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Community Noise Fundamentals

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Community Noise Fundamentals D-1 Appendix D Community Noise Fundamentals BACKGROUND FREQUENCY WEIGHTING Three aspects of community noise are Many rating methods exist to analyze important in determining subjective response: sound of different spectra. The simplest method is generally used so that measure- 1) Level (i.e., magnitude or loudness) of ments may be made and noise impacts read- the sound. ily assessed using basic acoustical instrumen- tation. This method evaluates all frequencies 2) The frequency composition or spec- trum of the sound. by using a single weighting filter that pro- gressively de-emphasizes frequency compo- 3) The variation in sound level with time. nents below 1000 Hz and above 5000 Hz. This frequency weighting reflects the rela- Airborne sound is a rapid fluctuation tive decreased human sensitivity to low fre- of air pressure and local air velocity. Sound quencies and to extreme high frequencies. levels are measured and expressed in decibels This weighting is called A-weighting and is (dB) with 0 dB roughly equal to the thresh- applied by an electrical filter in all U.S. and old of hearing. international standard sound level meters. The frequency of a sound is a measure of the pressure fluctuations per second mea- NOISE EXPOSURE sured in units of hertz (Hz). Most sounds Noise exposure is a measure of noise do not consist of a single frequency, but over a period of time, whereas noise level is a are comprised of a broad band of frequen- single value at an instant in time. Although cies differing in level. The characterization a single sound level may adequately describe of sound level magnitude with respect to community noise at any instant in time, frequency is the sound spectrum. A sound community noise levels vary continuously. spectrum is often described in octave bands Most community noise is produced by many that divide the audible human frequency distant noise sources that produce a relative- range (i.e., from 20 to 20,000 Hz) into ten ly steady background noise having no iden- segments. Figure 6-A shows a range of sound tifiable source. These distant sources change spectra for various types of sound over the gradually throughout the day and include audible hearing range. traffic, wind in trees, and distant industrial activities. Superimposed on this slowly vary- ing background is a succession of identifiable noise events of brief duration. These include City of Cupertino General Plan D-2 Technical Appendix D: Community Noise Fundamentals nearby activities such as single vehicle pass- SUBJECTIVE RESPONSE TO bys or aircraft flyovers, which cause the com- NOISE munity noise level to vary from instant to The effects of noise on people can be instant. classified into three general categories: A single number called the equivalent • Subjective effects of annoyance, nui- sound level or Leq is used to describe noise sance, dissatisfaction. varying over a period of time. The Leq is the average noise exposure level over a period • Interference with activities such as of time (i.e., the total sound energy divided speech, sleep, and learning. by the duration). It is the constant sound level, which would contain the same acous- • Physiological effects such as anxiety or tic energy as the varying sound level, during hearing loss. the same time period. The Leq is useful in describing noise over a period of time with a The sound levels associated with com- single numerical value. munity noise usually produce effects only in the first two categories. No universal mea- In determining the daily measure of sure for the subjective effects of noise has community noise, it is important to account been developed, nor does a measure exist for the difference in human response to day- for the corresponding human reactions from time and nighttime noise. During the night- noise annoyance. This is primarily due to the time, exterior background noise levels are wide variation in individual attitude regard- generally lower than in the daytime. Most ing the noise source(s). household noise also decreases at night, and exterior noise intrusions become more An important factor in assessing a per- noticeable. People are more sensitive to son’s subjective reaction is to compare the noise at night than during other periods of new noise environment to the existing noise the day. environment. In general, the more a new noise exceeds the existing, the less accept- To account for human sensitivity to able it is. Therefore, a new noise source will nighttime noise, the Community Noise be judged more annoying in a quiet area than Equivalent Level (CNEL) is the adopted it would