2. Physiology

Spherical sound waves arriving at the ear entrance propagate in the external auditory canal (EAC) as plane waves. The pinna and the ear canal generate resonance phenomena around 3,000 Hz, with amplification reaching 15 dB or more at 2,700 Hz. The eardrum, ossicles and oval window then vibrate in succession.

These have to move from a gaseous medium (air) with low intrinsic acoustic resistance to a liquid medium (inner ear) with high intrinsic acoustic resistance, hence the need for impedance matching between the two media. The tympanic-ossicular chain performs this adaptation via two mechanisms:

• surface effect: concentration of forces from the large surface, about 0.5 cm ² , of the eardrum, to the small surface S ₂ , of the oval...