The effect of phase of differential vertical vibration at the seat and feet on discomfort

This experiment investigated whether the discomfort of seated subjects exposed to vertical vibration was influenced by the relative phase between vibration at the seat and the feet. Twelve seated subjects were exposed to sinusoidal 4 Hz vibration by means of two vibrators, one under the seat and the other under a footrest. A total of seventy combinations of vibration stimuli with seven phases (0, 30, 60, 90, 120, 150, 180 degrees) between the seat and the footrest and five acceleration levels (0.25, 0.4, 0.63, 1.0, 1.6 ms(-2) r.m.s.) were presented to subjects in two postures (with and without thigh contact with the seat). The subjects judged that the differential vibrations with greater phase difference caused greater discomfort. The subjects were most sensitive to phase changes at low magnitudes of vibration and with thigh contact. In the equation, psi = K phi(n), between the discomfort, psi, and the magnitude of vibration, phi, the exponent n had a maximum of 1.48 with thigh contact and 1.24 with no thigh contact, and decreased to approximately 1.0 with increasing phase between the seat and the feet. The results indicate that vibration discomfort is influenced by the phase between the seat and the feet, but that the effect depends on the magnitude of vibration and the posture of the body. (C) 1999 Academic Press.