Evanescent waves in simulated ear canals: Experimental demonstration and method for compensation

Area of Science:

• Acoustics
• Bioacoustics
• Signal Processing

Background:

• Evanescent waves are non-propagating sound waves near a source in a waveguide with a larger cross-sectional area.
• These waves can contaminate in-ear calibration of acoustic stimuli, leading to inaccurate measurements.
• Accurate acoustic source calibration is crucial for reliable ear-canal impedance measurements.

Purpose of the Study:

• To investigate the characteristics and impact of evanescent waves on acoustic calibrations.
• To explore methods for reducing evanescent waves in acoustic measurements.
• To improve the accuracy of in-ear calibration and ear-canal impedance measurements.

Main Methods:

• Measurements were conducted using an otoacoustic-emission (OAE) probe inserted into long tubes of varying diameters.
• The effect of probe tube advancement on evanescent wave amplitude was analyzed.
• The sound source aperture shape was modified to alter acoustic flow and reduce evanescent waves.

Main Results:

• Evanescent wave amplitude increases with frequency and is strongly dependent on tube diameter.
• Modifying the sound source aperture significantly reduced evanescent wave pressure.
• Reduced evanescent waves led to lower errors in Thévenin-equivalent source pressure and impedance calibrations.

Conclusions:

• Evanescent waves pose a significant challenge to accurate in-ear acoustic calibration.
• Modifying the sound source aperture is an effective strategy to mitigate evanescent waves.
• An alternative calculation method accounting for evanescent waves can improve calibration quality and ear-canal impedance measurement accuracy.