## 4 Discussion
The testing environment for the current study also could have contributed to overall higher noise floors than previously recorded in clinical and patient rooms where it was only the tester and the patient [51; 53]. Often, family members in addition to the parents were present during testing, as were nursing staff. It was stressed that this was a \"hearing test,\" but often the quiet environment did not remain stable throughout testing. Still, as the noise floor was the least at the highest frequencies tested, further study is warranted to maximize DPOAE responses at frequencies up to 16 kHz in newborns. While a stimulus frequency ratio of 1.22 was used in the current study, narrower ratios have been proposed as generating larger DPOAE levels at higher frequencies in adults [5]. Additionally, higher stimulus levels result in larger DPOAE levels [48; 78]. So, if the goal is to generate a DPOAE at the highest frequencies possible for monitoring purposes, stimulus parameters required to maximize HF DPOAE recordings in newborns need to be determined as this can in turn benefit repeated recordings. To determine if HF DPOAE levels are repeatable in newborns, initial measurements were made and then the probe was removed and reinserted. Probe removal has been reported to be the greatest source of variability in DPOAE measures [35]. For monitoring purposes, several measurements are made over time and compared to a baseline measure. One metric used for making comparisons across trials is to examine the average absolute differences between trials. In the current study, average absolute differences between trials collapsed across frequency (2-16 kHz) were 3.64 (SD = 3.27) and 3.59 (SD = 2.84) for FPL and SPL calibrations, respectively. These results in newborns are similar to those reported in children aged 3 to 6 and 10 to 12 years collapsed across frequency (2-16 kHz) with average absolute differences between trials of 3.56 (SD = 3.68) and 3.37 (SD = 3.73), respectively [49; 52], utilizing an alternate calibration technique to better control stimulus levels. If only frequencies between 10 through 16 kHz are examined for newborns, the average absolute difference between trials was 2.82 (SD = 2.68) for FPL and 3.43 (SD = 2.62) for SPL calibrations. These results are less variable than for young, normal-hearing adults using SPL calibration [5] and more variable than cystic fibrosis patients monitored betweenotoxic treatments using alternate calibration techniques [51] with average absolute differences between trials of 5.15 (SD = 4.40) and 1.96 (SD = 2.19), respectively. It should be noted that the newborns in this study only had DPOAEs measured in one session compared to the other literature referenced for comparison which involved data collection minimally across several hours, but more often at least two different days. Previously, the calibration method used has been shown to influence DPOAE repeatability measured in children, normal-hearing adults, and patients, but is inconclusive in newborns. However, FPL did not perform any worse than SPL calibrations, as overall DPOAE levels, noise floors, and average differences between trials for all frequencies were similar for both methods. This warrants further exploration using FPL calibration techniques in newborns, especially at frequencies known to be affected by standing waves. Despite the inconclusive results in newborns in regard to repeated measurements benefiting from using FPL calibration (Figure 3), the results are promising and merit further study, especially at higher frequencies. Average absolute differences between trials for 12 through 16 kHz were 2.31 (SD = 2.57) and 3.87 (SD = 2.82) for FPL and SPL, respectively. [PERSON] et al. [24] reported that SPL calibration is less able to overcome errors related to probe placement in an enclosed ear canal. For adults, standing wave effects created by closing off the ear canal result in alterations of DPOAE levels for frequencies between 3 and 7 kHz [5].