Morteza Khaleghi

Digital holographic measurements of shape and three-dimensional sound-induced displacements of tympanic membrane

Abstract. Acoustically induced vibrations of the tympanic membrane (TM) play a primary role in the hearing process, in that these motions are the initial mechanical response of the ear to airborne sound. Characterization of the shape and three-dimensional (3-D) displacement patterns of the TM is a crucial step to a better understanding of the complicated mechanics of sound reception by the ear. Sound-induced 3-D displacements of the TM are estimated from shape and one-dimensional displacements measured in cadaveric chinchillas using a lensless dual-wavelength digital holography system (DWDHS). The DWDHS consists of laser delivery, optical head, and computing platform subsystems. Shape measurements are performed in double-exposure mode with the use of two wavelengths of a tunable laser, while nanometer-scale displacements are measured along a single sensitivity direction with a constant wavelength. Taking into consideration the geometrical and dimensional constrains imposed by the anatomy of the TM, we combine principles of thin-shell theory together with displacement measurements along a single sensitivity vector and TM surface shape to extract the three principal components of displacement in the full-field-of-view. We test, validate, and identify limitations of this approach via the application of finite element method to artificial geometries.

Three-dimensional vibrometry of the human eardrum with stroboscopic lensless digital holography.

Abstract. The eardrum or tympanic membrane (TM) transforms acoustic energy at the ear canal into mechanical motions of the ossicles. The acousto-mechanical transformer behavior of the TM is determined by its shape, three-dimensional (3-D) motion, and mechanical properties. We have developed an optoelectronic holographic system to measure the shape and 3-D sound-induced displacements of the TM. The shape of the TM is measured with dual-wavelength holographic contouring using a tunable near IR laser source with a central wavelength of 780 nm. 3-D components of sound-induced displacements of the TM are measured with the method of multiple sensitivity vectors using stroboscopic holographic interferometry. To accurately obtain sensitivity vectors, a new technique is developed and used in which the sensitivity vectors are obtained from the images of a specular sphere that is being illuminated from different directions. Shape and 3-D acoustically induced displacement components of cadaveric human TMs at several excitation frequencies are measured at more than one million points on its surface. A numerical rotation matrix is used to rotate the original Euclidean coordinate of the measuring system in order to obtain in-plane and out-of-plane motion components. Results show that in-plane components of motion are much smaller (<20%) than the out-of-plane motions’ components.

Miniaturization as a key factor to the development and application of advanced metrology systems

Recent technological advances of miniaturization engineering are enabling the realization of components and systems with unprecedented capabilities. Such capabilities, which are significantly beneficial to scientific and engineering applications, are impacting the development and the application of optical metrology systems for investigations under complex boundary, loading, and operating conditions. In this paper, and overview of metrology systems that we are developing is presented. Systems are being developed and applied to high-speed and high-resolution measurements of shape and deformations under actual operating conditions for such applications as sustainability, health, medical diagnosis, security, and urban infrastructure. Systems take advantage of recent developments in light sources and modulators, detectors, microelectromechanical (MEMS) sensors and actuators, kinematic positioners, rapid prototyping fabrication technologies, as well as software engineering.

In-plane and out-of-plane motions of the human tympanic membrane

Computer-controlled digital holographic techniques are developed and used to measure shape and four-dimensional nano-scale displacements of the surface of the tympanic membrane (TM) in cadaveric human ears in response to tonal sounds. The combination of these measurements (shape and sound-induced motions) allows the calculation of the out-of-plane (perpendicular to the surface) and in-plane (tangential) motion components at over 1,000,000 points on the TM surface with a high-degree of accuracy and sensitivity. A general conclusion is that the in-plane motion components are 10-20 dB smaller than the out-of-plane motions. These conditions are most often compromised with higher-frequency sound stimuli where the overall displacements are smaller, or the spatial density of holographic fringes is higher, both of which increase the uncertainty of the measurements. The results are consistent with the TM acting as a Kirchhoff-Loves thin shell dominated by out-of-plane motion with little in-plane motion, at least with stimulus frequencies up to 8 kHz.

Simultaneous full-field 3-D vibrometry of the human eardrum using spatial-bandwidth multiplexed holography

Abstract. Holographic interferometric methods typically require the use of three sensitivity vectors in order to obtain three-dimensional (3-D) information. Methods based on multiple directions of illumination have limited applications when studying biological tissues that have temporally varying responses such as the tympanic membrane (TM). Therefore, to measure 3-D displacements in such applications, the measurements along all the sensitivity vectors have to be done simultaneously. We propose a multiple-illumination directions approach to measure 3-D displacements from a single-shot hologram that contains displacement information from three sensitivity vectors. The hologram of an object of interest is simultaneously recorded with three incoherently superimposed pairs of reference and object beams. The incident off-axis angles of the reference beams are adjusted such that the frequency components of the multiplexed hologram are completely separate. Because of the differences in the directions and wavelengths of the reference beams, the positions of each reconstructed image corresponding to each sensitivity vector are different. We implemented a registration algorithm to accurately translate individual components of the hologram into a single global coordinate system to calculate 3-D displacements. The results include magnitudes and phases of 3-D sound-induced motions of a human cadaveric TM at several excitation frequencies showing modal and traveling wave motions on its surface.

