Reliability of pure-tone thresholds at high frequencies

Standing waves, in the ear canal are created by the interference of two plane waves of the same frequency traveling in opposite directions. One manifestation of this phenomena is that any lateral motion of the transducer produces a change in the length of the canal and therefore, in general, a change in the sound level at the eardrum. Because of the smaller wavelength at the higher frequencies, the changes produced by movement of the transducer will be larger for the higher frequencies than for the lower frequencies. Thus, when the transducer is removed and replaced between successive threshold estimates, it is expected, from physical principles alone, that the intrasubject reliability of the estimates should be better at the lower frequencies. Previous studies, however, have shown that the intrasubject reliability of threshold estimates is nearly the same at both the low (0-8 kHz) and the high (8-16 kHz) frequency regions. A possible explanation for these anomalous results is proposed. While it is found that transducer placement increases the variability of threshold estimates more at the higher frequencies, it has also been found that conventional headphones reduce the size of the standing wave ratios compared with a hard wall termination. An additional factor is the steeper slope of the psychometric function at the higher frequencies. The combination of these factors produces a standard deviation for threshold estimates that is only about 1 dB larger at the high than at the low frequency region.