Frequency-resolution measurements with notched noises for clinical purposes

Knowledge of the frequency resolving power of the pathological ear can be helpful for good hearing-aid prescription. In particular, the slope of the low-frequency edge of the auditory filter will determine the extent of upward spread of masking. The most precise method for measuring this filter is the "notched-noise" procedure developed in the early 1980s. However, at this moment, the notched-noise protocol is too time consuming for clinical use. In this paper we developed a protocol that is applicable in the clinic. In the first part of the study we investigated the minimum number of threshold measurements necessary for a reliable estimation of the filter parameters. For this purpose we analyzed 99 filters originally obtained with 13 threshold measurements. The influence of reducing the number of notch widths used to obtain filter shapes on the variability of filter parameters was investigated. Subsets of seven and five notch widths gave about the same standard deviations of the differences between the parameters obtained with the subset and the parameters obtained with the full set of 13 thresholds, while subsets of four and three notch widths gave significantly higher variability. However, because small deviations of one threshold determination can give rise to a large change in filter parameters, especially for filters with flatter skirts, it is strongly recommended to reduce the variability by measuring thresholds twice (test and retest). In the second part of the study we investigated a faster method for measuring thresholds. The forced-choice paradigm normally used in notched-noise procedures was replaced by a Békésy paradigm. The Békésy paradigm did not significantly increase intra-individual standard deviations, but did reduce the measuring time by more than 50%. In conclusion, the new procedures reduce the measurement time needed to obtain reliable estimates of auditory-filter parameters by a factor of about 5. The new protocol lasts about 15 min for each filter measurement, which appears to be acceptable for clinical use, at least for difficult-to-fit hearing aid users.