The apparent mass of the seated human body in the fore-and-aft and lateral directions

Abstract

Apparent mass frequency response functions of the seated human body have been measured in the fore-and-aft and lateral directions using random vibration in the frequency range 0.25–20 Hz. The measurements were made with a rigid seat so as to measure the horizontal force transmitted to the body. Eight subjects were used to investigate the apparent mass of the seated body with and without backrest contact for a range of three vibration magnitudes. The body appeared to have two heavily damped modes of vibration when there was no backrest contact. The first mode had a resonance frequency at about 0·7 Hz for both the fore-and-aft and the lateral directions. The second mode was less pronounced than the first, particularly for the lateral direction, and had a resonance frequency in the region of 1.5–3 Hz. The resonance frequency of the second mode of vibration, but not the first, decreased with increasing magnitude of vibration. There appeared to be only one mode of vibration when the motion of the upper body was restrained with a backrest, with a resonance frequency in the region of 3·5 Hz for the fore-and-aft direction and at about 1·5 Hz for the lateral direction. The effect of the backrest was particularly pronounced for the fore-and-aft direction.