Calibration of Distortion Product Otoacoustic Emission Probes

Metrology in Life Sciences

Distortion product otoacoustic emissions, measured in the human ear canal, are ear-generated signals elicited in response to two pure-tone stimuli. They provide an objective and noninvasive method to assess the function of the outer hair cells within the cochlea, especially for non-cooperative patients (e.g. babies or small children). Although the presence of otoacoustic emissions indicates healthy outer hair cells, their absence should be interpreted with great caution, since this might not always be the result of an inner-ear pathology. A dysfunction in the middle-ear chain might also inhibit measurable emissions. However, even under the assumption of an intact middle-ear chain, calibration errors might obstruct the detection of otoacoustic emissions. One of the reasons for these errors lies in standing waves instigated within the ear canal. At higher frequencies an adult ear canal becomes more and more acoustically long compared to the wavelength. The interference of incident and reflected signals creates a standing wave pattern within the ear canal. Hence, the actual sound pressure at the tympanic membrane can be underestimated when measuring in the vicinity of a sound pressure node. The purpose of this project is to thoroughly assess the calibration errors of distortion product otoacoustic emission probes by means of finite-element simulations as well as through measurements in couplers simulating average human ear canals. The accuracy of the estimation of the sound pressure at the tympanic membrane from measurements at the probe microphone position is to be investigated applying transmission line models of average ear canals of adults and babies. The quantification of calibration errors and contributions to a more reliable calibration procedure comprise the core of this research.

TU Braunschweig
Ph.D. student: