Ear-canal standing waves and high-frequency sound calibration using otoacoustic emission probes

Low-noise microphones designed to measure otoacoustic emissions from the human ear canal typically sample the sound field in the canal some 15–20 mm away from the eardrum. The input sound levels to the middle ear are usually defined as the sound-pressure level “at the eardrum.” However, the well-known phenomenon of standing waves produces a spatially nonuniform pressure for frequencies above 2–3 kHz. This phenomenon is demonstrated using physical simulation with a commonly used emission probe and the interpretation of the results are confirmed by simple acoustical theory. In addition, it is shown large (±20 dB) errors in the estimated eardrum sound pressure result from varying the position of the sound source in the canal. Theory, physical simulations and measurements in human ear canals are used to characterize the errors using various calibration procedures commonly employed in otoacoustic emission studies. It is predicted that the reliability of measurements of high-frequency stimulated otoacoustic emissions will be improved if the stimulus sound-pressure levels were measured near the eardrum.