Structural Floor Vibration and Sound Isolation Design for a Magnetic Resonance Imaging System

This is a case study about noise and vibration control problems, and the design constraints and solutions for a proposed installation of a magnetic resonance imaging system (MRI) in an existing medical research facility. Manufacturer's data indicated that airborne sound level emissions over a broad frequency span could exceed permissible noise criteria for nearby occupied rooms. The building structure also required reinforcement to accommodate the MRI magnet's concentrated load, but invasive disturbance to a transgenic research mouse vivarium on the floor below was prohibited. The structure borne vibration paths needed attenuation or isolation. Design parameters included structural strength, stiffness and the specific platform resonant frequency (non-coincident with known vibration sources or building structure). In addition, acoustical containment was required for anticipated noises from the magnet room, to prevent excessive or annoying and distracting noise in the MRI control room or other adjacent (but unrelated) research, animal holding and office spaces. Structural “de-tuning” and architectural “decoupling” concepts were employed. A resonant frequency criterion was recommended for the new structural floor design. A combination of vibration spectrum analysis, dynamic analyses of alternate structural concepts and existing physical conflict constraints led to the design of an independent platform floor above the existing building floor. Post construction floor vibration measurements were compared to earlier measurement data to show quantitative change in performance. The modifications satisfied acoustical criteria and occupants' subjective evaluations.