Characterization of Acoustically-Induced Forces of the Human Eardrum

Human eardrum or Tympanic Membrane (TM) is a thin structure located at the boundary between outer and middle ears. Shape, deformations, and thickness of the mammalian TMs have been studied by several groups; however, sound-induced forces of the TM, and the question of “how large the forces produced by acoustic waves are along the manubrium at the input to the middle-ear ossicular system?” have not been fully answered. In this paper, sound-induced forces in the human TM are measured at different tonal frequencies and at several points on its surface. A calibrated force sensor with a resolution of 0.5 μN is used with a 3D nano-positioner, enabling accurate placing of the sensor at points of interests on the TM surface. A closed-loop control system is designed and implemented in order to realize constant preload of the sensor at all the measuring points. Concomitant to the force measurements, time-averaged and three-dimensional stroboscopic holographic interferometry are used to compare the modal shape of the sound-induced motion of the TM before and after the presence of the force sensor. The preliminary results show that the maximum sound-induced forces at the umbo occurs at frequencies between 1.5 and 2.3 kHz, whereas the maximum forces for locations on the surface of the TM occurs at around 5–6 kHz.

Study of the Transient Response of Tympanic Membranes Under Acoustic Excitation

Characterization of the transient response of the human Tympanic Membrane (TM) subjected to impulse acoustic excitation is important in order to further understand the mechanics of hearing. In this paper, we present results of our initial investigations of the transient response of an artificial fully-constrained circular membrane as a simplified model of the TM. Two different optical methods used in our investigations are Laser Doppler Vibrometery (LDV) and Pulsed Double-Exposure Digital Holography (PDEDH) for single-point and full-field-of-view measurements of displacements, respectively. Applying Hilbert Transformation methods to the measured displacements allows determination of the transient characteristics of the membrane, including damping ratios and time constants, which are also computed and compared with corresponding FEM models. We expect to use this method in the investigation of the transient response of TM of specific species.

Thin-Shell Behavior of Mammalian Tympanic Membrane Studied by Digital Holography

The acousto-mechanical-transformer behavior of the Tympanic Membrane (TM) is defined by its shape, 3D displacements, and mechanical properties. In this paper, we report the quantification of these characteristics by full-field-of-view optoelectronic techniques. Due to geometrical constraints imposed by the ear canal, however, 3D displacement measurements with multiple sensitivity vectors in holographic interferometry or 3D Laser Doppler Vibrometry (LDV) have limited applications for testing in vivo. Therefore, we seek alternative methods to perform 3D measurements. In our work, we hypothesize that the TM behaves as a thin-shell, so that the principal components of vibration are parallel to the TM’s shape normal vectors, which allows the estimation of the 3D components of displacement with only 1D component of displacements and shape information. Full-field-of-view measurements of the TM are obtained with our digital holographic system, with shape measured in two-wavelength mode and 1D displacements measured in single-wavelength mode. The theoretically-estimated 3D components of displacement are then compared with those measured by methods of multiple sensitivity vectors. Preliminary data suggest that the thin-shell hypothesis is applicable for estimation of the 3D acoustically-induced vibrations of the TM excited at low and mid frequency ranges.

Long-Term Effects of Cyclic Environmental Conditions on Paintings in Museum Exhibition by Laser Shearography

To better evaluate current condition standards commonly used for the exhibition of canvas paintings, it is necessary to have a quantitative technique capable of measuring degradation components induced by changes in temperature and relative humidity, as well as the effects of ambient vibration and the thermomechanical effects of museum lighting. This paper presents advances in our development of a customized laser shearography system for temporal characterization of in-plane displacements of canvas paintings when subjected to changes in exhibition conditions. The shearography system performs concomitant measurements of gradients of displacement along two orthogonal shearing directions and is synchronized with a thermal IR camera to provide thermal maps of the area being analyzed. Recent innovations incorporated into the system include a real-time temporal phase unwrapping algorithm, and high-resolution Fast Fourier Transform (FFT) methods to calibrate applied shearing levels that allow a wide range of measuring resolutions. Examples will be presented that illustrate the system’s capabilities to detect cracks in the paint surface and measure and map associated strain vectors as a function of changes in condition parameters. Included are representative results of continuous 30 h recordings on American nineteenth century oil on canvas painting. Multi-domain data has been combined and correlated using the shearography and IR data from our system, temperature and humidity data from the museum’s climate control system, as well as activity log from museum’s security system.

Attenuating the ear canal feedback pressure of a laser-driven hearing aid

Microphone placement behind the pinna, which minimizes feedback but also reduces perception of the high-frequency pinna cues needed for sound localization, is one reason why hearing-aid users often complain of poor sound quality and difficulty understanding speech in noisy situations. In this paper, two strategies are investigated for minimizing the feedback pressure (thereby increasing the maximum stable gain, MSG) of a wide-bandwidth light-activated contact hearing aid (CHA) to facilitate microphone placement in the ear canal (EC): (1) changing the location of the drive force and its direction at the umbo, and (2) placing an acoustic damper within the EC to reduce the feedback pressure at the microphone location. The MSG and equivalent pressure output (EPO) are calculated in a 3D finite element model of a human middle ear based on micro computed tomography (micro-CT) images. The model calculations indicate that changing the umbo-force direction can decrease feedback pressure, but at the expense of decreased EPO. However the model shows improvements in MSG without sacrificing EPO when an acoustic damper is placed in the EC. This was verified through benchtop experimentation and in human cadaver temporal bones. The results pave the path towards a wide-bandwidth hearing aid that incorporates an EC-microphone design.