Effects of Fixed Versus Variable Reference Frequencies on Psychophysical Judgments of Vibration

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Abstract
Equal magnitude contours for vibration were obtained by two methods from ten seated subjects exposed to sinusoidal vibrations in three axes. In the traditional “fixed reference frequency” method, subjects were asked to adjust the amplitude of vibration in order to match for equal vibratory magnitude of a variable stimulus (of from 1 to 30 Hz) to a fixed reference vibration stimulus. The reference stimulus corresponded to the ISO 1–4 min “reduced comfort boundary” level; i.e., 6 Hz at 0.9 m/s2 (rms acceleration) for vertical (±gz) axis and 1 Hz at 0.62 m/s2 rms acceleration for the lateral (±gy) and longitudinal (±gx) axes. In the modified “progressive matching” method, subjects were exposed initially to the same reference stimulus and matched the adjoining stimulus vibratory frequency magnitude to it. Then, however, the newly matched vibration frequency was used as the next reference stimulus, the subjects matched the adjoining frequency magnitude to this “new” reference stimulus, etc. over the same range of frequencies (from 1 to 30 Hz). The advantages and shortcomings of the two methods are discussed.

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