Golden Gate Parking Garage A noise control case study Client: Vibro-Acoustics Website: vibro-acoustics.com Vibro-Acoustics’ noise control solution silences local concerns over constructing an underground parking garage in Golden Gate Park. Background Located in the heart of San Francisco, the Golden Gate Park Music Concourse underground parking garage is integrated tightly between the de Young Museum of Fine Art and the California Academy of Sciences. This 800-car garage is virtually invisible within the 1,000 acre Golden Gate Park. To eliminate the buildup of toxic fumes, the garage relies on two exhaust ventilation systems, each of which includes its own 200,000 cfm axial fan, along with a number of smaller propeller fans. The two fans are set in mechanical rooms and work to draw air out of the parking garage. In an early phase of this project, two 10 ft long standard low-frequency silencers were specified based on an acoustical analysis for noise control. These silencers were to be installed on both the intake and discharge sides of the fans to control noise within and outside of the garage. Challenges Revisions to the project’s original plans involved changes to the garage’s mechanical rooms. As a result, the silencers originally specified for the fans became unsuitable for the space in the rooms. Another concern was the potential of creating high pressure drop. Changing the fans in an attempt to address the pressure drop would have raised horsepower requirements, increasing energy consumption and cost. While a noise criterion of only 60 dBA was required inside the garage, the outdoor requirement presented a greater challenge. At peaks hours of operation, the existing background noise levels near the exhaust discharges must not be allowed to increase; therefore, an outdoor noise criterion of 45 dBA was established. Meanwhile, the local community, concerned by the effect the garage and other noise sources may have on park events and wildlife, challenged to halt its construction. This political backdrop put greater pressure on the project team to meet the noise criteria. Solution Partnering with both the acoustical consultant and mechanical engineer, Vibro-Acoustics used their Integrated Systems Approach to perform a system analysis and helped redesign the noise control solution. As guaranteed, the new solution met the required noise criteria of 45 dBA (outdoor) and 60 dBA (indoor) and overcame the space and static pressure drop restrictions. On the intake side, acoustical louvers were installed to help reduce noise entering the garage, improving safety in the parking area. Absorption material was added in the fan rooms to control sound level buildup. For the exhaust fan intake, Vibro-Acoustics provided dissipative silencers to reduce noise and ease fan airflow. On the discharge side, 9 ft axial cone silencers were selected to facilitate the airflow through the two systems. To address the noise in the underground tunnels, Vibro-Acoustics provided a circular tunnel silencer design and used a culvert pipe casing to meet the structural requirements. These silencers were suitable for direct burial and prefabricated for low pressure drop, completing the attenuation required to meet the noise criterion. This system solution helped the design and construction teams meet all system requirements, and allowed the garage to obtain an occupancy permit to open for use.