Human body surface vibrations induced by high-level low-frequency pure tones were measured at the chest and the abdomen. At the same time, the subject rated the unpleasantness that he had just perceived during the exposure to low-frequency noise stimulus. Examining the relationship between the measured vibration and the rating score of the unpleasantness revealed that the unpleasantness was in close correlation with the vibration acceleration level (VAL) of the vibration measured. Taking previous results into account, this finding suggests that noise-induced vibrations primarily induce vibratory sensations and through the vibratory sensation or together with some other factors, secondarily contribute to the unpleasantness. The present results suggest that in evaluating high-level low-frequency noise, the effect of vibration should be taken into account.
To understand the basic characteristics of the human body vibration induced by low frequency noise and to use it to evaluate the effects on health, we designed a measuring method with a miniature accelerometer and carried out preliminary measurements. Vibration was measured on the chest and abdomen of 6 male subjects who were exposed to pure tones in the frequency range of 20 to 50 Hz, where the method we designed was proved to be sensitive enough to detect vibration on the body surface. The level and rate of increase with frequency of the vibration turned out to be higher on the chest than on the abdomen. This difference was considered to be due to the mechanical structure of the human body. It also turned out that the measured noise-induced vibration negatively correlated with the subject's BMI (Body Mass Index), which suggested that the health effects of low frequency noise depended not only on the mechanical structure but also on the physical constitution of the human body.
The thresholds of perception of vibration by recumbent men are a significant index for the governmental agency concerned with regulation of vibration exposure. The thresholds of perception of continuous sinusoidal vibrations and single and multiple bursts of sinusoidal vibrations in the horizontal and vertical directions were investigated in recumbent men. We found that the threshold curves, as a function of the frequency, had different characteristics for vertical vibrations than for horizontal vibrations in the recumbent position. The threshold values for pulsed vibrations became smaller (easier to perceive) as the bursts were elongated until saturation occurred at 4 s duration. Threshold values for pulse durations longer than 4 s were the same as those for continuous vibration. This suggests the existence of temporal integration of sensation of vibrations as in psychoacoustics.
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