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The efficiency of a seat in reducing vibration depends on the characteristics of the vibration, the dynamic characteristics of the seat, and the dynamic characteristics of the person sitting on the seat. However, it is not known whether seat cushions influence the dynamic response of the human body, whether the human body influences the dynamic response of seat cushions, or the relative importance of human body nonlinearity and seat nonlinearity in causing nonlinearity in measures of seat transmissibility. This study was designed to investigate the nonlinearity of the coupled seat and human body systems and to compare the apparent mass of the human body supported on rigid and foam seats. A frequency domain model was used to identify the dynamic parameters of seat foams and investigate their dependence on the subject-sitting weight and hip breadth. With 15 subjects, the force and acceleration at the seat base and acceleration at the subject interface were measured during random vertical vibration excitation (0.25–25 Hz) at each of five vibration magnitudes, (0.25–1.6 ms−2 r.m.s.) with four seating conditions (rigid flat seat and three foam cushions). The measurements are presented in terms of the subject's apparent mass on the rigid and foam seat surfaces, and the transmissibility and dynamic stiffness of each of the foam cushions. Both the human body and the foams showed nonlinear softening behaviour, which resulted in nonlinear cushion transmissibility. The apparent masses of subjects sitting on the rigid seat and on foam cushions were similar, but with an apparent increase in damping when sitting on the foams. The foam dynamic stiffness showed complex correlations with characteristics of the human body, which differed between foams. The nonlinearities in cushion transmissibilities, expressed in terms of changes in resonance frequencies and moduli, were more dependent on human body nonlinearity than on cushion nonlinearity. [ABSTRACT FROM PUBLISHER]

A combined lumped-parameter and multi-body system dynamic model of the human body-seat system has been constructed with masses and moments of inertia and with linear translational and rotational springs and dampers. The model was developed in four steps by minimising the sum-of-least-squares error between laboratory measurements and model predictions of the fore-and-aft driving point apparent mass and the fore-and-aft transmissibility of a car backrest. Good agreement was achieved between model predictions and both the median measured driving-point apparent mass and the median measured backrest transmissibility with six subjects. The model was capable of representing the measured apparent masses and predicting the backrest transmissibility with the individual subjects. It was also capable of predicting the backrest transmissibilities of two different car seats. A sensitivity study was conducted and the effects of the model parameters on the peak moduli and corresponding frequencies of the apparent mass and the backrest transmissibility are presented. [ABSTRACT FROM AUTHOR]