The onset of nonlinear growth of middle-ear responses to high intensity sounds

The human tympanic membrane (TM) and ossicles are generally considered to act as a linear system as they conduct low and moderate level environmental sounds to the cochlea. At intense stimulus levels (> 120 dB SPL) there is evidence that the TM and ossicles no longer act linearly. The anatomical structures that contribute to the nonlinear responses and their level and frequency dependences are not well defined. We used cadaveric human ears to characterize middle-ear responses to continuous tones between 200 and 20,000 Hz with levels between 60 and 150 dB SPL. The responses of the TM and ossicles are essentially sinusoidal, even at the highest stimulus level, but grow nonlinearly with increased stimulus level. The umbo and the stapes show different nonlinear behaviors: The umbo displacement grows faster than the stimulus level (expansive growth) at frequencies below 2000 Hz, while the stapes exhibits mostly compressive growth (grows slower than the stimulus level) over a wide frequency range. The sound pressure level where the nonlinearity first becomes obvious and the displacement at that level are lower at the stapes than at the umbo. These observations suggest the presence of multiple nonlinear processes within the middle ear. The existence of an expansive growth of umbo displacement that has limited effect on the stapes compressive growth suggests that the ossicular joints reduce the coupling between multiple nonlinear mechanisms within the middle ear. This study provides new data to test and refine middle-ear nonlinear models.