Predicting the dynamic performance of seats
The transmission of vibration to people through seats was studied with the final objective of being able to predict the dynamic performance of seats. It was necessary to investigate both the dynamic response and the subjective response of a person sitting on a seat, in addition to the dynamic response of the seat itself. Initial experiments were conducted to investigate the effect of variables, such as the magnitude of the vibration and the posture of the body, on the transmission of vibration through a seat. Seat transmissibilities measured in a laboratory with vertical-only vibration were shown to be similar to those measured in a vehicle with multiple vibration. Frequency response functions were determined for the transmission of multiple axis translational vehicle vibration to the cushion and the backrest of a seat. The apparent mass of the seated body was measured as a pre-requisite for the prediction of seat transmissibilities. It was found that the resonance frequency of the body decreased when the magnitude of the vibration was increased. Increased muscle tension tended to have the opposite effect. The apparent masses of sixty people - men, women and children - were very similar once they were normalised to remove the effect of the different static weights on the seat. Some correlations with the body characteristics were significant - such as that between the normalised apparent mass at resonance and total body mass. A method of measuring the dynamic stiffness of a seat using a rigid indenter instead of a person to load the seat was developed. The results, in conjunction with measurements of the apparent mass of the body, were used to predict seat transmissibilities. Predicted seat transmissibilities were shown to be similar to transmissibilities that were measured with a person sitting on the seat. It was investigated how the weight of a person and the magnitude of the vibration could affect predicted seat transmissibilities. The discomfort caused by vibration transmitted to a seated person was measured using a novel method of adjustment. It was investigated whether the discomfort caused by simultaneous vertical and fore-and-aft vibration could be predicted from the discomfort that would have been caused if the vibration in each axis had occurred separately. A root-sum-square summation procedure was found to be the best.