## 2 Methods
### Procedures  
Following consent, testing was performed in the mother's hospital room while the newborn was swaddled in their bassinet or occasionally in a parent's arm. Despite only one family per hospital room, the testing environment was not ideal as family members and nursing staff would come and go, visitors did not always abide by the request to keep noise at a minimum, and no incentives were given to assist in the data collection process. Thus, the test team had little control over the test environment. Newborns were tested while awake and quiet or during natural sleep and testing took approximately 20 to 40 min. Stimulus parameters were chosen to mimic typical clinical procedures. DPOAE testing was conducted at discrete frequencies (f\({}_{2}\) = 2, 4, 8, 10, 12, 14, and 16 kHz) using an f\({}_{2}\)/f\({}_{1}\) = 1.22. Stimulus levels were L\({}_{1}\)/L\({}_{2}\) = 62/52 dB FPL and 65/55 dB SPL depending on the method chosen for calibration, FPL or SPL. At the start of the session, adequate probe insertion was assessed by determining if the probe was leaking and if a \(\frac{1}{2}\) wave resonance of greater than 8 kHz, typical for an adult ear with a shorter ear canal length, was measured. SysRes software was used to ensure the probe was not leaking by presenting a chirp stimulus and examining the frequency response. If a drop in pressure was noted below 1 kHz, a leak was suspected and the probe was reinserted. The \(\frac{1}{2}\) wave resonance of the newborn ear canal was recorded using EMAV software and a chirp stimulus. Measured resonances ranged between 9.5 and 11.6 kHz. Once an adequate probe insertion was obtained, discrete frequency sweeps (7 frequencies) were collected starting with SPL and ending with FPL calibration techniques. Two authors were present for data collection. One operated the equipment while the other monitored the probe in the newborn's ear. If the probe shifted, data collection was paused until probe placement resulted in the same \(\frac{1}{2}\) wave resonance as initially measured. Typically, it took under two minutes to collect the data for each discrete frequency sweep. Following the discrete frequency sweep measurement with SPL and FPL calibrations, the probe was removed completely and the testing was repeated, first ensuring adequate probe placement with \(\frac{1}{2}\) wave resonances matched from the previous recording. Thus, test-retest repeatability for each calibration method was assessed by completely removing the probe from the newborn's ear canal and reinserting, resulting in a total of four trials (two discrete frequency sweeps using dB FPL and two discrete frequency sweeps using dBSPL). If time permitted and the newborn remained quiet, SOAEs were collected two times, but without probe removal.  
Initial testing occurred using the smallest silicone eartip (3-5 mm Flanged Sanibel Red Silicone) available for use with the ER10X probe, herein referred to as red eartip. Upon initial evaluation of the data, it was realized that many newborns had DPOAEs that would not be classified as present, see data analysis, in comparison to a pilot study using the ER10B+ and a 3 or 4 mm silicone eartip. It was determined that the red eartip used in the current study was slightly different than the one used previously with the ER10B+. After consultations with those that measure DPOAEs in newborns (personal communication [PERSON] & [PERSON], 2018), we chose to modify a yellow silicone eartip (3 mm Flanged Sanibel Yellow Silicone), herein referred to as yellow eartip. The yellow eartip was modified by trimming the base to allow a snug fit on the ER10X probe. A deeper and tighter insertion in the ear was achieved with the smaller yellow eartip. See Figure 1 for a photographic reproduction of the eartips used in this study.