Frequency-dependence of psychophysical and physiological responses to hand-transmitted vibration.

This invited paper reviews experimental studies of the frequency-dependence of absolute thresholds for the perception of vibration, equivalent comfort contours, temporary changes in sensation caused by vibration, and reductions in finger blood flow caused by hand-transmitted vibration. Absolute thresholds depend on the contact conditions but for a typical hand grip the thresholds show greatest sensitivity to acceleration around 125 Hz. The frequency-dependence of discomfort caused by hand-transmitted vibration depends on vibration magnitude: similar to absolute thresholds at low magnitudes, but the discomfort at higher magnitudes is similar when the vibration velocity is similar (at frequencies between about 16 and 400 Hz). Hand-transmitted vibration induces temporary elevations in vibrotactile thresholds that reflect the sensory mechanisms excited by the vibration and are therefore highly dependent on the frequency of vibration. Hand-transmitted vibration reduces finger blood flow during and after exposure; when the vibration velocity is similar at all frequencies there is more vasoconstriction at frequencies greater than 63 Hz than at lower frequencies. A single frequency weighting cannot provide a good indication of how all effects of hand-transmitted vibration depend on vibration frequency. Furthermore, a single frequency weighting provides only an approximate indication of any single response, because many factors influence the frequency-dependence of responses to hand-transmitted vibration, including the magnitude of vibration, contact conditions, and individual differences. Although the frequency weighting in current standards extends from 8 to 1,000 Hz, frequencies greater than 400 Hz rarely increase the weighted value on tools and there is currently little psychophysical or physiological evidence of their effects.