Study on the relationship between unpleasantness and perception of vibration in the head of subjects exposed to low-frequency noise

Induction of vibratory sensation is one of the unique characteristics of low-frequency noise, and the vibratory sensation could be a significant contributor to unpleasantness of persons exposed to low-frequency noise. We previously found that the head was the most sensitive part of the body to vibratory sensation induced by low-frequency noise, and have been studying the perceptual characteristics of "vibration perceived in the head". The aim of the present study was to examine the relationship between the "vibration perceived in the head" and the subjective unpleasantness of persons exposed to low-frequency noise. Subjects were exposed to 6 low-frequency pure tones (16, 20, 25, 31.5, 40, and 50 Hz) under 2 background noise conditions (with/without 35 dB(A) pink noise). Hypothesizing that the unpleasantness of a person in performing a mental task while being exposed to low-frequency noise could be categorized as either of 4 grades ("not annoying", "slightly annoying", "very annoying", and "too annoying to work"), we measured the sound pressure levels corresponding to 3 boundaries between the unpleasantness grades. For comparison, the hearing threshold levels, the threshold levels for "vibration perceived in the head", and the threshold levels for unpleasantness were also measured under the same experimental conditions. As a result, the threshold levels for unpleasantness were higher than those for "vibration perceived in the head" at all test frequencies. But the frequency-dependences in the two threshold levels were very similar to each other. In addition, the boundary level between "not annoying" and "slightly annoying" under the "No B.G.M." condition was close to the threshold level for "vibration perceived in the head". These results supported our basic hypothesis that the perception of "vibration perceived in the head" contributes to the subjective unpleasantness.