Thresholds for the perception of vibration of the back: effect of backrest inclination

Drivers, passengers, and patients in various forms of transport are supported by backrests at angles between the vertical and the horizontal. The effect of backrest inclination on the perception of vibration has not been previously reported, and there are no standardised guidelines that can be used to optimise the design of vehicles or seats so as to minimise the probability of perceiving vibration in these postures. This study was designed to determine the effect of backrest inclination on absolute thresholds for the perception of vibration applied normal to the back (i.e. fore-and-aft vibration with an upright backrest, and vertical vibration with a fully reclined backrest). Thresholds were determined using an up-and-down transformed response (UDTR) method at frequencies from 2.5 to 25 Hz at four backrest angles: 0 (upright), 30, 60 and 90 (recumbent). Inclining the backrest raised thresholds at frequencies between 4 and 8 Hz. It is concluded that the frequency weighting Wc provides a useful prediction of the perception of fore-andaft vibration with an upright backrest. Frequency weightings Wb or Wk might be used to predict the frequency-dependence of vibration perception with inclined backrests, but only at frequencies greater than about 8 Hz.