## 4 Discussion  
Previously, HF (up to 16 kHz) DPOAEs have been measured in normal-hearing children [48; 49; 52] and young adults [5; 48; 50; 78]. However, DPOAE levels are reduced at higher frequencies compared to conventional frequencies and are not always present for every individual at the highest frequencies tested [5; 48; 50; 78]. Newborns compared to children and adults have different ear canal and middle ear characteristics [8; 15], which can influence DPOAE measures. Thus, it was unknown if HF DPOAEs could be evoked in newborns and if newer calibration techniques that offer more stability and less variability would influence measurements. DPOAEs up to 16 kHz are measurable in some newborns and the average DPOAE levels are comparable across frequencies for both calibration methods and trials and decline at frequencies greater than 8 kHz (Figure 2). Declining DPOAE levels as frequency is increased have been reported previously in term newborns, children, and adults [5; 7; 8; 48; 49; 50; 52]. Unlike previous studies which measured DPOAEs out to 16 kHz and demonstrated the largest DPOAE levels occurring around 4-5 kHz in children (\(\geq\)3 years) and adults [5; 48; 50; 52], the newborns in this study showed a peak DPOAE level at 8 kHz. This peak in DPOAE level occurring at a higher frequency in newborns, no matter which calibration method was used, could be reflective of non-adult-like outer and middle ears. However, given the unequal number of data points across the frequencies, further evaluation is needed to determine if this trend remains in a larger sample size. In term newborns, [PERSON] and [PERSON] [8] report DPOAE levels peaking around 1 kHz (17 dB SPL) and declining as frequency increased to approximately 6 dB SPL slightly less than 4 kHz. In addition, [PERSON] et al. [7] report mean DPOAE levels of 13.6 and 9.5 dB SPL at 2 and 4 kHz, respectively, declining to 5.8 dB SPL at 8 kHz. Mean DPOAE levels in the current study at 2 and 4 kHz were less than 5 dB SPL, whereas levels at 8 kHz approached 5 dB SPL. Thus, it might not be that our data peaks at 8 kHz, but that our data declines at 2 and 4 kHz compared to other term newborns and this should be explored further. The average noise floor is greatest at the lowest frequencies tested and declines at the highest frequencies tested, similar to previous reports [7; 48; 49; 52]. The higher noise floor at the lowest frequencies tested would hinder the identification of lower level DPOAEs at these frequencies. When data were deemed unusable because the criteria for a present DPOAE were not met, the predominant reason for not meeting the inclusion  
Figure 5: An example of spontaneous otoacoustic emissions (SOAEs) across frequency recorded from one newborn. There were 10 repeatable SOAEs identified for this newborn. Trial 1 is shown in red and trial 2 is shown in black. criteria at the lowest frequencies tested was due to SNR values less than 6 dB and with elevated noise floors it is difficult to obtain high SNR values. The fewest data points for analysis did occur at the lowest and highest frequencies tested. At the highest frequencies evaluated, both SNR and decreased DPOAE levels were responsible for data being classified as not present. To illustrate this point further, if the average noise floor for one trial and one calibration method for all 26 newborns were calculated at 16 kHz and then all data for that trial and calibration method were compared to the average noise floor not the individual newborn's noise floor, approximately six more newborns would have had present DPOAEs. While the average noise floors are not too high in those newborns with present DPOAEs (Figure 2), individual noise floors for each newborn could have had a spike that deemed the data not present. In adults and children at the highest frequencies tested (up to 16 kHz), average noise floors are less and DPOAE levels are higher than were recorded in the current study [5; 48; 50; 52].