1,147 research outputs found
Effect of seating on exposures to whole-body vibration in work-vehicles
No full textThe vibration isolation efficiency of seating has been evaluated in 100 work vehicles in 14 categories (cars, vans, lift trucks, lorries, tractors, buses, dumpers, excavators, helicopters, armoured vehicles, mobile cranes, grass rollers, mowers and milk floats). Seat isolation efficiency, expressed by the SEAT value, was determined for all seats (67 conventional seats and 33 suspension seats) from the vertical acceleration measured on the floors and on the seats of the vehicles.For most categories of vehicle, the average SEAT value was less than 100%, indicating that the average seat provided some attenuation of vibration. However, there were large variations in SEAT values between vehicles within categories. Two alternative vibration frequency weightings (Wb from BS 6841, 1987; Wk from ISO 2631, 1997) yielded SEAT values that differed by less than 6%. Overall, the SEAT values determined by two alternative methods (the ratio of r.m.s. values and the ratio of vibration dose values) differed by less than 4·5% when using weighting Wb, although larger differences may be expected in some situations. The median SEAT value for the suspension seats was 84·6%; the median SEAT value for the conventional seats was 86·9% (based on weighting Wb and the ratio of r.m.s. values).Predicted SEAT values were obtained assuming that each seat could be interchanged between vehicles without altering its transmissibility. The calculations suggest that 94% of the vehicles investigated might benefit from changing the current seat to a seat from one of the other vehicles investigated. Although the predictions are based on assumptions that will not always apply, it is concluded that the severity of whole-body vibration exposures in many work environments can be lessened by improvements to seating dynamics
The transmission of translational seat vibration to the head-I. Vertical seat vibration
No full textVibration in the three translational (fore-and-aft, lateral and vertical) and the three rotational (roll, pitch and yaw) axes of the head has been measured during exposure to whole-body random vibration. Using an instrumented bar gripped between the teeth, the influence of variations in bite grip and bite-bar mass on movements of the head were found to be small up to a mass of 375 g. The repeatability of measures of seat-to-head transmissibility within a single subject and the variability in transmissibility across a group of twelve subjects have been determined with two seating conditions: a rigid seat with a backrest and the same seat with no backrest. Seat-to-head transmissibilities associated with vertical seat vibration are presented at frequencies up to 25 Hz for all six axes of head vibration both with and without a backrest. Head motion occurred principally in the fore-and-aft, vertical and pitch axes of the head. The backrest increased the magnitude of head vibration in most cases. Intra-subject variability was generally small compared to inter-subject variability.</p
Transmission of yaw seat vibration to the head
No full textThe transmission of yaw-axis vibration to the heads of seated subjects has been investigated at frequencies below 5 Hz. The variability between and within subjects and the effects of backrest contact, visual environment and the position of the centre of rotation have been investigated. The subjects sat on a rigid flat seat and were exposed to random motion at a magnitude of 1·0 rad/s2r.m.s. (root-mean-square) for 2 min. Head motion was measured in six axes using a light-weight bite-bar held between the teeth. Twelve male subjects participated in a study of the effect of backrest contact and visual conditions and one male subject participated in a repeatability study. A "back-on" posture (subject's back in contact with the seat backrest) increased the frequency of maximum transmissibility from 2 to 3 Hz compared with a "back-off" posture. There was little change in transmissibility with the subjects sitting with their eyes open compared to their eyes closed. With increasing separations between a subject and the centre of rotation (at six distances from 0 to 500 mm with the subject facing outwards) there were large increases in lateral acceleration at the head
Attenuation of airborne noise by wet and dry neoprene diving hoods
No full textThe insertion losses of five neoprene diving hoods of varying thicknesses (2 mm–9 mm) were measured in one-third octave bands using a Kemar manikin in a diffuse broadband noise field. The insertion losses were measured in air for both dry and wet hoods. The insertion loss was calculated as the sound level in each frequency band measured with the hood, minus the corresponding sound level measured without the hood. The insertion losses were similar for both ears of the manikin. Both wet and dry hoods neither attenuated nor amplified sound below 250 Hz. Between 315 Hz–1250 Hz, the insertion loss of each hood was negative, displaying a broad resonance with a gain of 6–8 dB. In this frequency range the hood acts as a mass-spring system, resonating like a drum skin when stretched over the ears. Above 1000 Hz, the insertion loss increased with frequency (10 dB per octave), reach-ing a maximum of 5000 Hz–6000 Hz. Wetting each hood did not significantly affect the insertion loss; the ‘drum-skin’ resonance frequency was marginally lower with a wet hood, and insertion losses may be marginally greater between 1000 Hz– 10 000 Hz. The resonance frequency decreased with increas-ing thicknesses of hood, and the insertion loss at frequencies above the resonance increased with hood thickness.</p
The transmission of translational seat vibration to the head-II. Horizontal seat vibration
No full textThe second part of this study of the six axes of head motion caused by translational seat vibration is concerned with the effect of fore-and-aft (x-axis) and lateral (y-axis) seat vibration. Seat-to-head transmissibilities have been determined at frequencies up to 16 Hz for each of the three translational and three rotational axes of the head during exposure to random vibration of the seat. Repeatability measures within a single subject and studies of the variability across a group of twelve subjects have been conducted with two seating conditions: a rigid seat with a backrest, and the same seat with no backrest. Fore-and-aft seat motion mainly resulted in head motion within the mid-sagittal plane (x-z plane). Without the backrest, transmissibilities for the fore-and-aft, vertical and pitch axes of the head were greatest at about 2 Hz. The backrest greatly increased head vibration at frequencies above 4 Hz and caused a second peak in the transmissibility curves at about 6 to 8 Hz. Lateral seat motion mainly caused lateral head motion with a maximum transmissibility at about 2 Hz. The backrest had little effect on the transmission of lateral vibration to the head. For both axes of excitation inter-subject variability was much greater than intra-subject variability.</p
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