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HomeMy WebLinkAboutCT 79-04; RANCHO LA QUESTA; NOISE EVALUATION; 1981-04-24-• HILLIARD & BRICKEN-----81 - 13 - 57 ---.... John HIiiiard • Consulting Acoustl~al & Energy Engineers • Gordon Brlcken April 24, l98l Mr. Bob. McClaire Ayres Construction Company Drawer A Huntington Beach, California 92648 Subject: Noise Evaluation, Tract 7~-4, in the City of Carlsbad~ Dear Mr. McClaire: At your request we have evaluated the noise conditions in Tract 79.--4 .. The s·ite is· located along El Camino Real. Lots 38 and 39 are ·adjacent to El Camino Real as noted in Exhibit l. Design Nois·e Cotid:itions· The design noise conditions· is based on a ten (10) year design year. There are no forecasts available so it is necessary to construct one Based on an evaluation of historic growth trends. The basic data is listed in table l. Table l Traffic Volume on El Camino Real 1976 1978 1980 8300 9188 l4,650 The average four year growth is 20% per year. It is unlikely this will be sustained. A reasonable average growth rate is l0-l2% a year. On this basis in the ten (.10). year range would be 38,943 to 49,730 ADT. A Design value of 40,000 ADT was selected for the study. / 1538 East Fourth Street D Franklin Square D Santa Ana, California 92701 D (714) 835-3728 Bob McClaire A Ayres Construc~n April., 1981 Page~ This is also the capacity of the street at "C" level service. With a 40,000 ADT, 45 mph average speed and estimate 4% trucks, the reference noise level is 73.5 CNEL 50' for CNEL. Applicable Noise Criteria The City of Carlsbad adheres to the requirements of Table 2. Table 2 (.J. l Applicable Noise Criteria Exterior Interior 65 CNEL 45 CNEL Cl}_ See Noise Rating Methods, Application Note #6 for explanation of acoustical terminology. Mitigation Exterior The s.i.te i.s approximately 30.1• below street grade as. noted on· Exhibi.t 1. There is shelding from the slope "i.n the order of J.O -. 12 dB. The schedule noi~e le~el at· 6• along the site grade will be 55 CNEL. This· is below-the 65 CNEL limit. No furture mitigation i's requi'red. 'Interior Buildings of standard construction yield 20 dBA noise reduction with closed windows and 10 dBA with open windows. Design interior values will be 35 CNEL with closed windows and 45 CNEL with open windows. Even the open case conforms to the crite • ~-......_ ditional mitigation is required. Prepared by: ~,)_ < 1/' (7 Gordo~~ --..--. C:,;-:: Yi.ce President , r /kmt Enclosures: Exhibit l Application Note #6 .. --··· .- JOOLIED ~- \ \ \ '\ \ I I \ BP..OW D\ lt: 1-t DEBRI'? FW:K' PER '?TD. DVQ. @3\ -· . q_ . Exhibit J.·. - TRACT 79-4 Carlsbad Newport Shores Builders --· ---- \ .·,~ I \ \ \ \ \ '\ \ ·, I~\\-\\ \~'\') ,- '. \ \ .\ '_,___,....--: ,: \ \ \ \~/ \ \ ~}} ... a;)'· t't) .., \ ~ I ' "> ;:.f>, "';,,,(,• ,,;( .. I I I -APieCATION NOTE .. #6 HILLIARD & BRICKEN---------- John HIiiiard • Consulting Acoustical Engineers • Gordon Brlcken NOISE RATING METHODS The A-weighted decibel of "A" scale on the sound level meter is most often used in .the measurement of noise because the weighting characteristics of this scale approximates the subjective response of the human ear to a broad frequency band noise source by discriminating against the very low and the very high frequencies of the audible spectrum. Since community noise is seldom constant,.varying from moment to moment and throughout the day, the "A" weighted noise level needs to be further described to provide meaningful data. The Environmental Protection Agency, Federal Department of Transportation, foreign countries and private consultants are now using three time-exceeded percentile figures to describe noise: 1. L90 is the noise level which is exceeded 90% of any sample time period (such as 24 hours) and is used to describe the background or ambient noise level. 2. Lso is the noise level which is exceeded 50% of the time. It is the median level. 3. LlO is the noise level which is exceeded 10% of the time and is a good descriptor of fluctuating noise sources such as vehicular traffic. It indicates the near-maximum levels which occur from grouped single events. Being related to the subjective annoyance to community noise, it is a good design tool in the planning of acoustical barriers. More recent noise assessment methods are ·based on the equivalent energy concept where Leq !x) represents the average energy content of a fluctuating noise source over a sample period. The subscript (x) represents the period in which the energy is computed and measured. Current practice references the quantity to either one hour, 8 hours or 24 hours. When referenced to ·one hour, Leg is also sometimes called HNL (Hourly Noise Level). 1538 East Fourth Street □ Franklin Square □ Santa Ana, California 92701 □ (714) 835-3728 1 ,.. NOISE RATING METHODS PAGE 2 - Since Le is the summation of the functional products of noise leiel and duration, many combinations of noise level,. duration time and time history. can make up the same L value. Thus, an L (24) equals 50 means only that tff~ aver~ge noise levelqis 50 dB.· During the 24 hour period there can be times when the noise level is higher than 50 dB and many times when it is lower. If the period of the raeasurement is only a single event, the energy content is not averaged. The energy expression for a single event is simply the sum of the functional product of the noise level and duration. time of the event .. This term ~s called Le or SENEL {Single Event Noise Exposure Level). The sum.~ation of L values averaged over one hour is Leg(l) -Leg(B) and Leq -~24 ), etc.). Leq is further refined into Ldn (Level Day-Night) and CNEL (Community Noise Equivalent Level) where noises that occur during certain hours of the day are weighted (or penalized) because they-are considered subjectively more annoying during these time ~eriods: 1. Ldn is the sound level in dBA which corresponds to the average eriergy content of the noise being • measured over a 24-hour period including a 10 dBA weighting penalty for sound levels which occur during the nighttime hours of 10PM to 7AM. This is a rating method recommended by the Environmental Protection Agency because it takes into account those subjectively more annoying noise events which occur during the sleeping hours. 2. CNEL is the sound level in dBA.which corresponds to the average energy content of the noise being measured over a 24-hour period including a 3 dBA weighting penalty for noises that occur during the evening hours of 7PM to l0Pm, plus a 10 dBA penalty for noise events that occur during the nighttime hours of 10PM to 7AM. For typical highway vehicular traffic situations, computer analysis has shown that CNEL and Ldn co~relate within 0.5 dBA. The percentile figures L10 , L50 and L90 can be·directly scaled from a graphical recoraing of the Measured noise over a particular time period. They are also convenient_ to implement in automatic measurement equipment .. Energy parameters Le, Leq, La and CNEL require expensive and complicated equipment ~hat has only recently become available on the commercial market. As a result, engineers have devised ways of estimating Leq (and hence Lan) using standard instrumentation and methods.