The Transmission of Vibration to the Occupants of a Car Seat with a Suspended Back-Rest

Abstract

A new type of car seat has been designed in which the back-rest is able to move freely in the vertical direction. The design allows the back-rest to follow the vertical vibration of the back of an occupant of the seat. This is intended to decrease any strain on the lumbar part of the spinal column caused by a different vertical motion at the seat squab and the seat back. The transmission of vertical (z axis) vibration from the base of this seat to the seat surface, to the back-rest and to the backs and heads of seat occupants has been measured. Measurements were made with the seat back fixed, to prevent vertical motion relative to the seat frame, and with the seat back free to move. Vibration transmissibilities from the seat base to the seat surface were characterized by a resonance at frequencies in the region of 4 Hz. The resonance frequencies were slightly higher with the fixed back-rest than with the moving back-rest. The transmissibilities at frequencies above 4 Hz were generally higher with the fixed back-rest. Measurements with no contact between the seat occupants and the seat back indicated that the dynamic properties of the human body and the seat cushion were less influenced by the moving seat back than by the fixed seat back. Differential motion between the seat surface and seat back was smaller with the moving seat back than with the fixed back.