Background:
If calibration errors are excluded, the detection of distortion product otoacoustic emissions (DPOAE) reveals a healthy cochlear function. While the commonly used in-the-ear calibration provides an efficient way to adjust the stimulus levels for each test ear, it fails at higher frequencies due to standing waves within the ear canal. Slight unintentional changes in the probe position might lead to calibration errors, affecting the DPOAE results.

Material and Methods:
In this study, we made use of this “drawback” by deliberately varying the probe position within the ear canal and analysing the DPOAEs for two insertion depths: “intermediate” and “shallow”. We performed DPOAE measurements up to a stimulus frequency of 12 kHz in four human test subjects.

Results:
Varying the probe insertion depth had a marginal effect (<2 dB) on the DPOAE levels below 4 kHz. However, above this frequency, the probe insertion depth variation led to the absence of detectable DPOAEs at some frequencies, or yielded ambiguous DPOAE levels (> 20 dB) for some subjects above 10 kHz.

Conclusions:
We suggest accepting DPOAEs as valid indicators for cochlear function, if detected for at least one insertion depth below 8 kHz. For higher frequencies, DPOAEs are accepted only if the stimulus frequencies are not in the vicinity of any notch frequency in the ear canal transfer function. The deliberate variation of the probe insertion depth within the ear canal provides a simple way to increase the reliability of the DPOAE pass/refer results.