Jeffrey Tao Cheng, Ph.D.
Instructor of Otology and Laryngology, Harvard Medical School
We are using newly developed laser holography techniques to measure motion of the entire surface of the tympanic membrane (TM), or the eardrum, in both magnitude and phase in a number of mammalian species in our lab. Our study can address basic science questions, such as how does the TM interact with forward sound waves traveling down the ear-canal over a broad auditory frequency range (20 Hz ~20 kHz for human)? What role does the TM play in receiving and transmitting acoustic energy into the middle-ear ossicle and cochlea in normal hearing? Up to date, several different theories or models have been postulated to describe the function of the TM, such as catenary lever theory of a curved membrane by Helmholtz (1868), stiff plate membrane by von Bekesy (1941), modal motion at high frequency by Tonndorf and Khanna (1970), transmission line models by Puria and Allen (1988) and summed modal resonance model by Fay et. al (2006). Recently our group by using stroboscopic holography to characterize TM surface motions in response to forward acoustic stimuli has come to a hypothesis that sound-induced TM motion can be interpreted as the combination of modal motion and traveling-wave-like motion, where the relatively low-order modal motion plays a more important role for coupling acoustic energy into mechanical vibration of the middle-ear ossicular chain even at high frequency. A similar conclusion has been reached by the Columbia group (De La Rochefoucauld and Olson, 2010) using scanning Laser Doppler Vibrometery to measure surface motion of the TM on the gerbil. To further our understanding on the working mechanism of the TM, we are also investigating how the TM reacts to the reverse mechanical stimulation of the ossicles produced by, for example, an active middle-ear implant. Furthermore, how does TM motion produced by "sound" generated within the inner or middle ear affect the ear-canal sound pressure? The results from this study could help define the mechanical properties of the TM or other middle-ear structures for the modeling analysis of the ear.
Our study also has clinical impact. A detailed quantitative analysis of TM surface motion by holography on pathological ears, such as TM perforation, ossicular chain fixation or interruption, reconstructed middle-ear with various prostheses, could benefit diagnoses of middle-ear diseases and help plan for pre-operative treatment as well as post-treatment evaluation.