Dynamic response of the coupled human body and seat in vertical and fore-and-aft direction

Abstract

In many environments vibration is transmitted to a person through a seat. Seats can be designed to reduce the discomfort and the injuries caused by vibration. The efficiency of a seat in reducing vibration depends on the characteristics of the vibration, the characteristics of the seat, and the characteristics of the person sitting on the seat (Griffin, 1990). This research was designed to investigate several aspects of the transmission of vertical and fore-and-aft vibration through polyurethane foams used in seat construction. The research programme was focused on two experiments. The first experiment was designed: (i) to investigate non-linearities in the seat and the human body in the vertical direction and their contributions to seat transmissibility; (ii) to compare the vertical apparent mass of the human body on rigid and soft seats; (iii) to measure and model the vertical dynamic stiffness of polyurethane foam seat cushions and investigate how the dynamic stiffness depends on vibration magnitude and subject characteristics (i.e. sitting weight, and hip breadth). The second experiment was designed: (i) to investigate the dependence of fore-and-aft seat cushion transmissibility on vibration magnitude, foam stiffness and contact with a backrest; (ii) to compare the fore-and-aft apparent masses of the human body on rigid and soft seats; (iii) to measure and model the dynamic stiffness of polyurethane foam seat cushions in the fore-and-aft direction, compare the fore-and-aft and vertical dynamic stiffness of foam, and investigate how fore-and-aft dynamic stiffness depends on subject sitting weight and hip breadth; (iv) to study the linear and non-linear effects of simultaneous vertical and fore-and-aft vibration and investigate whether single-axis transmissibility and single-axis models can be used to predict seat cushion transmissibility in multi-axis vibration environments. Fifteen subjects attended the two experiments. In the first experiment, the vertical force and vertical acceleration at the seat base and vertical acceleration at the seat-subject interface were measured during random vertical vibration excitation (0.25 to 25 Hz) at each of five vibration magnitudes (0.25 to 1.6 ms-2 r.m.s.), with four seating conditions (rigid flat seat and three foam cushions). The measurements are reported in terms of the subject apparent mass on the rigid and foam seat surfaces, and the transmissibility and dynamic stiffness of each of the foam cushions. A frequency domain model was used to identify the dynamic parameters of the foams and to investigate their dependence on subject sitting weight and hip breadth. In the second experiment, the vertical and fore-and-aft forces and accelerations at the seat base and the vertical and fore-and-aft accelerations at the seat-subject interface were measured during random vibration excitation (0.25 to 25 Hz) in fore-and-aft and vertical directions. Using three acceleration magnitudes in each direction (0, 0.25 and 1.0 ms-2 r.m.s.) eight different combinations of vertical and fore-and-aft excitation were investigated with three seating conditions (rigid flat seat and two foam cushions), with and without contact with a rigid vertical backrest. Both the human body and the foams showed nonlinear softening behaviour, which resulted in nonlinear cushion transmissibility in both the vertical and the fore-and-aft direction. The nonlinearities in vertical cushion transmissibility, expressed in terms of changes in resonance frequencies and moduli, were more dependent on human body nonlinearity than on cushion nonlinearity. The vertical 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. Fore-and-aft apparent mass was strongly dependent on the use of the backrest. Fore-and-aft apparent masses on rigid and soft seats had similar shapes. The vertical and fore-and-aft dynamic stiffness of foam was found to be nonlinear with vibration magnitude and showed complex correlations with the characteristics of the human body. Foams were stiffer in the horizontal direction than in the vertical direction. Linear cross-coupling between vertical and fore-and-aft transmissibility was found: a small part of the vertical (or fore-and-aft) vibration at the seat base contributes to fore-and-aft (or vertical) vibration at the subject-seat interface. Nonlinear cross-coupling was found in seat transmissibility and foam dynamic stiffness: the softening of the seat-subject system in one axis is affected by the vibration in the perpendicular direction. The author believes that this research increased the current state of knowledge of the dynamics of the seated human body and polyurethane foams and so it represents a step forward in the understanding of the mechanisms involved in the vibration isolation provided by seats.