Laserfiche WebLink
2 <br />SETTING <br />Fundamentals of Environmental Noise <br />Noise may be defined as unwanted sound. Noise is usually objectionable because it is disturbing or <br />annoying. The objectionable nature of sound could be caused by its pitch or its loudness. Pitch is <br />the height or depth of a tone or sound, depending on the relative rapidity (frequency) of the <br />vibrations by which it is produced. Higher pitched signals sound louder to humans than sounds with <br />a lower pitch. Loudness is intensity of sound waves combined with the reception characteristics of <br />the ear. Intensity may be compared with the height of an ocean wave in that it is a measure of the <br />amplitude of the sound wave. <br />In addition to the concepts of pitch and loudness, there are several noise measurement scales which <br />are used to describe noise in a particular location. A decibel (dB) is a unit of measurement which <br />indicates the relative amplitude of a sound. The zero on the decibel scale is based on the lowest <br />sound level that the healthy, unimpaired human ear can detect. Sound levels in decibels are <br />calculated on a logarithmic basis. An increase of 10 decibels represents a ten-fold increase in <br />acoustic energy, while 20 decibels is 100 times more intense, 30 decibels is 1,000 times more <br />intense, etc. There is a relationship between the subjective noisiness or loudness of a sound and its <br />intensity. Each 10 decibel increase in sound level is perceived as approximately a doubling of <br />loudness over a fairly wide range of intensities. Technical terms are defined in Table 1. <br />There are several methods of characterizing sound. The most common in California is the A- <br />weighted sound level (dBA). This scale gives greater weight to the frequencies of sound to which the <br />human ear is most sensitive. Representative outdoor and indoor noise levels in units of dBA are <br />shown in Table 2. Because sound levels can vary markedly over a short period of time, a method for <br />describing either the average character of the sound or the statistical behavior of the variations must <br />be utilized. Most commonly, environmental sounds are described in terms of an average level that <br />has the same acoustical energy as the summation of all the time-varying events. This energy- <br />equivalent sound/noise descriptor is called Leq. The most common averaging period is hourly, but <br />Leq can describe any series of noise events of arbitrary duration. <br />The scientific instrument used to measure noise is the sound level meter. Sound level meters can <br />accurately measure environmental noise levels to within about plus or minus 1 dBA. Various <br />computer models are used to predict environmental noise levels from sources, such as roadways and <br />airports. The accuracy of the predicted models depends upon the distance the receptor is from the <br />noise source. Close to the noise source, the models are accurate to within about plus or minus 1 to 2 <br />dBA. <br />Since the sensitivity to noise increases during the evening and at night -- because excessive noise <br />interferes with the ability to sleep -- 24-hour descriptors have been developed that incorporate <br />artificial noise penalties added to quiet-time noise events. The Community Noise Equivalent Level <br />(CNEL) is a measure of the cumulative noise exposure in a community, with a 5 dB penalty added <br />to evening (7:00 p.m. - 10:00 p.m.) and a 10 dB addition to nocturnal (10:00 p.m. - 7:00 a.m.) noise <br />8.A. - Page 118