If the physical characteristics of several sounds or subjects' psychological factors differ, the unpleasantness that subjects feel towards a sound seems to differ even if the loudness of the sound are equal. Such unpleasantness of sound has been shown to provoke several kinds of stress responses. The present study attempt to examine this.
For the study, the unpleasantness of four kinds of sounds with equal-A-weighted sound levels (70dB(A)) was quantified. The noises included that of scratch a fryingpan with a metal ladle (Fryingpan Noise: FN), scratch a flosted glass with a metal ladle (Glass Noise: GN), a synthesizer (Synthesizer Noise: SN) and white noise (WN). Psychophysical methods of quantification included the rating scale and the paired comparisons. The order of unpleasantness was FN>GN>WN>SN or the most unpleasant was FN.
EEGs were then recorded during a 30-minute exposure to sound of FN, WN, SN stimuli, respectively. At rest, with the eyes closed, 40 minute EEGs were recorded for the control (CTRL). Changes in EEGs due to unpleasantness were estimated by FFT spectral analysis and the usefulness of certain spectral parameters was examined.
Changes in EEG were found to depend not on the kind of stimulus, but on the subjects' psychological responses depending on whether they regarded a stimulus as unpleasant or not. When a subject indicated discomfort, the alpha wave decreased and slow waves increased. Parameters such as log (%
x/(100-%
x)) [%
x: relative power in
x-band] were deemed excellent representation of changes in EEG.
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