Factors affecting the dynamic response of the body and the vibration transmitted through seats

Abstract

The vibration transmitted through a seat is influenced by the dynamics of the seat and thedynamics of the occupant. The principal objective of this thesis is to understand how the dynamicsof the body and factors affecting the dynamics of the body influence the vibration transmittedthrough seats. Previous studies have shown that the apparent mass of the body and seattransmissibility are affected by the seating environment (e.g. vibration input spectra, backrest,hands position, foot position) and variability between people (i.e. physical characteristics), butthese effects have not previously been systematically explored for realistic seating conditions.The apparent masses of 12 subjects were measured during exposure to random vertical vibration(from 0.125 to 40 Hz) to investigate the effects of the seat backrest, the footrest and steeringwheel, and input spectra. In a rigid seat with no backrest, there were resonances in the apparentmass of the body around 5 and 10 Hz (with 1.0 ms-2 r.m.s broadband vibration). In the same seatwith a rigid backrest, the median resonance frequency in the apparent mass increased from 5.47 to6.35 Hz as the backrest was reclined to 30 degrees in 5 degrees increments; with a 100-mm foambackrest, the median resonance frequency decreased from 5.18 to 4.49 Hz as the backrest wasreclined to 30 degrees. When subjects held a steering wheel, the mass supported on the seatsurface decreased and there was an additional resonance at 4 Hz in the apparent mass. Movingthe steering wheel away from the body reduced the apparent mass at resonance and increased theapparent mass around the 4 Hz resonance. As the feet moved forward, the mass supported on theseat surface increased, indicating that the backrest and footrest supported a lesser proportion ofthe subject weight. Applying force (0, 50, 100, 150, 200 N) to either the steering wheel or thefootrest reduced the apparent mass at resonance and decreased the mass supported on the seatsurface. Narrowband inputs at ½-octave intervals (from 1 to 16 Hz) presented at five magnitudes(0.25, 0.4, 0.63, 1.0 and 1.6 ms-2 r.m.s.) showed that the extent of nonlinearity previously observedwith broadband vibration was frequency-dependent: the magnitude of vibration at frequencies lessthan 4 Hz had the greatest effect on the apparent mass at resonance, while vibration atfrequencies less than 8 Hz had the greatest effect on the resonance frequency.A simple lumped parameter model was used to demonstrate that changes in the apparent masswith backrest contact, backrest inclination, hand position, foot position and vibration magnitudecould be closely represented by changing the parameters in the model. Trends in modelparameters, the damping ratios, and the damped natural frequencies were identified as a functionof the model variables.A study was designed to determine how the physical characteristics of 80 seated adults (41 malesand 39 females aged 18 to 65) affected their apparent mass and the transmission of vibrationthrough a seat. Multiple regression models showed that while the strongest predictor of the verticalapparent mass at 0.6 Hz, at resonance, and at 12 Hz was bodyweight, weight was not stronglyassociated with seat transmissibility. A lumped parameter seat-person model was used to showthat the dynamic stiffness of the seat increased with increased loading so as to compensate forincreases in apparent mass associated with increased sitting weight. As age increased from 18 to65 years, the apparent mass resonance frequency increased by up to 1.7 Hz. This change wasgreater than the 0.9-Hz increase in resonance frequency between sitting without a backrest andsitting with a backrest reclined to 15° and greater than the 1.0-Hz reduction in resonance frequencywhen the magnitude of vibration increased from 0.5 to 1.5 ms-2 r.m.s. Subject age was much thestrongest predictor of the seat transmissibility resonance frequency and the transmissibility atresonance. The model was used to show that changes in the seat transmissibility with age could bepredicted from changes in the apparent mass with age