13 November 2025
Tyler Wood
Quarterra
114 Sansome Street, Suite 225
San Francisco, CA 94104
[email protected]
Subject: Willow Apartments
Environmental Noise Study
Salter Project 24-0542
Dear Caroline:
We have conducted an environmental noise study for the project. The purpose of the study is to determine the noise environment at the project site, compare the measured data with applicable standards, and propose initial mitigation measures as necessary. This report summarizes the results of our study based on the 16 June 2025 site plan (see attached) and updates to the building design (e.g., room and window sizes).
PROJECT CRITERIA
California Building Code (Title 24)
Section 1206.4 of the California Building Code (CBC) requires that the indoor noise level in residential units of multi-family dwellings not exceed DNL1 45 dB.
City of Hercules
Zoning Ordinance Chapter 13-31.300.11 states2:
● Indoor noise levels for new housing units are not to exceed DNL 45 dB, which is in agreement with Title 24.
1 DNL (Day-Night Average Sound Level) – A descriptor for a 24-hour A-weighted average noise level. DNL accounts for the increased acoustical sensitivity of people to noise during the nighttime hours. DNL penalizes sound levels by 10 dB during the hours from 10 PM to 7 AM. DNL is sometimes written as Ldn. 2 https://www.codepublishing.com/CA/Hercules/#!/Hercules13/Hercules1331.html Willow Apartments
13 November 2025
Environmental Noise Study
● Single-event noise (e.g., train passbys) should not exceed Lmax3 50 dB in bedrooms and 55 dB in other habitable rooms.
● Outdoor noise levels should not exceed DNL 60 dB for residential areas. DNL 65 dB might apply at the private balconies, at the City’s discretion.
● The outdoor noise goal is DNL 70 dB when the noise source is a railroad.
CALGreen
CALGreen Code Section 5.507.4 addresses acoustical issues for non-residential spaces. If a building is exposed to an exterior Leq(h)4 of 65 dB during any hour of operation, the building envelope must reduce the interior noise environment to Leq(h) of 50 dB in occupied areas. We have assumed this will apply at the clubhouse.
NOISE ENVIRONMENT
The project site is along Willow Avenue, east of Interstate 80 (I-80), south of State Route 4 (CA-4), and north of the Burlington Northern Santa Fe (BNSF) railway. The noise environment at the site is predominantly controlled by traffic on Willow Avenue, the freeways, and the railway.
To quantify the existing noise environment, we conducted three long-term noise measurements between 6 and 8 January 2025 (see Figure 1 for the measurement locations and measured noise levels). The noise monitors were at a height of 12 feet above grade, except for the on-site monitor, which was about 7-feet above grade.
A traffic analysis5 has been provided for this project. However, the study does not include future cumulative traffic predictions (only “with project” increases). Therefore, we have added 1 dB to our measured noise levels to account for future traffic increases6.
3 Lmax30 (Typical Maximum Sound Level) – There is no standardized metric to quantify “typical” maximum sound levels in an environment (instead of the absolute maximum sound level for a measurement period). The metric Lmax30 comes from a paper by Rob Greene (“Max Level Intrusive Noise Limit: 1982 National Conference on Environmental and Occupational Noise”). It is based on the logarithmic average of the noisiest 30 percent of single events (e.g., train passbys, aircraft flyovers).
4 Leq(h) – The equivalent steady-state A-weighted sound level that in an hour would contain the same acoustic energy as the time-varying sound level during that hour.
5 Traffic Study from Hexagon Transportation Consultants dated 7 October 2025. 6 The California Department of Transportation (DOT) assumes a traffic volume increase of three-percent per year, which corresponds to a 1 dB increase in DNL over a ten-year period.
Willow Apartments
13 November 2025
Environmental Noise Study
RECOMMENDATIONS
Interior Noise (Code)
Using the 10 April 2025 entitlement submittal drawings and the 16 June 2025 site plan, we calculated the window and exterior door STC7 ratings needed to meet the project criteria, as shown in Figure 2. For our calculations, we have assumed the following:
● All spaces will have hard-surfaced (non-carpeted) flooring ● Ceilings will be 9-feet high
● The exterior wall achieves minimum STC 45 (equivalent to 7/8-inch-thick stucco over wood sheathing) The recommended STC ratings are for full window and door assemblies (glass and frame) rather than just the glass itself. Tested sound-rated assemblies should be used. For reference, typical one-inch glazing assemblies (two 1/4-inch-thick panes with 1/2-inch airspace) typically achieve an STC rating of 32. STC ratings above 32 should include laminated glass. STC ratings above 38 might require an IGU greater than one-inch thick. This will vary depending on the window manufacturer.
Where windows need to be closed to achieve an indoor DNL of 45 dB, an alternative method of supplying fresh air (e.g., mechanical ventilation) should be considered. This applies to all locations. This issue should be discussed with the project mechanical engineer.
