Sound-induced displacement of the Tympanic Membrane (TM) is the first stage in the forward transformation of environmental sound to sound within the cochlea, while displacement of the TM induced by mechanical motions of the ossicular chain is the last stage in the reverse transformation of cochlea generated sound to clinically valuable oto-acoustic emissions (OAEs) measured in the ear canal. However, our knowledge of the workings of the TM in both forward and reverse sound transmissions is limited. Although recent studies suggest complex TM surface motions in response to ear-canal sound at high frequency are consistent with multiple waves co-existing on the TM surface, the contributions of different TM displacement waves to excitation of the ossicular chain and subsequent sound transmission to inner ear are unclear. Furthermore, little is known of how the TM responds to ossicular motions produced by inner-ear generated sound or mechanical stimulation of the ossicles. There is also a lack of data describing spatial sound-pressure distributions near and far from the TM even though it is known that there are significant non-uniformities in TM motion in both forward and reverse sound transmission. This study aims to: (1) Characterize TM surface motions in response to forward stimulation by sound generated within the ear canal and reverse mechanical stimulation produced by an active middle-ear implant;(2) Produce detailed spatial profiles of sound pressure near and far from the TM that will be correlated with detailed TM surface motions in forward and reverse stimulation;and (3) Quantify the relationship between (a) TM surface motions and sound energy transmission through the middle ear to the cochlear excitations and (b) Ossicular motion and the sound transformation by the TM in reverse stimulation. We employ a newly developed stroboscopic holographic interferometer to measure displacement amplitude and phase in response to different stimuli at over 300000 points on the TM surface, together with a computer-controlled microphone positioning system to systematically sample the sound pressure within the ear canal both near (within 1 mm) and far (up to 10 mm) from the TM surface. Accomplishing these aims will: (i) Quantify the different wave types, wave amplitude and wavelength of TM surface motions produced by forward and reverse stimulation;(ii) Better define the contributions of different TM surface waves to sound transmissions in both directions;(iii) Better describe the action of the TM in clinically useful oto-acoustic emission measurements;and (iv) Investigate the clinical utility of backward driven ear-canal sound pressure measurements in the evaluation of active middle-ear prostheses that drive the intact ossicular chain or the round window.
Understanding how the eardrum responds to forward (normal) sound stimuli and reverse mechanical stimuli (from oto-acoustic emissions or active middle-ear implants) will define the role of the normal eardrum. A detailed picture of sound pressure in space near the eardrum will tell us: whether irregularities in eardrum motion significantly affect the ear-canal sound field during normal stimulation, and how such irregularities affect the ear-canal sound pressures produced by sound generated within the inner or middle ear. The later question is significant to the use of oto-acoustic emissions in hearing diagnosis and the tests of middle-ear implants.
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