Discomfort caused by multiple-input vibration at the hands, the seat and the feet

Abstract

Ride quality in vehicles can be influenced by multiple-input vibration: vibration simultaneously transmitted to the body via multiple vibrating surfaces, such as steering wheels, seats, and floors. Standardised methods of predicting the discomfort of multiple-input vibration (ISO 2631-1, 1997) do not take into account vibration of the hands or the phase between vibration input to the hands, the feet, and the seat. This research seeks to understand the mechanisms associated with discomfort caused by multiple-input vertical vibration, examining the effect of vibration frequency, vibration magnitude, input location, and phase between vibration at the hands, the seat, and the feet, at frequencies between 2 and 12.5 Hz.

A total of four psychophysical laboratory experiments were conducted. Three studies were designed to expand knowledge of the absolute and relative sensitivity to vibration at the inputs, or between the three inputs, by determining: (i) absolute thresholds for perception of vibration (minimum levels of vibration that can be detected) at the hands and at the feet (Experiment 1); (ii) equivalent comfort contours for vibration (levels of vibration that produce similar discomfort across the range of frequencies) at the hands and at the feet (Experiment 2); and (iii) the equivalence of vibration sensation (vibration magnitudes required to produce equivalent discomfort) between the hands and the feet (from 2 to 12.5 Hz), between the hands and the seat (from 4 to 12.5 Hz), and between the seat and the feet (from 4 to 12.5 Hz) (Experiment 3). The findings indicate similar absolute thresholds at the hands and the feet between 2 and 5 Hz, with the thresholds having constant velocity. At frequencies between 5 and 12.5 Hz, absolute thresholds have constant acceleration at the feet while continuing to have constant velocity at the hands: an indication that different mechanisms may be involved in the detection of threshold levels of vibration at the hands and the feet between 5 and 12.5 Hz. Contours of equivalent comfort at the hands and the feet between 2 and 12.5 Hz resemble the shapes of the absolute thresholds, with little dependence in vibration magnitude. The relative magnitudes of vibration required to produce equivalent discomfort at the hands and the feet, or at the hands and the seat, depend on the frequency of vibration and indicate greater sensitivity to vertical vibration at the seat than at the hands and the feet over the frequency range investigated. A clear difference in the frequency-dependence of sensitivity to vertical vibration at the seat from those at the hands and the feet suggests a separate mechanism for detecting supra-threshold vibration of the seat.

The final study (Experiment 4) concerned the perception and discomfort of multiple-input vertical vibration with various phases between vibration at pairs of inputs, so as to determine: (a) difference thresholds (just noticeable differences, JNDs) for the detection of phase differences, (b) the effect of phase on discomfort, and (c) the localisation (i.e. body location) at which phase differences are detected. The results suggest that phase differences between the hands and the feet at frequencies greater than 5 Hz are unlikely to be detected, and that any phase differences between the hands and the feet over the range 2 to 12.5 Hz are unlikely to influence discomfort. Phase differences between the seat and the hands, or between the seat and the feet, over the range
4 to 12.5 Hz were detected by some subjects and increased discomfort up to 50% (when changing phase only, with no change in vibration magnitude). Over the three inputs (i.e. the hands, the feet, and the seat) sensitivity to vertical vibration was greatest at the seat and least at the hands. The phase between the inputs can affect discomfort, but only when vibration is applied at the seat. Changes in discomfort due to vibration frequency, vibration magnitude, or phase differences between the inputs may be partly associated with changes in the body locations experiencing greatest discomfort, due to different paths for the transmission of vibration into the body. The research has increased understanding of how multiple-input vibration applied simultaneously at the hands, the seat, and the feet contribute to produce an overall sensation of discomfort.

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