Subjective effects of combined-axis vibration: III. Comparison of Y-axis and Y-plus-yaw vibrations.

Two psychophysical matching experiments (Exps. III and IV in a series) were conducted to compare the perceived intensity of Y-axis and Y-plus-yaw vibrations. Seated subjects matched their perceptions of the intensity of single-axis vibrations in the Y-axis, or combined-axis vibrations made up of Y-axis and yaw motions, by adjusting the intensity of a sinusoidal, 5 Hz, Z-axis response vibration. Stimulus vibrations were sinusoidal at 3.15, 4, 5, 6.3, and 8 Hz. For each frequency, both types of stimulus vibrations were presented at three acceleration levels related to three axis-to-seat distances for the yaw motions. In Exp. III the subjects were seated forward of the rotation axis, and in Exp. IV they were seated aft of the axis. For both experiments the results showed that as frequency increased the acceleration of the matching response decreased for both types of stimuli. In addition, as stimulus acceleration (axis-to-seat distance) increased, response acceleration showed substantial increases at every frequency. However, the effects of vibration type were dependent on the location of the subject. In Exp. III the combined-axis stimuli produced higher response accelerations, but in Exp. IV matching responses were the same for both vibration types.