Nonlinearity in the vertical transmissibility of seating: the role of the human body apparent mass and seat dynamic stiffness

Abstract

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 coupled seat and human body systems and 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 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 The measurements are presented 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.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.Keywords: seat transmissibility; nonlinearity; apparent mass; polyurethane foam; dynamic stiffness Published as: Nonlinearity in the vertical transmissibility of seating: The role of the human body apparent mass and seat dynamic stiffness

Effects of seating on the discomfort caused by mechanical shocks: Measurement and prediction of SEAT values

“…In addition to the model not predicting the body leaving the seat, the model did not predict the non-linearity of the biodynamic responses evident in the reduction in the resonance frequency with increasing magnitude of vibration (Figure 3). Although the measured and predicted SEAT values shown in Figure 7 have encouraging agreement, they would likely be closer if the parameters of the model had been adjusted to take account of the nonlinearity in the biodynamic responses of the body to vertical vibration and mechanical shocks (Matsumoto and Griffin, 2005;Zhou and Griffin, 2017b;Tufano and Griffin, 2013;Fairley and Griffin, 1989).…”

“…A three degree-of-freedom model was used to represent the coupled seat and human body system ( Figure 2). The parameters of the model m 1 , m 2 , and m 3 represent masses of the human body, k 1 and k 2 and k 3 represent stiffnesses of the block of foam and the human body, c 1 and c 2 and c 3 represent the damping of the block of foam and the human body (Wei and Griffin, 1998;Tufano and Griffin, 2013).…”

Development of a Five‐Degree‐of‐Freedom Seated Human Model and Parametric Studies for Its Vibrational Characteristics

“…All springs assumed to be massless and frictionless were in nonstretched and nonrotated conditions at the initial configuration. Although seat foam has nonlinear viscoelastic properties [27][28][29], it has been reported that such properties do not significantly affect the transmissibility of human body vibrations in practice [13]. It was also assumed that because deformations of the human back and buttock tissue after sitting on a seat are sufficiently small, their nonlinearity could be disregarded.…”

“…e natural frequencies and frequency responses of a seated human are decided depending on body weight, length of human segments, and sitting posture (e. g., slouched or erect) [6][7][8][9][10]. In addition, dynamic characteristics of a seated human involve nonlinearities such as the frequency softening phenomenon, in which the first natural frequency decreases as the magnitude of the excited vibrations increases [11][12][13]. Mansfield and Griffin [11] inferred that complex human factors such as muscle forces and the bending or buckling of a spine could give rise to these nonlinearities.…”

An Effective Technology to Evaluate Vibration Comfortability of Human Body Based on Near-Infrared Spectroscopy

“…9 When the vibration frequency is close to the inherent frequency of the body's internal organs, or the two frequencies are consistent, the resonance will occur. It will influence the human normal work of each system of the body directly, 10,11 and also lead to body's discomfort and reduce the operational severity.…”