Apparent mass of the human body in the vertical direction: effect of seat backrest

Apparent mass frequency response functions of the seated human body have been measured with random vibration in the vertical direction at frequencies up to 20 Hz. A group of eight subjects was used to investigate some factors (footrest, backrest, posture, muscle tension, vibration magnitude) that may affect the apparent mass of a person; a group of 60 subjects (24 men, 24 women and 12 children) was used to investigate variability between people. Relative movement between the feet and the seat was found to affect the apparent mass at frequencies below resonance, particularly near zero-frequency. The resonance frequency generally increased with the use of a back rest, an erect posture and, in particular, increased muscle tension; but there was considerable intersubject variability in the changes. The magnitude of the vibration had a consistent effect: the resonance frequency decreased from about 6 to 4 Hz when the magnitude of the vibration was increased from 0.25 to 2.0 ms-2 r.m.s. The apparent masses of all the subjects were remarkably similar when normalized with respect to sitting weight. However, there were statistically significant correlations between apparent mass and some body characteristics (such as weight and age)

Laboratory methods for testing the performance of acoustic rail dampers

International audienceSeveral types of rail damper are now commercially available to reduce noise radiated by railway track. These dampers, attached to the rail between sleepers, increase the attenuation with distance of vibration transmitted along the rail (decay rate). Currently, there are no standardized procedures to measure the effectiveness of these dampers without the need for their installation in a track. Track decay rates of damped ‘freely’ supported rails have been measured using two proposed methods: (i) for short rails (4-6m length), at low frequency from the modal properties of the rail, and at high frequencies directly from point and transfer response functions (FRFs) at either end of the rail; and (ii) for long rails (15-32m), by integrating decay rates derived from FRFs measured at intervals along the rail. Results from four test institutes show generally good agreement between the two methods for three different damper designs. However, at some frequencies substantial inter- and intra- method variability is evident. Sources of this variability are identified and are discussed. Further tests conducted on a 32m test track show that decay rates of damped track can be reasonably determined by summing decay rates of the ‘free’ damped rail and those of the undamped track

A variable parameter single degree-of-freedom model for predicting the effects of sitting posture and vibration magnitude on the vertical apparent mass of the human body

Models of the vertical apparent mass of the human body are mostly restricted to a sitting posture unsupported by a backrest and ignore the variations in apparent mass associated with changes in posture and changes in the magnitude of vibration. Using findings from experimental research, this study fitted a single degree-of-freedom lumped parameter model to the measured vertical apparent mass of the body measured with a range of sitting postures and vibration magnitudes. The resulting model reflects the effects of reclining a rigid backrest or reclining a foam backrest (from 0 to 30 degrees), the effects of moving the hands from the lap to a steering wheel, the effects of moving the horizontal position of the feet, and the effects of vibration magnitude (from 0.125 to 1.6 ms–2 r.m.s.). The error between the modelled and the measured apparent mass was minimised, for both the apparent masses of individual subjects and the median apparent masses of groups of 12 subjects, for each sitting posture and each vibration magnitude. Trends in model parameters, the damping ratios, and the damped natural frequencies were identified as a function of the model variables and show the effects of posture and vibration magnitude on body dynamics. For example, contact with a rigidbackrest increased the derived damped natural frequency of the principal resonance as a result of reduced moving mass and increased stiffness. When the rigid backrest was reclined from 0 to 30°, the damping decreased and the resonance frequency increased as a result of reduced moving mass. It is concluded that, by appropriate variations in model parameters,a single degree-of-freedom model can provide a useful fit to the vertical apparent mass of the human body over a wide range of postures and vibration magnitudes. When measuring or modelling seat transmissibility, it may be difficult to justify an apparent mass model with more than a single degree-of-freedom if it does not reflect the large influences of vibration magnitude, body posture, and individual variability