be in a noisier location. standard in California. CNEL values are typically computed by energy summation of Knowledge of the following relation- hourly noise level values, with the proper ships is helpful in understanding how changes adjustment applied for the period of evening in noise and noise exposure are perceived. or night. The CNEL is computed by assess- ing a 5-dB penalty for evening (i.e., 7:00 pm • Except under special conditions, a to 10:00 pm) noise and a 10-dB penalty for change in sound level of 1 dB cannot nighttime (i.e., 10:00 pm to 7:00 am) noise. be perceived. Noise exposure measures such as Leq and CNEL are A-weighted, with units expressed • Outside of the laboratory, a 3-dB in decibels (i.e., dB). change is con sidered a just-noticeable difference. City of Cupertino General Plan Noise Monitoring Summary D-3 • A change in level of at least 5 dB is What follows is a summary of each required before any noticeable change meter’s location and measurement details. in community response would be expected. Meter #1 (E) • A 10-dB change is subjectively heard The first meter was placed along as an approx imate doubling in loud- Stevens Creek Boulevard, on the north side ness and almost always causes an of the street between North Wolfe Road and adverse community response. Finch Ave., adjacent to the Vallco Financial Center SW. The meter was approximately 60 feet from the center of Stevens Creek NOISE MONITORING SUMMARY Boulevard, the primary noise source. The Vallco Fashion Park is located just northwest Overview of this monitoring location. Existing noise conditions in Cupertino were measured at six locations for twenty-four Meter #2 (A) hours. These measurements commenced on The second meter was placed along the December 18, 2001 and ended on December south side of Stevens Creek Boulevard just 19, 2001. The six monitoring locations were east of De Anza Boulevard, a commercial chosen by the City of Cupertino. area. The meter was sited on a telephone pole adjacent to the Symantec Building. The weather conditions during the There was construction occurring on the measurement period are summarized in south side of Stevens Creek Boulevard at Table D-1. De Anza. Stevens Creek Blvd. contains six lanes of traffic at the monitoring location, and was the primary noise source. Table D-1. Date High Temp. Mean Temp. Low Temp. Winds Conditions December 18 55° 50° 45° 0 – 5 mph Clear December 19 57° 50° 43° 5 – 15 mph Cloudy City of Cupertino General Plan D-4 Technical Appendix D: Community Noise Fundamentals Meter #3 (A2) Meter #5 (C) The third meter was placed along South The fifth meter was placed along the Stelling Road at Tomki Court, just north of west side of Foothill Boulevard just north of the Highway 85 overpass. The meter was Silver Oak Way. At the measurement loca- positioned on a telephone pole at a height of tion there are six lanes of traffic (three in eight feet, with the pole being located five feet either direction), with residences flanking from the roadway. The primary noise source both sides of Foothill. There were a large is traffic along South Stelling with secondary number of trucks traveling to and from the noise sources being Highway 85 and the pub- quarry located further south on Foothill. lic park across the street. There were approximately five trucks per minute in both directions counted at 12 pm. Meter #4 (B) The primary noise source was the traffic on Foothill Blvd. The fourth meter was placed along the west side of North Stelling Road between Highway 280 and Homestead Road. This Meter #6 (H) area is primarily commercial, with a shop- ping center across the street and a church The sixth meter was placed along the and some residences on the west side of south side of Bollinger Road just west of Stelling Rd. The meter was located on a Miller Avenue. This area is residential yet telephone pole at a height of eight feet. The Bollinger can be considered a major street. primary noise source was vehicular traffic The primary noise source was traffic along along North Stelling. Bollinger Road. HOMESTEAD ROAD 4 280 85 STELLING RD De ANZA BLVD WOLFE RD 5 BLVD FOOTHILL STEVENS CREEK BLVD 1 2 E V A AVE U McCLELLAN A T ROAD N A BLANEY AVE T MILLER BOLLINGER RD 3 6 BUBB ROAD Meter DNL CNEL RAINBOW DRIVE #1 68 dB 69 dB #2 71 dB 72 dB PROSPECT ROAD #3 73 dB 73 dB #4 72 dB 72 dB #5 76 dB 76 dB #6 72 dB 73 dB City of Cupertino General Plan Noise Monitoring Summary D-5 Hourly Noise Level Table D-2 is a summary by hour of the noise levels measured at each location. Table D-2. Hour Location 1 Location 2 Location 3 Location 4 Location 5 Location 6 Stevens Creek Stevens Creek S. Stelling Stelling Foothill Blvd. Bollinger Blvd. at Vallco Blvd.
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