Human Response to Vibration

Dumper operators are exposed to whole body vibration (WBV) in the course of their work. The exposure to WBV in a coal mine in Central India was investigated through measurement of the magnitude of vibration and exposure time. The vibration magnitude along the dominant Z-axis ranged from 0.644 to 1.82 m/s2 in terms of root mean square acceleration. When evaluated in conjunction with their average daily exposure time of 5 h, all dumpers caused elevated health risk for their operators according to guidelines in ISO 2631-1:1997 WBV standard. Health risks were re-affirmed when vibration dose value (VDV) was applied as an additional tool for risk assessment. Forty dumper operators and 20 controls from the same mine were selected for detailed studies. The control population was not exposed to WBV. An epidemiological study was carried out to determine the prevalence of musculoskeletal disorders (MSD) related to WBV exposure. It was observed that the problem of low back pain was significantly higher (85%) in the exposed population as compared to controls (20%). Similarly, pain in the ankle (37.83%), shoulder (30%) and neck (37.5%) was higher among exposed personnel as compared to the control population (5, 0 and 15%, respectively). A significant degradation in quality of life among the exposed subjects was also observed.

Previous vibrotactile research has provided little or no definitive results on the discrimination and identification of important pitch aspects for musical performance such as relative and absolute pitch. In this thesis, psychophysical experiments using participants with and without hearing impairments have been carried out to determine vibrotactile detection thresholds on the fingertip and foot, as well as assess the perception of relative and absolute vibrotactile musical pitch. These experiments have investigated the possibilities and limitations of the vibrotactile mode for musical performance.

Over the range of notes between C1 (32.7Hz) and C6 (1046.5Hz), no significant difference was found between the mean vibrotactile detection thresholds in terms of displacement for the fingertip of participants with normal hearing and with severe/profound hearing impairments. These thresholds have been used to identify an optimum dynamic range in terms of frequency-weighted acceleration to safely present vibrotactile music. Assuming a practical level of stimulation ≈10dB above the mean threshold, the dynamic range was found to vary between 12 and 27dB over the three-octave range from C2 to C5. Results on the fingertip indicated that temporal cues such as the transient and continuous parts of notes are important when considering the perception of vibrotactile pitch at suprathreshold levels.

No significant difference was found between participants with normal hearing and with severe/profound hearing impairments in the discrimination of vibrotactile relative pitch from C3 to C5 using the fingertip without training. For participants with normal hearing, the mean percentage of correct responses in the post-training test was greater than 70% for intervals between four and twelve semitones using the fingertip and three to twelve semitones using the forefoot. Training improved the correct responses for larger intervals on fingertips and smaller intervals on forefeet. However, relative pitch discrimination for a single semitone was difficult, particularly with the fingertip. After training, participants with normal hearing significantly improved in the discrimination of relative pitch with the fingertip and forefoot. However, identifying relative and absolute pitch was considerably more demanding and the training sessions that were used had no significant effect.