## 4 Discussion  
This work has revisited some basic properties of HT and DPOAEs, with a focus on EHF. The novel aspect is the acquisition of DPOAEs in the 10-16 kHz (EHF) range using a commercial device, as opposed to existing studies which have relied mostly on custom equipment [33, 37]. The main finding here is that DPOAEs, for both the SF and EHF bands, decrease as thresholds become worse. In both bands, DPOAEs and HTs depend strongly on age, even for those subjects who are classed as having a normal SF audiogram. However, for the EHF band, responses seem to be especially affected by age. Finally, it was found that there seem to be only minor (and not significant) differences in HTs and DPOAEs in terms of either gender or left versus right ear, with the only significant factor affecting DPOAE response levels being the presence of SOAEs. This study has reproduced the well-known relationship between DPOAE level and HL, i.e., that DPOAE levels decrease as hearing becomes worse (e.g., [8, 34]). It has also shown that, even when hearing is within normal limits in the SF range, in the EHF range DPOAE levels shift downwards in parallel with poorer thresholds; for these subjects, a similar relationship holds in the SF range. Comparable findings have been seen in previous work (e.g., [11, 17, 18, 38]). In all our age groups, DPOAEs and HTs worsened with increasing age, a finding which is similar to previous studies (e.g., [39, 40]). This was true even in the G16 group, who had normal HTs over the whole tested range. The EHF range seems to be particularly affected by age, both for HTs and DPOAEs, as the correlation coefficients were higher for that range. Our data do not indicate whether the decline with age depends on age alone or whether additional factors like lifetime noise exposure are at play, as this aspect was not investigated here. Nevertheless, both HTs and DPOAEs in the EHF range may be particularly useful when investigating presbycusis. Previous studies have reported that there are significant differences in DPOAEs depending on ear and gender, i.e., higher levels for right ears and for women [41]. Although these tendencies can be seen in our dataset, they were not significant. They may be significant in a larger dataset, and they may be more pronounced with TEOAEs (e.g., [42]). In reporting gender differences, some older studies have described differences in DPOAEs for the SF range (e.g., [16, 41, 43]), whereas a more recent study reported no differences for the SF range but a significant difference for the EHF range [44]. The results obtained in the  
Figure 6: DPOAE response level and hearing threshold (HT) in group G16 when the group is split according to SOAE presence (SOAE+/SOAE-), gender (women/men), and ear side (right/left). Asterisk indicates a significant difference (\(p<0.05\)). present work fall more or less in the middle. A difference between men and women was apparent for SF, and widened for EHF, but was still not significant. As mentioned earlier, a unifying hypothesis could be that there is indeed a gender effect on DPOAEs, but it is small, and can only be distinguished with either very large datasets or data of high quality (as in [44]). We found that the effect of SOAEs on DPOAE response level was significant only for the SF range. This agrees with [PERSON] et al. [16]. In the EHF range, DPOAE response levels were slightly higher for ears with SOAEs than for those without, but the difference was not significant. Looking at HTs, there were no significant differences for either range between ears with or without SOAEs. Previous studies have shown some contradictory results in this area. Some have shown that in ears with SOAEs, HTs were better in the SF range than for ears without SOAEs but were no different in the EHF range [45]; other work has shown the opposite, i.e., better thresholds in the EHF range [16]. This discrepancy is hard to explain as the sizes of study groups were comparable between our work and the previously mentioned studies.