## 1 Introduction  
Otoacoustic emissions (OAEs) are low-level sounds generated by the active processes of the cochlea, namely outer hair cells (OHCs). Evoked OAEs are generated in response to auditory stimuli, while spontaneous OAEs naturally occur in the absence of stimulation. Distortion-product otoacoustic emissions (DPOAEs) are generated by presenting two closely spaced pure-tones simultaneously in the ear canal. The intermodulation distortion generated by the two pure tones at the level of the basilar membrane generates several new acoustic frequency components, the largest being 2f\({}_{1}\)-f\({}_{2}\) in humans [1,2]. To generate DPOAEs, the higher frequency evoking stimulus tone, f\({}_{2}\), and the lower frequency stimulus tone, f\({}_{1}\), must sufficiently overlap along the basilar membrane. This relationship of the evoking stimuli is dictated by the frequency ratio (f\({}_{2}\)/f\({}_{1}\)). In addition to the frequency ratio, f\({}_{1}\) and f\({}_{2}\) are presented at a given level, which can be equivalent (L\({}_{1}\) = L\({}_{2}\)), offset by a given amount (e.g., L1 = L2 + 10 dB), or one stimulus can be varied while the other is fixed (input/output function). Traditionally, discrete frequencies (\(\leq\)10 kHz) of the primary tones are varied with fixed levels (e.g., L1/L2 = 65/55 dB SPL) and ratio (1.22) for clinical applications. The 1.22 ratio is sufficient for determining whether DPOAEs are present over a wide frequency range despite the fact this ratio does not necessarily yield the largest DPOAE level at each tested frequency, which is especially true for higher frequencies [3; 4; 5]. Over the last four decades, DPOAE responses have been characterized at frequencies (\(\leq\)10 kHz) across the lifespan in those with and without hearing loss [6; 7; 8; 9; 10; 11; 12; 13]. Given DPOAEs provide objective frequency specific information regarding cochlear health; one of the major applications of this measurement lies in testing those who cannot provide reliable behavioral responses [14]. When comparing emissions measured in newborns and adults, findings indicate that the cochlear mechanisms responsible for generating emissions are mature and adult-like at birth, but differences in outer and middle ear properties can influence the forward transmission of signals [15]. DPOAE levels in infants are higher than those measured in adults, suggesting age-related differences [8; 16]. Specifically, newborns' ear canal and middle ear spaces are non-adult like. As the newborn ages, the inner two-thirds of the external auditory canal ossifies and the canal length increases resulting in increased canal stiffness and decreased resonance [17]. The tympan membrane rotates its angle more vertically and decreases in thickness [18; 19]. Middle ear mass decreases due to changes in ossicle bone density and reduction of middle ear mesenchyme [20]. These developments, occurring over the course of weeks to years, alter the acoustic properties of the newborn outer and middle ear, thus influencing OAE measures. Middle ear pathologies can reduce or extinguish OAE levels and should be monitored to ensure that any variability in OAE levels are due to cochlear changes only [16]. Another way to reduce DPOAE variability is to ensure accurate stimulus levels from one session to the next. Traditional calibration procedures involve placing a measurement probe at the entrance to the ear canal which creates standing waves, as not all of the sound is absorbed by the eardrum and some energy is reflected. The microphone in the measurement probe will record a pressure null relative to the eardrum pressure at approximately one-quarter of the wavelength of the stimulus frequency which corresponds to the distance between the measurement probe and the eardrum [21; 22]. This results in a discrepancy up to 15-20 dB between the sound pressures at the eardrum versus the measurement microphone [5; 22; 23].