Limits of Application of Human Body Dynamics in Assessing Vibration Comfort of Seats

It has been widely reported that the overall vibration comfort performance of static and dynamics seats is strongly influenced by the biodynamic behaviour of the seated human body. The contributions of the seated occupant to the overall vibration attenuation of the coupled seat-occupant system are experimentally investigated as functions of the nature of excitation, static and dynamic properties of the seat, and the sitting posture. The study involved two different seats with natural frequencies in the vicinity of 1.5 Hz and 4 Hz, which would characterize the low natural frequency suspension as well as high natural frequency seats employed in automobiles and some industrial vehicles. The vibration isolation properties of the seats are evaluated with a rigid mass and two human subjects under different vibration excitations, including swept sine, broad-band random and standardized vibration spectra of selected vehicles. The data acquired for the seat-occupant system are compared with those for the seat-mass system to identify the limits of applicability of the human body dynamics in assessing the vibration comfort performance of seats. It is concluded that the role of human body dynamics is relatively small in case of low natural frequency suspension seats, when subjected to either low frequency excitations such as those encountered in urban buses or high frequency excitations, such as those of the forklift trucks. The human body dynamics are also insignificant for high natural frequency seats subjected to low frequency excitations. The contributions due to human body dynamics are most significant for high natural frequency seats exposed to excitations near the natural frequencies of the seated occupant and the seat.