Stapes displacement and intracochlear pressure in response to very high level, low frequency sounds

The stapes is held in the oval window by the stapedial annular ligament (SAL), which restricts total peak-to-peak displacement of the stapes. Previous studies have suggested that for moderate (<130 dB SPL) sound levels intracochlear pressure (P_IC), measured at the base of the cochlea far from the basilar membrane, increases directly proportionally with stapes displacement (D_Stap), thus a current model of impulse noise exposure (the Auditory Hazard Assessment Algorithm for Humans, or AHAAH) predicts that peak P_IC will vary linearly with D_Stap up to some saturation point. However, no direct tests of D_Stap, or of the relationship with P_IC during such motion, have been performed during acoustic stimulation of the human ear. In order to examine the relationship between D_Stap and P_IC to very high level sounds, measurements of D_Stap and P_IC were made in cadaveric human temporal bones. Specimens were prepared by mastoidectomy and extended facial recess to expose the ossicular chain. Measurements of P_IC were made in scala vestibuli (P_SV) and scala tympani (P_ST), along with the SPL in the external auditory canal (P_EAC), concurrently with laser Doppler vibrometry (LDV) measurements of stapes velocity (V_Stap). Stimuli were moderate (∼100 dB SPL) to very high level (up to ∼170 dB SPL), low frequency tones (20-2560 Hz). Both D_Stap and P_SV increased proportionally with sound pressure level in the ear canal up to approximately ∼150 dB SPL, above which both D_Stap and P_SV showed a distinct deviation from proportionality with P_EAC. Both D_Stap and P_SV approached saturation: D_Stap at a value exceeding 150 μm, which is substantially higher than has been reported for small mammals, while P_SV showed substantial frequency dependence in the saturation point. The relationship between P_SV and D_Stap remained constant, and cochlear input impedance did not vary across the levels tested, consistent with prior measurements at lower sound levels. These results suggest that P_SV sound pressure holds constant relationship with D_Stap, described by the cochlear input impedance, at these, but perhaps not higher, stimulation levels. Additionally, these results indicate that the AHAAH model, which was developed using results from small animals, underestimates the sound pressure levels in the cochlea in response to high level sound stimulation, and must be revised.