The subjective equivalence between steering wheel vibration and sound was investigated in two experiments. In each experiment, 20 subjects were presented 64 pairs of simultaneous sound and vibration stimuli in a simulator which reproduced a realistic driving posture. The first used stimuli measured in an automobile when driving over a coarse asphalt road surface, while the second used data obtained by driving over a 1.0 cm square metal bar. In both experiments, the acceleration time histories were scaled to eight root mean square (r.m.s.) levels from 0.5 to 4.0 m/s(2) and the sound pressure time histories were scaled to eight levels from 85 to 106 dB sound pressure level (SPL). Test subjects were asked to indicate which stimuli, the sound or the vibration, was felt to be the "more unpleasant". Regression expressions of subjective equivalence were developed for both stimuli types, in both the unweighted (decibel versus r.m.s. acceleration) and the frequency-weighted (decibel(A) versus VDV) forms. Regression slopes were found to be in the range from 23.8 to 26.8, similar to values for whole-body vibration exposures. Subjective equivalence was, however, found to be 5-15 dB(A) lower for the coarse asphalt stimuli and 15-25 dB(A) lower for the square metal bar stimuli. All other conditions remain equal, the human response to the vibration was found to increase in relative importance with respect to the sound in the case of short duration, transient, square metal bar stimuli. For vehicle and vehicle component manufacturers, the proposed expressions of subjective equivalence between sound and vibration will permit a more accurate diagnosis of the sources of human discomfort. © 2005 Elsevier B.V. All rights reserved.
