Computer-assisted time-averaged holograms of the motion of the surface of the mammalian tympanic membrane with sound stimuli of 0.4–25kHz

Abstract

Time-averaged holograms describing the sound-induced motion of the tympanic membrane (TM) in cadaveric preparations from three mammalian species and one live ear were measured using opto-electronic holography. This technique allows rapid measurements of the magnitude of motion of the tympanic membrane surface at frequencies as high as 25 kHz. The holograms measured in response to low and middle-frequency sound stimuli are similar to previously reported time-averaged holograms. However, at higher frequencies (f > 4 kHz), our holograms reveal unique TM surface displacement patterns that consist of highly-ordered arrangements of multiple local displacement magnitude maxima, each of which is surrounded by nodal areas of low displacement magnitude. These patterns are similar to modal patterns (two-dimensional standing waves) produced by either the interaction of surface waves traveling in multiple directions or the uniform stimulation of modes of motion that are determined by the structural properties and boundary conditions of the TM. From the ratio of the displacement magnitude peaks to nodal valleys in these apparent surface waves, we estimate a Standing Wave Ratio of at least 4 that is consistent with energy reflection coefficients at the TM boundaries of at least 0.35. It is also consistent with small losses within the uniformly stimulated modal surface waves. We also estimate possible TM surface wave speeds that vary with frequency and species from 20 to 65 m/s, consistent with other estimates in the literature. The presence of standing wave or modal phenomena has previously been intuited from measurements of TM function, but is ignored in some models of tympanic membrane function. Whether these standing waves result either from the interactions of multiple surface waves that travel along the membrane, or by uniformly excited modal displacement patterns of the entire TM surface is still to be determined.

The Tympanic Membrane Motion in Forward and Reverse Middle-Ear Sound Transmission

“…3B1 bottom). While with piezo stimulation, the displacement amplitude and phase maps are more consistent with high order mode shapes of a circular membrane fixed at the rim and excited at one of its "natural frequencies" [8], where the TM is divided into many sub-regions with local displacement maxima and minima (Fig. 3B2 top), and the phase differs aboutź among these sub-regions (Fig.…”

“…The TM was painted with a suspension of ZnO powder in saline (90mg/cc) to increase the light reflected from the TM surface. The effect of painting on TM motion patterns is small [8]. We used a CF1 speaker (Tucker-Davis) coupled to the "holographic otoscope" to provide forward acoustic stimulation to the TM.…”

“…The displacement resolution can go as low as 10-20 nm [5], while the spatial resolution reaches around 300,000 points for the human TM with a surface area about 50 mm 2 [2]. Uniform sound pressure applied over the TM surface produces displacement patterns of the TM that can be classified as either simple, complex or or-dered [8]. The complex and ordered TM surface motions observed at medium and high frequencies (>3 kHz) are consistent with a combination of standing-wave-like modal motions and traveling waves that co-exist on the TM surface [2,3].…”

Sound pressure distribution within natural and artificial human ear canals: Forward stimulation

“…With this small sample, only limited conclusions can be drawn about mean properties and the distribution of variations, but this sample provides a good indication of which features are common among ears and what sorts of variations might be observed in a larger population. Similarly, the spatial resolution of our measurements is much coarser than the resolution of tens of microns for TM motion studies (e.g., Rosowski et al, 2009), but is adequate to establish general trends in sound pressure distribution.…”

“…It is commonly understood that the motion of the TM surface in response to sound can be quite complicated and varies among different frequency ranges (Tonndorf and Khanna, 1972;Rosowski et al, 2009;Cheng et al, 2010Cheng et al, , 2013. In the human, TM surface motion changes from a simple to a complex regime տ4 kHz and to an ordered regime տ8 kHz .…”