By Paul Sherman, CCE
I started life as an electronics tech, picking up some RF and audio expertise. Eventually (at 34) I earned my EE. After graduating, I ended up in clinical engineering, which is basically ensuring that all medical technology in a hospital plays well together and performs in the best interests of patients for many years. This environment is a lot more challenging than you’d think, especially with buildings that are 70+ years old and age of the equipment varies.
In my first job, my first assignment was to help solve the problem of a very loud, very strange noise coming from the equipment room of the new CT scanner. The equipment room was adjacent to the patient waiting room and the noise was loud enough to make everyone in the room sit as far from the wall as possible. The other clinical engineering staff and the equipment manufacturer had been struggling with the problem for a couple of months.
Much of a hospital’s medical equipment (including this CT) is on emergency power. By code, emergency power has up to 10 seconds to transfer and come up to speed. But a random loss of power to a CT, especially during an exam is not acceptable. In order to “smooth” the transition, this installation used a motor-generator (M-G) to provide interim power.
The equipment room was basically a computer room, with a raised floor. The M-G sat in the middle of the equipment room, spinning away. When I walked into the room, I not only heard the sound, I felt it in my feet. It was not a 60 cycle sound, the pitches were different. After walking around, feeling and listening I found the same sound coming from the motor generator. Inspecting the unit, I saw that it sat on feet, with the body of the unit about two inches off the floor. I had a thought - vibrating item (M-G) a few inches away from a large flat chamber (the raised floor) - could it be that the M-G and space under the floor were at resonance?
To test the theory, I needed to disrupt the energy transfer. Scrounging around in the shop area, I found some one-inch foam rubber floor mats, the perfect size to fit under the motor-generator. I took a few mats up to the room. We slipped one under the M-G, the sound level dropped immediately, one more and it disappeared. Problem solved - the mats stayed in place until the CT was replaced.
I guess they just needed a fresh set of eyes (and ears) to solve the problem.
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