Subjective effects of combined-axis vibration: II. Comparison of X-axis and X-plus-pitch vibrations

Seated subjects matched their perceptions of the intensity of single-axis vibrations in the X axis, or combined-axis vibrations made up of X-axis and pitch 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 vibration were presented at three acceleration levels related to three axis-to-seat distances for the pitch vibrations. Results showed that Z-axis response accelerations were essentially constant across frequency. However, matching responses were significantly higher for X-plus-pitch than for X-axis vibrations. These findings are in contrast to those of a previous experiment involving Y-axis and roll vibrations, and are probably due to additional input from the seat back for X and pitch motions. The two experiments do agree on the importance of the distance of the subject from the axis of rotation for angular motions. In both experiments, as stimulus acceleration (axis-to-seat distance) increased, response acceleration increased substantially at every frequency.