Hearing thresholds for pure tones from 2 kHz to 28 kHz were measured. A 2AFC procedure combined with a 3-down 1-up transformed up-down method was employed to obtain threshold values that were less affected by listener's criterion of judgment. From some listeners, threshold values of 88 dB SPL or higher were obtained for a tone at 24 kHz, whereas thresholds could not be obtained from all participants at 26 kHz and above. Furthermore, thresholds were also measured under masking by a noise low-pass filtered at 20 kHz. At frequencies above 20 kHz, the difference of threshold values between with and without the masking noise was a few decibels, indicating that the tone detection was not affected by subharmonic components that might have appeared in the lower frequency regions. The results of measurement also showed that the threshold increased rather gradually for tones from 20 to 24 kHz whereas it increased sharply from 14 to 20 kHz.

The statistical distribution of normal hearing thresholds for pure tones of frontal incidence under binaural, free-field listening conditions was estimated as a function of frequency. First, the form of threshold distribution was investigated with threshold measurement data of the present study and those of other studies. Analytical results indicate that the threshold distribution has a form of normal distribution for the frequency range from 25 Hz to 16 kHz. Second, under the assumption of normality, standard deviations of thresholds were calculated for the frequency range from 25 Hz to 18 kHz by combining available threshold data of different studies. A supplementary experiment showed that thresholds at frequencies above 16 kHz were measurable with high reliability. These results illustrate the profile of our auditory sensitivity more accurately because they account for individual differences.

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The hearing threshold for low frequency (LF) tones was measured in a pressure field to investigate the effects of ageing on hearing sensitivity. Participants were young adults around 20 years old and older adults over 60. Measurement results showed that the older listeners had a higher threshold, on average, than the young listeners. The difference of median thresholds between these two groups was about 10 dB at every measurement frequency. Furthermore, hearing abnormalities other than age-related hearing loss showed no great effect upon the LF thresholds. Comparison of LF thresholds and audiograms (i.e. mid and high frequency hearing) of the older listeners revealed only moderate correlation between them. These results suggest that older people retain good hearing sensitivity in the LF region, in contrast to their often-degraded sensitivity at higher frequencies. Therefore, in LF noise evaluation, we should carefully regard the possibility that older listeners can perceive a low level LF noise.