Hypothesis: The sound-induced motion of the tympanic membrane has features that are most consistent with modal responses to a uniform stimulus.
Background: Conceptual models of the coupling of tympanic membrane motion to the ossicular chain can be classified as either modal responses to a uniform stimulation of the entire membrane or traveling wave models in which sound energy is captured at the membrane's rim and travels along the surface to the umbo. The stroboscopic holography technique we use can separate strongly modal or traveling wave-dominated motions of the tympanic membrane surface.
Methods: We use computer-aided optoelectronic holography with stroboscopic illumination to measure the magnitude and phase of the sound-induced motion of more than 40,000 points on the surface of the tympanic membrane in cadaveric human temporal bones. Our techniques are sensitive to motions of the membrane as small as 0.01 µm and allow determinations of membrane displacement at frequencies as large as 20 kHz.
Results: We report clear signs of both modal tympanic membrane responses and traveling waves on the human tympanic membrane. Modal responses are seen throughout the frequency range, whereas the traveling waves are most apparent between 2 and 8 kHz. In general, the magnitudes of the traveling waves are small compared with the modal magnitudes.
Conclusion: Much of the motion of the tympanic membrane is well approximated by modal motions of the tympanic membrane surface. This conclusion has implications for eardrum pathology and its treatment.