Frequency dependence and harmonic distortion of stapes displacement and intracochlear pressure in response to very high level sounds

Previous reports have suggested that intracochlear pressures (P_IC) measured at the base of the cochlea increase directly proportionally with stapes displacement (D_Stap) in response to moderately high (<130 dB SPL) level sounds. Consistent with this assumption, we have reported that for low frequency sounds (<1 kHz), stapes displacement and intracochlear pressures increase linearly with sound pressure level (SPL) for moderately high levels (<130 dB SPL), but saturate at higher exposure levels (>130 dB SPL). However, the magnitudes of each response were found to be frequency dependent, thus the relationship between D_Stap and P_IC may vary at higher frequencies or higher levels.

In order to further examine this frequency and level dependence, measurements of D_Stap and P_IC were made in cadaveric human temporal bones prepared with a mastoidectomy and extended facial recess to expose the ossicular chain. P_IC was measured in scala vestibuli (P_SV) and scala tympani (P_ST) simultaneously with SPL in the external auditory canal (P_EAC) and laser Doppler vibrometry (LDV) measurements of stapes velocity (V_Stap). Consistent with prior reports, D_Stap and P_SV increased proportionally with sound pressure level in the ear canal up to a frequency-dependent saturation point, above which both D_Stap and P_SV showed a distinct deviation from proportionality with P_EAC, suggesting that their relationship may remain constant at these high frequencies. Likewise, while the asymptotic value, and SPL at which saturation occurred were frequency dependent in both D_Stap and P_SV, the reduction in gain with increasing SPL above this level was constant above this level at all frequencies, and the magnitude of responses at harmonics of the driving frequency increased with increasing level, consistent with harmonic distortion via peak clipping. Importantly, this nonlinear distortion shifts the energy arriving at the inner ear to higher frequencies than are present in incident stimulus, thus exposing the high frequency sensitive components of the auditory system to more noise than would be expected from measurement of that stimulus on its own. Overall, responses suggest that the cochlear representation of very high-level air conducted stimuli is limited by nonlinearities in the middle ear, and that this peak limiting leads to increased high frequency cochlear exposures than are present in the driving stimulus.