Defining the Line of Practicality: an Investigation into the Impacts of Detailed Source Modeling and Preliminary Site Investigation when Specifying Mechanical Noise Control Measures

Acoustic barriers are the most commonly recommended means of noise mitigation for mechanical sources located at roof level. Why? They are predictable, generally cost effective, and are typically the simplest means to satisfying receptor limits in an acoustic model. In addition, acoustic barriers generally yield no functional impact to the mechanical sources they are designed to mitigate. What is commonly overlooked are the structural complexities and implications in integrating a rooftop barrier system to an existing roof structure. Lateral wind loads, seismic loads, and in some cases snow loads, can turn a seemingly cost-effective and adequate acoustic solution in to a very costly and intrusive endeavor. This paper will highlight the mitigation of several induced draft cooling towers at Toronto Western Hospital, to illustrate how to evaluate the practicality of acoustic screens versus direct-to-source mitigation methods. The original specified solution at this location was an 8m high, full perimeter acoustic rooftop barrier. The significant structural implications rendered this approach impractical. The modified approach was a plenum silencer solution that achieved the specified dynamic insertion loss while maintaining functional maintenance access and designed mechanical operation. No structural enhancement was required, yielding a more cost-efficient solution to the facility's noise concerns.