75D - PH ZONING ORD AND CUP CALIFORNIA PALMS

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