Equivalent sensation curves of simultaneous lateral and vertical sinusoidal whole-body vibration.

Background: To evaluate the effects of whole body vibration on discomfort, the sensitivity to different frequencies as well as to different vibration axes must be regarded. This experimental study investigates the significance of lateral relative to vertical motions in the sensation of dual axis sinusoidal vibrations in sitting posture. The results are discussed in the context of the evaluation procedure proposed in the international standard ISO 2631 (13).

Methods: The experiments, in which 31 volunteers (16 female, 15 male, 19 51 yr) participated, used the method of adjustment. The subjects compared a single axis reference motion (aw = 1.25 ms(-2) rms) with a dual axis test motion of the same frequency. The magnitude of the test signal's component along the reference axis was kept constant at a fractional level of the reference magnitude (10, 25, 50, 75 or 90%). The component perpendicular to the reference axis was adjusted by the subjects until the test signal was rated as being equally strong as the reference. Both vertical and lateral reference motions were applied. The frequencies used were 1.6, 3.15, 6.3 and 12.5 Hz.

Results: The shape of the resulting right-downwards bent equivalence curves was reasonably well fitted using the frequency weightings and evaluation procedure of ISO 2631. However, there were considerable quantitative discrepancies for frequencies above 1.6 Hz with an underestimation of the effects of lateral vibrations by a factor of 1.5-2. Therefore, it is concluded that lateral vibrations above 1.6 Hz need more weight in the evaluation of discomfort caused by multi-axis whole body vibrations.