In the past, sensitive hospital equipment like MRIs were often located in the building basement on a grade-supported slab, and away from vibration-producing equipment.

Recently, I have seen a trend where such equipment is located on higher floors in the hospital, closer to the patients.

In buildings with rooftop heliports, there is a possibility that vibrations induced by helicopter operations could affect sensitive equipment on the uppermost floors of the building. Fortunately the helicopter event is usually short-lived and disturbance can usually be handled administratively.

However, if one wants to predict the helicopter-induced vibrations to evaluate potential disturbance, then the dynamic forces exerted by the helicopter’s downwash need to be known.

Unfortunately, there is little information available regarding the low-frequency pressures associated with helicopter operations.

Because of this, my colleagues and I conducted a series of measurements at a helipad site with a Eurocopter BK-117C1 life flight helicopter and found that the dynamic forces at the blade passage rate were about 2% of the helicopter’s weight.

Jeffrey A. Zapfe, PhD, is Director, Noise Vibration Group, at Acentech. For more information, visit