HomeMy WebLinkAboutSound Engineer Report10400 Overland Road #211 -- Boise, ID 83709
www.mullinsacoustics.com 208-514-6264
September 2, 2020
Kameron Nauahi, Assistant Project Manager
BRIGHTON CORPORATION
12601 W. Explorer, Suite 200
Boise, ID 83713
Subject: Bainbridge Subdivision #7 project #3828
Phase 11 Sound Wall Analysis
Dear Kameron,
As requested, I have reviewed your project design in terms of the City of Meridian’s
sound wall requirements. This portion of the project is a subdivision with 172 residential
lots, fronting along West Chinden Boulevard / State Highway 20/26 and just east of
North Tree Farm Way.
This letter is an update from September 2018 to cover the newer phase #11 of the
project. The barrier as described herein will meet the noise abatement
requirements expressed in the Meridian code.
REQUIREMENTS
The City of Meridian requires a ten foot high berm or barrier wall along major roadways
for most noise-sensitive land uses, including residential. This is a blanket requirement,
without determining whether traffic noise mitigation is actually needed for a given
project. The specific text is:
D. Noise abatement shall be required for residential and other noise sensitive
uses including, but not limited to, education institutions, churches or places of
religious worship, libraries and/or hospitals adjoining state highways: (Ord. 10-
1439, 1-12-2010, eff. 1-18-2010)
1. The applicant shall provide traffic noise abatement by constructing a berm or a
berm and wall combination approximately parallel to the state highway.
2. The top of the berm or berm and wall in combination shall be a minimum of
ten feet (10') higher than the elevation at the centerline of the state
highway.
3. If a wall is proposed, the wall shall meet the following standards:
MULLINS
ACOUSTICS
architectural
environmental
and industrial
noise control
a. Wall materials shall be impervious concrete or stucco or other
appropriate sound attenuating material.
b. Intermittent breaks in the berm or berm and wall in combination will
degrade the function and shall not be allowed.
c. The applicant shall not construct a monotonous wall. In order to achieve this
standard, the applicant may choose one or both of the following variations:
(1) The color and/or texture of the wall shall be varied every three hundred (300)
linear feet. This could include murals or artwork.
(2) The wall shall be staggered every three hundred (300) linear feet subject to
subsection D3b of this section that prohibits breaks in the wall.
4. The director may approve alternative compliance as set forth in chapter 5,
"Administration", of this title where the applicant has a substitute noise
abatement proposal in accord with ITD standards and prepared by a
qualified sound engineer. (Ord. 05-1171, 8-30-2005, eff. 9-15-2005)
Acoustically relevant text is shown in bold type.
A brief review was made of ITD noise criteria published on-line and 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
These are the typical feasibility requirements for evaluating highway noise barriers, as
used by most transportation and highway departments around the country.
BARRIER
The project will include a property line noise barrier as part of its design, as required by
the city. This barrier will run along the Chinden Boulevard frontage. The barrier will be
approximately 94 feet away from the road centerline, about 52 feet from the nearest
edge of a traffic lane, and the setback of the nearest home pads is about 150 feet from
the road centerline.
This segment of Chinden Boulevard is one lane in each direction, a left turn lane
eastbound onto North Tree Farm Way, a right turn lane eastbound onto Tree Farm
Way, plus bike lanes on each shoulder. The posted speed limit is 50 mph.
This segment of Chinden is mostly level. The property itself where homes will be
located will have a final grade roughly 4 feet lower than the road centerline across its
entire width.
The barrier location is shown on the latest landscape plan (sheet L1.1, stamped
October 2019). The heavy black line on this sheet shows the location and extent of the
barrier atop the berm, and shows berm contours. Details of the berm and fence are
shown as Detail 7 and Detail 6 on sheet L1.5. These drawings confirm the overall 10
foot height of the combined berm/barrier.
The roadside noise barrier will be a combination of a four foot high berm and a six foot
high wood fence. The cedar fence boards will be 7/8” thick and overlapped by 1” to
avoid gaps per notes on sheet L1.5. The barrier top edge will parallel the slight slope of
this road segment to maintain a consistent height.
