effect of changes in sound <br />pressure levels. Ambient sounds generally range from 30 dBA (very quiet) to 100 dBA (very loud). Changes <br />of 1 to 3 dB are detectable under quiet, controlled conditions, and changes of less than 1 dB are usually not <br />discernible (even under ideal conditions). A 3 dB change in noise levels is considered the minimum change <br />that is detectable with human hearing in outside environments. A change of 5 dB is readily discernible to <br />most people in an exterior environment, and a 10 dB change is perceived as a doubling (or halving) of the <br />sound. <br />Table 1 Noise Perceptibility <br />Change in dB <br />Noise Level <br />±3 dB <br />Threshold of human perceptibility <br />± 5 dB <br />Clearly noticeable change in noise level <br />± 10 dB <br />Half or twice as loud <br />± 20 dB <br />Much quieter or louder <br />Page 2 <br />PlaaWorkr <br />75C-221 <br />