By Kate Dydak
Creating spaces that provide appropriate acoustics is crucial to the function of a school building. While acoustic solutions are often straightforward, sometimes they are not. In the case of D.C .International School (D.C.I), a creative solution was needed to ensure optimal learning without disrupting the budget.
D.C.I is a D.C. public charter school serving 1,750 middle and high school students. Because it was located in D.C., it was required to achieve LEED Gold for Schools by the D.C. Green Building Act—a point system that includes acoustic pre-requisites. The school was also challenged by limited physical space to provide fields and parking, and to meet stringent stormwater and historic preservation requirements. This impacted the placement of all communal spaces—especially the gymnasium, which needed to be full-size to serve the school’s growing sports programs. With a larger campus, D.C.I could have built a free-standing gymnasium with fewer acoustic considerations; such adjacency decisions are made in the feasibility/concept stage of design. However, space was limited and the decision was made to site the gymnasium over the science classrooms—and the challenge was on to find an acoustic solution.
The key challenge introduced by siting the gymnasium over classrooms was noise transfer. The gym was slated to be in use almost every period of the day and after school, meaning structure-borne noise would be continuously transmitted through the floor and into the learning spaces below.
The original acoustic solution proposed by the architect was a floating slab. The base building slab would be poured, and then a second slab poured, which would create an air gap between the two slabs intermediated only by springs. This solution would eliminate noticeable structure-borne noise transmission from the gym floor to the classrooms below. However, when the solution was priced by the general contractor, it added half a million dollars to the budget—an amount the school wanted to invest in other items.
This initiated a value engineering process to see if other viable acoustic solutions could be put in place. One solution, proposed by the design team led by Perkins Eastman D.C., was to move the sound isolation from the floor of the gymnasium to the ceiling of the science classrooms below. This would involve a more standard single slab construction for the gymnasium floor, and separating all suspended elements of the spaces below—ceiling, pipes, mechanical units — from the underside of the slab using springs. A spring-supported, 3-layer gypsum board isolation ceiling installed between these would further deaden noise from the gymnasium above. The acoustician had installed a similar spring-isolated acoustic ceiling at a former university client in a gym over conference rooms, and the contractor estimated that this would provide a 25% cost savings over the floating slab.
In order to choose, D.C.I toured several sites to listen in person to the impacts of varying acoustical treatments. One site visit was to a charter school that had not significantly invested in acoustical treatments between its gymnasium and the offices and music classroom below. While the sound was not noticeable in the music classrooms, staff members expressed frustration at sound transmission into their offices.
In contrast, the impact of basketballs in the gym could not be heard at the aforementioned university’s conference center—even when the collegiate athletes were actively practicing in the gymnasium above. Visiting convinced D.C.I’s Executive Director, Mary Shaffner, that this less expensive spring-isolated solution was the right answer for D.C.I.
Still, there were challenges that arose with this below-slab approach. Per code, the fire alarm sprinkler pipes could not be suspended from springs. There were also specific constructability questions that had to be worked out at later coordination meetings between the contractor and architect. The team had to find a solution that provided the acoustical separation necessary while meeting the contractor’s provided estimate as closely as possible. One compromise included attaching the above-ceiling fan coil units and sprinkler pipes with rigid connections to the gym’s concrete deck but then installing gypsum-backed acoustic ceiling tiles in lieu of standard ceiling tiles. This ultimately achieved the same level of sound isolation while saving money, as the springs were more expensive than the tiles.
The contractor provided a mock-up of the ceiling assembly system for the architect and acoustician to review prior to full installation. During the actual installation, both the architect and the acoustician conducted site visits to ensure the ceiling was being installed per the discussed specifications.
Once installed, the acoustician returned to the building to test the system. These tests showed that structure-borne noise from the gymnasium above did not exceed the level of 45 dB—the equivalent of background mechanical noise. A level of 70 dB is generally where teachers have to use raised voices, so the school’s investment in a ceiling acoustic solution resulted in not having to install any sort of vocal amplification in the science classrooms. Most importantly, it allowed both spaces to be used simultaneously. As D.C.I’s executive director Shaffner stated: “I am really impressed at the lack of sound in our Science Labs. Two PE classes can be going on in the gym and labs are going on in our science classes undisturbed.”
While D.C.I faced an extreme acoustic design challenge, the process the school went through to identify the solution helped it balance competing strategic priorities and find the best value for the cost. As existing sites are built out and become more constrained, as LEED for Schools certification becomes more prevalent, as more schools invest in amplification systems to meet the needs of students with auditory challenges, and certainly as technology becomes further integrated into classrooms, more and more schools will be faced with decisions on their level of acoustic investment. Best practices like considering adjacencies early in the process, investigating multiple options through site visits, and working with an experienced acoustician, will help.
Kate Dydak is an Assistant Project Manager with Brailsford & Dunlavey, serving as an owner’s representative for charter school construction projects in Washington, D.C.