Loading...
CC - Sound Study - 5.1910400 Overland Road #211 -- Boise, ID 83709 www.mullinsacoustics.com 208-514-6264 May 19, 2017 Scott Harrop, Executive Pastor Rockharbor Church 6437 Tree Haven Way Meridian, ID 83646 Subject: Noise Berm at Church Property project # 3720 Dear Pastor Harrop: You asked me to evaluate the City of Meridian’s requirement for a ten foot high noise berm along the Chinden Boulevard frontage of the church property. The city generally requires a ten foot high berm or barrier wall along major roadways for most noise-sensitive land uses. The apparent intent is to protect homes and backyards from excess traffic noise when they are built near major roadways. When exposed to significant traffic noise levels, both the outdoor use space (backyards) and indoor spaces in homes benefit from some shielding provided by a berm or noise barrier wall. The ordinance also states that, “…..The director may approve alternative compliance…. where the applicant has a substitute noise abatement proposal…...”. (ORD 10-1439, section D, paragraph 4) I have reviewed what was published on-line for ITD noise criteria as referenced in the Meridian ordinance. The key points are: • mitigation must provide at least 5 dB of noise reduction at protected properties • cost per benefitted receptor must be less than $24,500 • only necessary when increased exposure to traffic noise is greater than 3 dB Per ITD form 1057 Optional Non-Barrier Noise Abatement Checklist, “… insulation can be provided to achieve at least 5 dB of reduction inside the structure….”. This is a checklist (attached) that is used to decide whether noise mitigation measures are worthwhile and feasible. There is section on that form labeled “Reasonableness” which asks whether: • the cost of implementation meets the cost-per-benefitted residence criterion • the benefitted residents endorse the measurements • the measure achieves the design goal The barrier will likely give 5 dB of reduction at the church facade. However, noise reduction is not really needed and is better achieved indoors using other methods (windows). Noise reduction from the berm / barrier will certainly be less than 5 dB at the nearby homes due to geometry. Since the church is a single non-residential receiver MULLINS ACOUSTICS architectural environmental and industrial noise control and the barrier will cost far more than $25K, it fails the cost test. Any desired indoor noise reduction is easily achieved using closed windows and a mechanical ventilation system for noise-impacted sides of the building (south and east). In my professional opinion, a berm of this configuration and location would not provide meaningful protection from traffic noise for either the church building or for the nearest homes to the north located along N. Tree Farm Way. Such a berm or barrier would be an unnecessary and significant expense to the church while providing little meaningful acoustical benefit. Berms or noise barriers work best when located fairly close to either the noise source or the receiver location. The receiver should be located within the “shadow zone” of the barrier, well shielded from noise sources. When a berm / barrier is placed fairly far from either the source or the receiver, its effect quickly degrades. The nearest residential property line for homes to the north along N. Tree Farm Way are 785 feet minimum from the proposed berm centerline. Given this geometry, my calculations indicate that the 10 foot berm / barrier would not provide significant shielding for these homes from traffic noise, beyond the setback distance. The berm would simply be too far away, and traffic noise would diffract over the top negating most of the potential noise attenuation. As the location of interest gets further away from the barrier, sound curves (diffracts) over the top, and the barrier attenuation value declines noticeably. In a residential neighborhood, parallel rows of homes provide secondary shielding effects as the primary barrier effect declines with distance. That will not occur in this case. The nearest homes are located 800 feet away (or more) from Chinden Boulevard, so are exposed to noticeably less traffic noise than closer locations. The site is essentially flat. The church facade has a setback of about 225 feet to the center of the proposed berm location / landscape strip along Chinden. The berm centerline would be roughly 82 feet from the centerline of Chinden Boulevard. In this case the nearest structure of concern is the church building itself. There is a parking lot between the church and the proposed berm location. Neither the church building nor the parking lot is particularly noise-sensitive. The southern side of the church building shows offices along the edge. Classrooms are located along the east edge of the building, with a 288 foot minimum setback from the berm centerline, and only a partial view of Chinden Boulevard and its traffic noise. The berm would provide some noise shielding for the ground floor of the church building itself, but at 225 feet between the berm and the facade, the effect would be limited. Most of any effect that might be achieved is a function of the inherent setback distance from Chinden Boulevard. If noise reduction or shielding were desired for the interior of the church, that goal is much more effectively achieved at the envelope of the building using proper window selection. Dual-pane thermal windows as typically used in the southern Idaho climate for energy efficiency are more than adequate to reduce traffic noise indoors to meet acceptable levels for residential space. There is no need for special sound-rated windows or other mitigation measures as part of the building design. The only caveat is that the windows must remain closed to keep noise out, so adequate ventilation is needed for indoor spaces without opened windows (windows can be operable, but must be considered as “normally closed” for noise control purposes). For reference, this approach is acceptable to HUD for residential projects involving federal funds, and for the FAA when mitigating airport noise that impacts residential neighborhoods. I reiterate my professional opinion that the berm / barrier as configured would provide little or no acoustical benefit to the church, and none for the nearby homes. Please let me know if there are any questions about this information. Sincerely, Earl Mullins, PE Principal Acoustical Consultant Attached: sample ITD form 1057 Optional Non-Barrier Noise Abatement Checklist ITD 1057 (Rev. 05-11) Idaho Transportation Department itd.idaho.gov Key Number Project Number Project Name Receptor Number(s) Feasibility Yes No 1. Traffic management measures can be implemented which conform to the purpose of the project and achieve at least a 5 dBA reduction in sound levels at the majority of impacted receptors 2. Alignment alteration can be implemented which conforms to design standards and achieves at least a 5 dBA reduction in sound levels at the majority of impacted receptors 3. There is an Activity Category D land use n the project area and insulation can be provided which achieves a 5 dBA reduction in sound levels inside the structure If the response to all of the above is No, the measures are not feasible. Do not continue with the checklist. If the response to any of the above is Yes, the measure is feasible. Consider the following. Reasonableness Yes No 1. The cost of implementation meets the cost per benefitted residence criterion 2. The benefitted residents endorse the measure(s) 3. The measure achieves the design goal Note - If the Noise Analyst is aware of considerations such as imminent land use changes, indirect impacts, noise sources other than those modeled, etc., note the considerations below.