Magnitude-dependence of equivalent comfort contours for fore-and-aft, lateral and vertical hand-transmitted vibration

The strength of sensation produced by vibration applied to the glabrous skin of the hand varies with the magnitude, frequency, and direction of the vibration and the contact conditions. With groups of 12 subjects gripping a cylindrical handle, this experimental study investigated perception thresholds (in the frequency range 8–315 Hz) and the strength of sensation caused by each of the three axes of hand-transmitted vibration (in the frequency range 8–400 Hz) at vibration magnitudes from threshold up to levels associated with discomfort and injury. In all three axes, acceleration thresholds for the perception of vibration showed a U-shaped frequency-dependence with greatest sensitivity around 80–160 Hz. At supra-threshold levels, the frequency-dependence of the equivalent comfort contours in each of the three axes was highly dependent on vibration magnitude. With increasing vibration magnitude, equivalent sensation approximated towards constant velocity, whereas with decreasing magnitudes the sensation became similar to the absolute perception threshold. This magnitude-dependence of equivalent comfort contours suggests differential mediation of psychophysical channels responsible for perception at different vibration magnitudes. The results imply that no single linear frequency weighting can provide accurate predictions of subjective judgments of discomfort caused by hand-transmitted vibration.