An office building had an open atrium that went from ground level up to the roof. In the center of the atrium was a large flowing fountain. Surrounding the fountain were meeting rooms. Each meeting room had a large plate glass window approximately 8 feet tall and 12 feet wide through which the individuals in the meeting room could view the fountain. The problem was that these large plate glass windows had high levels of vibration. The frequency of the vibration was 240 cycles/minute. The floors and walls were all tested and no significant level of the 240 cpm vibration was observed in any location. The doors were also tested and it was found that the doors that were not held tight against their latches also showed the presence of the 240 cpm vibration. It was concluded that the vibration was airborne and affected those items that had large surface areas and low stiffness values (plate glass windows and loose doors). The search began for the source of the 240 cpm stimulus. The most obvious location to look for airborne transmission of pressure pulses was in the air handler area in the penthouse located at the top of the building. The thing that made the search difficult was that the frequency was so low that it did not match any fan speed and certainly not any blade pass frequency. The breakthrough in solving the problem occurred while walking by an air handler and noticing that the pipe to the cooling coils in the discharge duct of the fan was vibrating. When a spectrum was taken on the pipe there was a match with the 240 cpm vibration on the window. When that particular air handler was shut down, the vibration on the windows several stories below immediately stopped. When the fan was opened up, it was discovered that the screws in the braces that connected the heating coil to the ductwork were missing. When the coil was impacted, its natural frequency was as would be expected 240 cpm. Without the supports, the coil was cantilevered off of the heating pipes resulting in a low natural frequency. When the fan was in operation, broadband flow energy excited this natural frequency causing the coil to vibrate in the duct at 240 cycles/ minute. Since the coil was approximately 6’ X 6’, it generated a significant amount of air pulsation energy. The pulses were fed directly into the atrium area where they excited the windows. Since the windows had a very large cross sectional area and very little stiffness, they responded with high levels of vibration.
