The growth and recovery of vibrotactile TTs caused by hand-transmitted repetitive shocks of various waveforms.

The effect of shock repetition rate on the temporary threshold shifts (TTSs) in vibrotactile perception at the fingers has been studied with shocks having exponential decays and hanning waveforms. The repetition rate of shocks delivered at the hand varied from 2 shocks per second up to 64 shocks per second. Subjects were exposed to vertical hand-transmitted repetitive shock vibration of equal energy contents. The magnitudes of the stimuli were adjusted to 2.8 ms-2 r.m.s. so that they were all equally severe according to current standards for assessing the severity of hand-transmitted vibration (e.g. International Standard 5349, 1986). It was found that the TTS in vibrotactile perception decreased with decreasing shock repetition rate for both waveforms. The relation between the TTS, the logarithm of exposure time, and the logarithm of the ratio between the frequency-weighted r.m.s. acceleration and the peak amplitude was described by the relation TTS = 16.1 + 3.9log10T + 17.9log10 (RMS/PEAK), where TTS is the temporary threshold shift (in decibels), T is the exposure time, and RMS is the frequency-weighted r.m.s. acceleration according to BS 6842 and ISO 5349, and PEAK is the peak amplitude of the frequency-weighted acceleration. This shows that current standards are not appropriate for the prediction of the changes in vibrotactile perception produced by shocks such as those on many hand-held percussive tools.