Agenda - 11-22-2004-c1 - Laserfiche WebLink

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18 Attachment 3 Environmental Sound Page 3 Outdoor Sound Propaeation In the simplest idealizedmodel, the sound leveloutdoors decreases 6 decibels foreach doubling ofdistance from the source. The farther one is from a sound source, the farther away one must go to notice a significant decrease in sound level. In the real world, the sound level often decreases more rapidly with distance. This is due to bazrieis blocking the line of'sight, absorption by the air, winds, and temperature vaziations in the atmosphere, The atmospheric effects become most important at distances beyond about 1000 feetfrom the sowce. Dry air absorbs sound and reduces its level rapidly over distance.. The normally humid environment in the southeastern U5 allows sound to travel further with less reduction in level. Downwind and under many night-time conditions (cooler air near the surface), sound waves that start upwazd will bend downward. Thus, the noise reduction benefits of barriers can be negated by these atrnospheric effects, Levels will be lower upwind and under most daytime conditions. Sound waves curve upwazd in the face of a wind or towazds cooler air higher in the atmosphere. For the same sound output at the source, distant levels can be over 20 decibels less than predicted by the distance effects alone, Sound Measurement The basic instrument for sound measurement is the sound levelmeter. This is a microphone connected to a voltmeter with some additional features, The microphone produces an electrical voltage signal proportional to the sound pressure variation, and the voltmeter is calibrated to read the sound level in decibels, Most meters have "fast" and "slow" responses, These aze effectively moving averages of the sound leveh The fast response corresponds more closely to the actual fluctuating sound heazd, with the needle sometimes moving very quickly or digital display changing rapidly, However, the slow response provides a slowly changing indication that is easier to read. It averages the sound over about a second, and commonly used for environmental sound measurements, The fast and slow response both introduce some averaging into the measured result Some meters have a special "impulse" response that is used primarily in Europe. VJhea a sound occurs very rapidly, such as a gunshot, 8 is sometimes desirable to measure the peak pressure without any averaging, This is called the "peak" level. It is much higher than the maximum level that might be measured with either the fast, slow, or impulse response, It 's often useful to know the level of'the sound at different frequencies over the audible range, An octave-band analyzer is most commonly used for this, It divides the audible frequency range into octave bands (similar to musical octaves) so sound energy in each band can be measured, These bands are identified by the center frequency of each band, The center frequency doubles for each higher frequency band. Octave-band measurements can be A-weighted for a better indication of the contribution of each octave to the A-weighted ]eveh However, they are not normally A-weighted ifused for comparison with criteria based upon unweighted data, For diagnosing the source of sound, a finer resolution of frequencies