## 2 Methods
### Equipment, Calibration, and Software  
An emission probe system with all capabilities necessary to reliably measure DPOAES up to 16 kHz was used. The signals for both the f\({}_{2}\) and f\({}_{1}\) channels were generated at the digital-to-analog converter of a MOTU 96 kHz Audio Firewire Interface (Cambridge, MA, USA), which was connected to a laptop, and presented through the Etiymotic Research ER-10X (Elk Grove Village, IL, USA), a low-noise DPOAE probe assembly with built-in sound pressure transducers positioned in the subject's ear canal. The probe assembly also housed the emission probe microphone (ER-10X), which recorded the sound pressure in the ear canal. The signal measured by the ER-10X microphone was amplified (20 dB) before being digitized by the analog-to-digital converter of the MOTU Audio Firewire Interface.  
Two methods of calibration were utilized. First, using EMAV software [76] the source pressure and impedance of the emission probe system were estimated by presenting a chirp stimulus in SPL to five cylindrical cavities of varying length (70.21, 55.32, 43.48, 37.9, and 31.50 mm) housed in the ER10X unit. Thevenin source characteristics are obtained by repetitively solving five linear equations to minimize the difference between measured and expected pressure responses. Error values of less than one for each sound source were achieved and this indicates a good calibration. Once the probe was placed in the newborns ear, the in-the-ear calibration phase of the test was performed and this determined the load pressure. The Thevenin source characteristics previously obtained are combined with the ear canal pressure measured in the newborn to calculate the load impedance, which is requiredto convert measured pressure in the ear canal to FPL [24; 25; 26]. Second, the stimulus and DPOAE levels were measured at the plane of the emission probe microphone near the entrance to the ear canal for conventional in-the-ear sound pressure level (SPL) calibration.  
EMAV software was used to collect DPOAE data. The software samples the ear-canal sound pressure in 46 ms time windows, accumulates these samples into two interleaved buffers, and averages each buffer in the time domain. The two subaverages are then subtracted (A \(-\) B) and Fast Fourier transformed to provide an estimate of the background noise floor. The grand average ([A \(+\) B]/2) is transformed to estimate the levels and phases of both stimuli and distortion products present in the ear canal. The level of the noise at the distortion-product frequencies is also estimated from the A-B spectrum. Sampling occurred, for a minimum of 4 s or longer, until one of two stopping rules were met: the noise floor at the distortion-product frequency was less than \(-\)20 dB SPL or until 4 s of artifact-free sampling had been averaged. To estimate system distortion, probe microphone measurements were made in an acoustic cavity (B&K 4157 ear simulator) with impedance characteristics similar to the human ear canal. System distortion (energy recorded in the test cavity at 2f\({}_{1}\)-f\({}_{2}\)) was at least 80 dB below the stimulus levels used in this study.  
Spontaneous otoacoustic emissions (SOAEs) were recorded using SysRes software[77] and the ER10X probe. SysRes is capable of recording real time responses and a sample rate of 32 kHz was used to record SOAEs. A total of 65,536 samples were recorded for a total of one average. No stimulus was used during the recording. A fast Fourier transform was performed on the saved average. Spectral averaging was used to decrease the variance of the noise and allow the measurement of small level SOAEs. In total there were 3639 frequencies evaluated ranging from 0.001 through 15.989 kHz (in 0.004395 Hz steps).