Based on a source distance of 25 feet to the nearest (future) traffic lane, 45 feet to the
nearest homes behind the barrier, and a ten foot barrier height, the predicted
attenuation is 14 dBA. This is a substantial degree of noise reduction, and will easily
meet ITD tests for effectiveness.
The barrier should extend along the full frontage of the project, shielding lots 6-15 from
traffic noise. Additionally an angled segment of barrier should extend along the rear of
lot #6 and along its western edge. Starting in the middle of lot #6 the overall height can
taper down to just the fence alone. No barrier is needed at lot #4 or for any locations
further south. (The drawing shows the fence extending to the southern corner of lot #3
anyway). The design on sheet L1.1 shows the proper placement and orientation of
property line barriers and berm details.
As the setback distance for lots increases beyond that of lot #4, traffic noise is expected
to fall off by at least 8 decibels due to added setback distance from the roadway. This is
a noteworthy reduction and comparable to the acoustical value of most barriers. From
lot #6 southward, no barrier is needed.
Lot #4 and all lots further south will be shielded from traffic noise both by the barrier and
by the intervening row of homes when completed. Rows of homes also act as an
effective noise barrier. These lots will further benefit from added setback distance from
Chinden Boulevard.
The future phase to the east (Bainbridge No. 12) will have a similar barrier requirement
and barrier design. That future barrier will protect lots 14-15 from traffic noise occurring
from beyond the eastern boundary of this property. A similar fence / berm will be
required for the adjoining property.
An effective traffic noise barrier may consist of a berm, a wall or fence, or any
combination of the two. The key elements of an effective noise barrier are:
• high enough to break line-of-sight between noise sources and receivers
• solid impermeable construction without air gaps
• mass of 1.0 psf or more (1/2” thick exterior grade plywood or comparable)
No barrier effect occurs until the sight lines between the noise source and the receiver
locations are interrupted. As soon as the direct line-of-sight is broken there is
immediately 5 dB of attenuation. Then each additional foot of barrier height yields
roughly another decibel of attenuation. In this case the LOS is about where the berm
ends and the fence begins.
Barrier effectiveness is always limited by the geometry (height and horizontal extent).
The material itself is almost never the limiting factor for barrier design. The maximum
real-world barrier gives about 17 dBA of noise reduction for very tall barriers placed very
close to either sources or receivers. In this case, 7/8” thick cedar fence boards weigh
about 1.7 pounds per square foot, so they have more than adequate mass to be an
effective noise barrier. Plywood of 1/2” thickness will stop about 27 dB of noise, as will
sheet metal that is at least 20 gauge. Barriers can be built using plywood, vinyl fencing,
cedar fence boards, 1/4” thick acrylic, or similar materials. Masonry and heavier
materials are not acoustically necessary.
Please let me know if there are any questions.
Sincerely,
Earl Mullins, PE
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PLANT SCHEDULE
KEY NOTES (TYPICAL)
PROJECT:
DATE:NO.ITEMDATE9233 WEST STATE STREET
BOISE, IDAHO 83714
PHONE (208) 639-6939
FAX (208) 639-6930REVISIONS
SHEET NO.
ENGINEERS . SURVEYORS . PLANNERS
CHECKED BY:
DRAWN BY:
DESIGN BY:
DRAWING STATUS:BAINBRIDGE SUBDIVISION NO. 11MERIDIAN, IDAHOLANDSCAPE PLAN FOR AGENCY REVIEW
L1.1
19-136
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PRELIMINARY- NOT FOR CONSTRUCTIONP:\19-136\CAD\LANDSCAPE\CONSTRUCTION PLANS\19-136 LANDSCAPE PLAN.DWG, ALYSSA YENSEN, 10/18/2019, DWG TO PDF.PC3, 22X34 L [PDF] LANDSCAPE PLAN
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W. SILVER RIVER LN.N. BOLSENA LN.N. BERNACCHI LN.N. TREE FARM WAYW. CHINDEN BLVD/ HIGHWAY 20/26
MATCH LINE - SEE L1.2 MATCH LINE - SEE L1.310/21/19