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period of time.A noise level is a measure of noise at a given instant in time.The noise levels presented in Figure
<br /> 4.13-1 are representative of measured noise at a given instant in time;however,they rarely persist consistently
<br /> over a long period.Rather,community noise varies continuously over time with respect to the contributing sound
<br /> sources of the community noise environment. Community noise is primarily the product of many distant noise
<br /> sources,which constitute a relatively stable background noise exposure,with the individual contributors
<br /> unidentifiable.The background noise level changes throughout a typical day,but does so gradually,
<br /> corresponding with the addition and subtraction of distant noise sources, such as traffic and atmospheric
<br /> conditions.What makes community noise constantly variable throughout a day,besides the slowly changing
<br /> background noise,is the addition of short-duration single-event noise sources(e.g., aircraft flyovers,motor
<br /> vehicles, sirens),which are readily identifiable to the individual.The most frequently used noise descriptors for
<br /> characterizing the community noise environment are summarized below:
<br /> ► L.,,(Maximum Noise Level):The maximum instantaneous noise level during a specific period of time.The
<br /> I.may also be referred to as the"peak(noise)level."
<br /> ` ► L.(Minimum Noise Level):The minimum instantaneous noise level during a specific period of time.
<br /> ► L.(Equivalent Noise Level): The energy mean noise level. The instantaneous noise levels during a specific
<br /> period of time in dBA are converted to relative energy values.From the sum of the relative energy values,an
<br /> average energy value is calculated,which is then converted back to dBA to determine the Leq.
<br /> ► Ld.(Day—Night Noise Level): The 24-hour L�q with a 10 dBA"penalty"for the noise-sensitive hours from
<br /> 10:00 p.m.to 7:00 a.m.The Ld.attempts to account for the fact that noise during this specific period is a
<br /> potential source of disturbance with respect to normal sleeping hours.
<br /> ► CNEL(Community Noise Equivalent Level):The CNEL is similar to the Ld.described above,but with an
<br /> additional 4.77 dBA"penalty"for the noise-sensitive hours from 7:00 p.m.to 10:00 p.m.,which are typically
<br /> reserved for relaxation,conversation,reading,and television.If using the same 24-hour noise data,the CNEL
<br /> is typically—0.5 dBA higher than the Ldn.
<br /> ► SEL(Single Event[Impulsive]Noise Level):The SEL describes a receiver's cumulative noise exposure from
<br /> a single impulsive noise event,which is defined as an acoustical event of short duration(0.5 second),such as
<br /> a backup beeper,an airplane traveling overhead,or a train whistle,and involves a change in sound pressure
<br /> above a defined value(usually approximately 40 dBA).
<br /> Negative Effects of Noise on Humans
<br /> Negative effects of noise exposure include physical damage to the human auditory system,interference,and
<br /> disease.Exposure to noise may result in physical damage to the auditory system,which may lead to gradual or
<br /> traumatic hearing loss. Gradual hearing loss is because of sustained exposure to moderately high noise levels over
<br /> a period of time as opposed to traumatic hearing loss,which is because of sudden exposure to extremely high
<br /> noise levels over a short period.However, gradual and traumatic hearing loss may both result in permanent
<br /> hearing damage.In addition,noise may interfere with or interrupt sleep,relaxation,recreation,and
<br /> communication.Although most interference may be classified as annoying,the inability to hear a warning signal
<br /> y may be considered dangerous.Noise may also be a contributor to diseases associated with stress, such as
<br /> hypertension,anxiety,and heart disease.The degree to which noise contributes to such diseases depends on the
<br /> noise frequency,bandwidth, level, and exposure time(Caltrans 1998). As indicated in Figure 4.13-1,continuous
<br /> exposure to sound levels of 85 dB or more is likely to degrade the hearing of most people over time.
<br /> EXISTING NOISE-SENSITIVE LAND USES
<br /> Noise-sensitive land uses include those land uses where there is an expectation for quiet or where low noise levels
<br /> are integral for operation.Noise-sensitive land uses typically include residential,school,hospital, day care,
<br /> College Park at Mountain House Specific Plan III Draft EIR EDAW
<br /> San Joaquin County 4.13-3 Noise
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