Interior Noise (Single-Event Noise)
To meet the single-event criteria, it will be necessary to provide upgraded exterior wall assemblies at the southern facades of Buildings 5 and 6. The exterior wall needs to achieve STC 55 (e.g. double-stud construction). With these upgrades, windows and exterior doors in these facades would need STC ratings up to STC 42, as shown in Figure 3.
Exterior Noise
The City has a DNL 60 dB goal for outdoor spaces. This applies at the clubhouse patio and dog park. It might be applied to private apartment balconies on a case-by-case basis (as determined by the City).
We expect noise levels at the clubhouse patio and the dog run to be up to DNL 73 dB. We recommend a sound wall on the west and south sides of the patio that blocks line-of-site to Interstate I-80 and the train line. A sound wall should also be considered at the north and west sides of the dog run. See Figure 4 for the location of the sound walls.
7 STC (Sound Transmission Class) – A single-number rating defined in ASTM E90 that quantifies the airborne sound insulating performance of a partition under laboratory conditions. Increasing STC ratings correspond to improved airborne sound insulation.
Willow Apartments
13 November 2025
Environmental Noise Study
We have estimated the noise level on the patio for different sound-wall heights. We have also calculated the average noise level on the patio during daytime hours (Leq(daytime)8). See Table 1 below for estimated noise levels on the patio with various wall heights.
Table 1: Noise Levels on Clubhouse Patio with Various Wall Heights Wall Height DNL Leq(daytime)
0 ft 73 dB 70 dB
6 ft 68 dB 65 dB
8 ft 67 dB 64 dB
10 ft 66 dB 63 dB
12 ft 64 dB 61 dB
We expect noise on private balconies to be between DNL 67 and 77 dB. However, due to the proximity to the freeway, mitigating the noise on all balconies below DNL 65 dB is not feasible without fully enclosing the balconies.
* * * This concludes our environmental noise study for the project. Please feel free to reach out with any questions.
Best,
SALTER
Zamar Bravo Tapia
Consultant
Eric Mori, PE
Executive Vice President
8 Leq(daytime) – Average sound level between the hours of 7am and 10pm.
PROJECT SITE
LT-2: DNL 75 dB, Leq(h) 74 dB
ST-1: DNL 70 dB, Leq(h) 69 dBLT-1: DNL 76 dB, Leq(h) 73 dB LT-3: DNL 74 dB, Leq(h) 71 dB
www.salter-inc.comSan FranciscoSan JoseHonoluluSeattleLos Angeles
Building 5Building 6
X01
X06X07
X12
= STC 28= STC 31= STC 34= STC 37= STC 40= STC 42
Building 1
Building 2Building 3Building 4
X06
X01X12
X07
X01X08
X04X05
X01
X06X07
X12
X01
X04X05 X08
STC RATINGS ARE FOR THE COMPLETE ASSEMBLY (E.G., GLASS, FRAME, AND OPERABLESECTIONS) BASED ON TEST REPORTS FROM AN NVLAP-ACCREDITED LABNOTE:www.salter-inc.comSan FranciscoSan JoseHonoluluSeattleLos Angeles
08.05.25 STC 55 UPGRADED WALL
Building 5Building 6
Building 1
Building 2Building 3Building 4
X06
X01X12
X07
X01X08
X04X05
X01
X06X07
X12
X01
X04X05 X08
X01
X06X07
X12
= STC 28= STC 31= STC 34= STC 37= STC 40= STC 42
GREYED-OUT AREASUNCHANGED FROMCODE-REQUIRED
STC RATINGS ARE FOR THE COMPLETE ASSEMBLY (E.G., GLASS, FRAME, AND OPERABLESECTIONS) BASED ON TEST REPORTS FROM AN NVLAP-ACCREDITED LABNOTE:www.salter-inc.comSan FranciscoSan JoseHonoluluSeattleLos Angeles
08.05.25 = Sound Wall
Dog Run
Clubhouse Patio
www.salter-inc.comSan FranciscoSan JoseHonoluluSeattleLos Angeles
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INTERSTATE 80INTERSTATECALIFORNIA80
WILLOW AVENUE
WILLOW AVENUE OFF-RAMP
CLUBHOUSEBLDG 6BLDG 5BLDG 4BLDG 3BLDG 2BLDG 1DRIVE 'A'DRIVE 'B'DRIVE 'B'DRIVE 'E'DRIVE 'D'DRIVE 'C'DRIVE 'F' 76 LEGEND:
SEE SHEET C.05 FORSTREET SECTIONS
12080400
SITE PLANC.04OF 13 SHEETSSHEET NO.CIVIL ENGINEERSSURVEYORSPLANNERSSAN RAMONWWW.CBANDG.COMROSEVILLE(925) 866-0322(916) 788-4456H:\4060-000\ACAD\TM\SITE PLAN.DWG SUBDIVISION 9715VESTING TENTATIVE MAPFOR CONDOMINIUM PURPOSESEMBLEM HERCULESCITY OF HERCULES CONTRA COSTA COUNTY CALIFORNIASCALE: 1"=40'DATE: JUNE 16, 2025
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