Robert Schofield, Richard McCarthy, Tyler Guidry, Genevieve Connolly, Ben Mannix, Jonathan Olson

Last summer, Genevieve identified the 35.4 Hz peak as likely from the office area air handler (80655). Recently, Ben was working a Data Quality Shift and reported that the peak had moved by tenths of a Hz and had increased dramatically in amplitude (82519).  This week I used a speaker to sweep in the region around 35 Hz and found that there was a sharp maximum in vibration coupling that happened to lie just where the HVAC peak has been for the last couple of months (See Figure). This meant that, if we could move it just a little in frequency, we would reduce the amplitude in DARM. We power some of the HVAC equipment with variable frequency drives (VFD), which convert the mains 60 Hz to other frequencies. We set the office area air handler VFD to 69 Hz, instead of the 72 Hz that it had been running at, and greatly reduced the amplitude of the peak in DARM (see Figure, page 2).