## 1 Introduction
Given that ear canal lengths and measurement probe insertions vary, the sound pressure discrepancy occurs at frequencies greater than 2 kHz and a single correction factor cannot be applied [5; 22]. Thus, alternative calibration methods have been explored to combat the issues of standing waves to provide better control over stimulus and emission levels [24]. Forward pressure level (FPL) is one such alternative where, in theory, the incident pressure is isolated allowing less susceptibility to standing waves in an enclosed ear canal, which in turn provides better accuracy of stimulus levels and consistent measurements [24; 25; 26]. With FPL, individual ear canal properties are accounted for while avoiding influences of reflected pressure from the eardrum [26]. It is suggested that FPL can be performed in any size ear canal and used to quantify stimulus levels up to 18 kHz [26]. To achieve FPL calibration, the incident and reflected signals in the ear canal need to be differentiated. So, the impedance characteristics of the source (measurement probe) and load (ear canal and eardrum) need to be determined. Using the Thevenin-equivalent, a simplification of a complex electrical circuit, both source pressure and impedance of the measurement probe is determined by presenting a wideband chirp stimulus to at least two known acoustic loads (e.g., cylindrical cavities of varying lengths). Then, the impedance of the load can be determined which allows the incident and reflected waves to be segregated. Technical and computational details pertaining to FPL calibration can be found in the literature [24; 25; 26; 27; 28; 29; 30]. The full extent of the influence of FPL on DPOAE measurements in newborns has not been established given the continued ear canal and middle ear development over weeks to years following birth. For example, the more flacied ear canal walls of newborns will impact how sound energy presented to the ear is reflected, which might influence whether FPL proves beneficial [31]. It is postulated that the calibration technique will have less influence on DPOAEs measured at frequencies \(\leq\)8 kHz versus those measured at higher frequencies (8-16 kHz) in newborns due to shorter ear canal lengths (\(\sim\)15 mm; [17; 32]) compared to adults (22-31 mm; [33; 34]), resulting in standing waves occurring at frequencies \(>\) 6 kHz in an enclosed ear canal [22]. Being attentive to ear canal and middle ear development, as well as calibration techniques used, can aid in reducing variability in serial DPOAE measures. Once DPOAE variability is minimized, repeatability of the measurement needs to be determined in order for DPOAE testing to be used in monitoring programs. Probe reinsertion is the single greatest source of variability in OAE measures [35]. Significant DPOAE level changes for immediate test-retest trials (no probe removal) or up to 5 to 10 days apart are defined as exceeding approximately 0.7 to 7 dB for frequencies between 0.5 and 5 kHz [36; 37; 38]. Greater variability is reported for higher frequency measurements where a change of 2.3 to 8 dB constituted a significant change for frequencies of 6-7 kHz [38; 39]. To increase the accuracy for estimating DPOAE short-term test-retest change expectations, a meta-analysis including data from 10 published studies was completed [40]. The 90% reference limit for DPOAE test-retest changes ranged from \(\pm\)3.76 to \(\pm\)5.63 dB for f\({}_{2}\) frequencies spanning 1 to 6 kHz. Additionally, reference limits increased slightly for the higher frequencies analyzed, 4 and 6 kHz [41]. DPOAE testing is used in serial monitoring programs due to its objective, quick, frequency-specific, and repeatable nature [40; 41; 42; 43; 44; 45; 46], but testing is restricted to frequencies \(\leq\)10 kHz due to a lack of commercially available equipment able to reliably produce stimulus tones \(>\)10 kHz. Ideally, cochlear health would be monitored at the highest frequencies producing a response as this is where damage initially occurs due to ototoxic drug exposures [47].