Analyses of the Tympanic Membrane Impulse Response Measured with High-Speed Holography

Abstract

The Tympanic Membrane (TM) transforms acoustic energy to ossicular vibration. The shape and the displacement of the TM play an important role in this process. We developed a High-speed Digital Holography (HDH) system to measure the shape and transient displacements of the TM induced by acoustic clicks. The displacements were further normalized by the measured shape to derive surface normal displacements at over 100,000 points on the TM surface. Frequency and impulse response analyses were performed at each TM point, which enable us to describe 2D surface maps of four new TM mechanical parameters. From frequency domain analyses, we describe the (i) dominant frequencies of the displacement per sound pressure based on Frequency Response Function (FRF) at each surface point. From time domain analyses, we describe the (ii) rising time, (iii) exponential decay time, and the (iv) root-mean-square (rms) displacement of the TM based on Impulse Response Function (IRF) at each surface point. The resultant 2D maps show that a majority of the TM surface has a dominant frequency of around 1.5 kHz. The rising times suggest that much of the TM surface is set into motion within 50 µs of an impulsive stimulus. The maps of the exponential decay time of the IRF illustrate spatial variations in damping, the least known TM mechanical property. The damping ratios at locations with varied dominant frequencies are quantified and compared.

Keywords: Frequency response function; High-speed digital holography; Impulse response function; Mechanical properties; Middle ear; Tympanic membrane.