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*:/ : 11-1 h WA <br />Fundamentals of Noise <br />ki101 M : <br />Noise is most often defined as unwanted sound; whether it is loud, unpleasant, unexpected, or otherwise <br />undesirable. Although sound can be easily measured, the perception of noise and the physical response to <br />sound complicate the analysis of its impact on people. People judge the relative magnitude of sound sensation <br />in subjective terms such as "noisiness" or "loudness." <br />Noise Descriptors <br />The following are brief definitions of terminology used in this chapter: <br />■ Sound. A disturbance created by a vibrating object, which, when transmitted by pressure waves through <br />a medium such as air, is capable of being detected by a receiving mechanism, such as the human ear or a <br />microphone. <br />■ Noise. Sound that is loud, unpleasant, unexpected, or otherwise undesirable. <br />■ Decibel (dB). A unitless measure of sound, expressed on a logarithmic scale and with respect to a <br />defined reference sound pressure. The standard reference pressure is 20 micropascals (20 µPa). <br />■ Vibration Decibel (VdB). A unitless measure of vibration, expressed on a logarithmic scale and with <br />respect to a defined reference vibration velocity. In the U.S., the standard reference velocity is 1 micro - <br />inch per second (1x10-6 in/sec). <br />■ A -Weighted Decibel (dBA). An overall frequencyweightedsound level in decibels that approximates <br />the frequency response of the human ear. <br />■ Equivalent Continuous Noise Level (IQ; also called the Energy -Equivalent Noise Level. The <br />value of an equivalent, steady sound level which, in a stated time period (often over an hour) and at a <br />stated location, has the same A -weighted sound energy as the timevaryingsound. Thus, the Ley metric is <br />a single numerical value that represents the equivalent amount of variable sound energy received by a <br />receptor over the specified duration. <br />■ Statistical Sound Level (L„). The sound level that is exceeded "n" percent of time during a given <br />sample period. For example, the Lso level is the statistical indicator of the timevaryingnoise signal that is <br />exceeded 50 percent of the time (during each sampling period); that is, half of the sampling time, the <br />changing noise levels are above this value and half of the time they are below it. This is called the <br />"median sound level." The Lio level, likewise, is the value that is exceeded 10 percent of the time (i.e., <br />near the maximum) and this is often known as the "intrusive sound level." The Lgo is the sound level <br />exceeded 90 percent of the time and is often considered the "effective background level" or "residual <br />noise level." <br />75C-220 <br />