Localised apparent masses over the interface between a seated human body and a soft seat during vertical whole-body vibration.
Vibration transmission through vehicle seats is usually predicted using the biodynamics of the seated human body measured with a rigid seat, however how coupling between the body and the seat would affect the biodynamics of the body is not considered. This study investigated how dynamic forces distributed over a soft seat compared to that over a rigid seat wtih vertical vibration excitation. Fourteen male subjects sitting on a rigid seat and on foam cushions of two different thickness with four different heights of footrest were exposed to vertical whole-body vibration between 0.5 and 20 Hz at 0.5 ms-2 r.m.s. Dynamic forces were measured beneath the ischial tuberosities, the middle thighs, and the front thighs and the transmissibility of the cushion was measured to the three locations. The resonance in the transmissibility of the cushion was found around 4 Hz to the ischial tuberosities but around 6-8 Hz to the front thighs. Differences between the apparent mass measured with the cushioned seat and the rigid seat decreased with increasing height of footrest. A multi-body dynamic model that can predict the dynamic forces beneath the ischial tuberosities and thighs was applied to identify the cause for the differences. It was suggested the differences can be caused by the variations in vibration over the surface of soft seats, especially when the contact area beneath the thighs was large, and by changes in the effective stiffness and damping of the human body when the contact area beneath the thighs was reduced.