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FULL PACKET_2007-07-16
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FULL PACKET_2007-07-16
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1/3/2012 4:38:39 PM
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7/11/2007 1:50:30 PM
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City Clerk
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Agenda Packet
Date
7/16/2007
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2012
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undeveloped areas, open space, and vegetated areas reduce noise at a rate of 4.S dBA per doubling of the distance. <br />These represe~ the extremes and most areas will actually contain a combination of hard and soft elemems with the <br />noise reduction placed somewhere in between these two factors. Unfortunately, the only way to actually determine <br />the absolute amount of attenuation that an area provides is through field measurement under operating conditions <br />with subsequent noise level measuremerrts conducted at varying distances from a constant noise source. (Note that <br />The Cou~y of Orange General Plan Noise Element uses the 4.S dBA reduction for all traffic noise impacts J <br />Objects that block. the line-of-sight attenuate the noise source if the receptor is low within the "shadow" of the <br />blockage (such as behind a sound wall). If a receptor is located behind the wall, but has a view of the source, the <br />wall will do liWe to reduce the noise. Additionally, a receptor located on the same side of the wall as the noise <br />source may experience an increase in the perceived noise level, as the wall may reflect noise back to the receptor <br />compounding the noise. <br />Several rating scales (or noise "metrics' exist to analyze adverse effects of noise, includingtraffic-generated noise, <br />on a community. These scales include the equivalent noise level (Leq), the community noise equivalem level <br />(CNEL), and the day/night noise level (Lda). Leq is a measureme~ of the sound energy level averaged over a <br />specified time period (usually 1-hour). Leq represems the amount of variable sound energy received by a receptor <br />over a time interval itl a single numerical value. For example, a 1 hour Leq noise level measuremem represents the <br />average amount of acoustic energy that occurred in that hour. <br />Unlike the Leq metric, the CNEL noise metric is based on 24 hours of measurement. CNEL also differs from Leq <br />in that it applies atime-weighted factor desigaed to emphasize noise events that occur during the evening and <br />nighttime hours (when quiet time and sleep disturbance is of particular concern). Noise occurring during the <br />daytime period (7:00 a.m. to 7:00 p.m.) receives no penalty. Noise produced during the evening time period (7:00 <br />p.m. to 10:00 p.m.) is penalized by 5 dBA, while nighttime noise (10:00 p.m. to 7:00 a.mJ is penalized by 10 dBA. <br />The Ldn noise metric is similar to the CNEL metric except that the period from 7:00 p.m. to 10:00 p.m. receives no <br />penalty. Both the CNEL and Ldn metrics yield approximately the same 24-hour value (within O.S dBA) with the <br />CNEL being the more restrictive (i.e., higher) of the two. <br />2.2 Vibration Fundamentals <br />Vibration is a trembling, quivering, or oscillating motion of the earth. Like noise, vibration is transmitted in waves, <br />but in this case through the earth or solid objects. Unlike noise, vibration is typically of a frequency that is felt <br />rather than heard. <br />Vibration can be either natural as in the form of earthquakes, volcanic eruptions, sea waves, landslides, etc., or <br />man-made as from explosions, the action of heary machinery, or heary vehicles such as tnteks or trains. Both <br />natural and man-matte vibration may 6e continuous such as from operating machinery, or transient as from an <br />explosion. <br />As with noise, vibration can be described by both its amplitude and frequency. Amplitude may be characterized in <br />three ways including displacement, velocity, and acceleration. Particle displacemem is a measure of fire distance <br />that a vibrated particle travels from its original position and for the purposes of soil displacemem is typically <br />measured in inches or millimeters, Particle velocity is the rate of speed at which soil particles move in inches per <br />second or millimeters per second. Particle acceleration is the rate of change in velocity with respect to time and is <br />measured in inches per second per second or millimeters per second per second. Typicatiy, particle velocity <br />(measured in inches or millimeters per second) and/or acceleration (measured in gravities) are used to describe <br />vibration. Table 1 presents the human reaction and effect on buildings to various levels of aontirtuous vibration. <br />Vibrations also vary in frequency and this affects perception. Typical construction vibrations fall in the 10 to 30 Hz <br />range and usually occur around 15 Hz. Traffic vibrations exhibit a similar range of frequencies. However, due to <br />their suspension systems, city buses often generate frequencies azound 3 Hz at high vehicle speeds. It is more <br />unccmmon, but possible, to measure traffic frequencies above 30 Hz. <br />2 <br />75D-122 <br />
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