Ear Canal Geometry Research Articles

Finite element and experimental modeling of jaw movement-induced deformations in the human earcanal

As ear-related technologies proliferate, optimizing comfort, retention, and battery life is crucial for enhancing user experience. A thorough understanding of the anatomical interaction between the temporomandibular joint (TMJ) and the earcanal during mouth-opening is essential. This study develops a finite element model and an experimental setup to investigate the biomechanical coupling between the TMJ and the earcanal. We analyze reverse-static deformations, focusing on cartilage-bone junction geometry, mandibular condyle location, and concha mobility. The earcanal geometry is assessed across five cross-sections with seven key dimensions measured. The results indicate that the deformations in cantilever-beam-like models closely match the reference geometry in both approaches, particularly in the lateral region. These findings suggest that a dynamic motion model of the earcanal, accurately simulating its behavior, is feasible.

Measurements of ear-canal geometry from high-resolution CT scans of human adult ears

Description of the ear canal’s geometry is essential for describing peripheral sound flow, yet physical measurements of the canal’s geometry are lacking and recent measurements suggest that older-adult-canal areas are systematically larger than previously assumed. Methods to measure ear-canal geometry from multi-planar reconstructions of high-resolution CT images were developed and applied to 66 ears from 47 subjects, ages 18–90 years. The canal’s termination, central axis, entrance, and first bend were identified based on objective definitions, and the canal’s cross-sectional area was measured along its canal’s central axis in 1–2 mm increments. In general, left and right ears from a given subject were far more similar than measurements across subjects, where areas varied by factors of 2–3 at many locations. The canal areas varied systematically with age cohort at the first-bend location, where canal-based measurement probes likely sit; young adults (18–30 years) had an average area of 44mm2 whereas older adults (61–90 years) had a significantly larger average area of 69mm2. Across all subjects ages 18–90, measured means ± standard deviations included: canals termination area at the tympanic annulus 56±8mm2; area at the canal’s first bend 53±18mm2; area at the canal’s entrance 97±24mm2; and canal length 31.4±3.1mm2.

Earmonitor

Earphones are emerging as the most popular wearable devices and there has been a growing trend in bringing intelligence to earphones. Previous efforts include adding extra sensors (e.g., accelerometer and gyroscope) or peripheral hardware to make earphones smart. These methods are usually complex in design and also incur additional cost. In this paper, we present Earmonitor, a low-cost system that uses the in-ear earphones to achieve sensing purposes. The basic idea behind Earmonitor is that each person's ear canal varies in size and shape. We therefore can extract the unique features from the ear canal-reflected signals to depict the personalized differences in ear canal geometry. Furthermore, we discover that the signal variations are also affected by the fine-grained physiological activities. We can therefore further detect the subtle heartbeat from the ear canal reflections. Experiments show that Earmonitor can achieve up to 96.4% Balanced Accuracy (BAC) and low False Acceptance Rate (FAR) for user identification on a large-scale data of 120 subjects. For heartbeat monitoring, without any training, we propose signal processing schemes to achieve high sensing accuracy even in the most challenging scenarios when the target is walking or running.

Body temperature measurement uncertainty arising from ear canal geometry and temperature gradients

We report the results of a numerical thermal model of the ear canal and tympanic membrane. The model was used to assess the uncertainty contributions in determining body temperature arising from lower ear canal temperature gradients. We find that for reasonable assumptions of such gradients the standard uncertainty contribution is < 0.1 °C.

Effects of mandibular motion on earplug function

Earplug attenuation depends on developing and maintaining an acoustic seal between the earplug and the walls of the ear canal. During use, earplugs may lose some of their effectiveness by loosening over time and with activity, thereby compromising the acoustic seal in the ear canal and allowing more sound to enter the ear canal from the sound field. Mandibular motion (as during talking, singing, chewing, breathing) provides a nearly continuous source of ear canal movement resulting from forces applied by the condyle of the mandible. Individual differences in ear canal geometry also may impact the effects of mandibular motion on earplug function. The effects of mandibular motion on earplug function were investigated using an acoustic test fixture that combined (1) individualized pinna and ear canal structure beyond the second bend, inclusive of the mandibular bump, with (2) a standardized ear simulator (GRAS RA0045) with ear canal extension and microphone (GRAS 40AH ¼”). Mandibular motion was simulated using a computer-controlled drive-pin with a silicone tip that articulated with the mandibular bump of the individualized silicon ear canal. Results indicated that as few as 50 cycles of mandibular motion could significantly compromise earplug function and that this compromise was highly subject-dependent.

Use of magnetic resonance image registration to estimate displacement in the human earcanal due to the insertion of in-ear devices.

In-ear devices are used in a wide range of applications for which the device's usability and/or efficiency is strongly related to comfort aspects that are influenced by the mechanical interaction between the device and the walls of the earcanal. Although the displacement of the earcanal walls due to the insertion of the device is an important characteristic of this interaction, existing studies on this subject are very limited. This paper proposes a method to estimate this displacement in vivo using a registration technique on magnetic resonance images. The amplitude, the location and the direction of the earcanal wall displacement are computed for four types of earplugs used by one participant. These displacements give indications on how each earplug deforms the earcanal for one specific earcanal geometry and one specific earplug insertion. Although the displacement due to a specific earplug family cannot be generalized using the results of this paper, the latter help to understand where, how much, and how each studied earplug deforms the earcanal of the participant. This method is revealed as a promising tool to investigate further acoustical and physical comfort aspects of in-ear devices.

Measurement of Core Body Temperature Using Graphene-Inked Infrared Thermopile Sensor.

Continuous and reliable measurements of core body temperature (CBT) are vital for studies on human thermoregulation. Because tympanic membrane directly reflects the temperature of the carotid artery, it is an accurate and non-invasive method to record CBT. However, commercial tympanic thermometers lack portability and continuous measurements. In this study, graphene inks were utilized to increase the accuracy of the temperature measurements from the ear by coating graphene platelets on the lens of an infrared thermopile sensor. The proposed ear-based device was designed by investigating ear canal geometry and developed with 3D printing technology using the Computer-Aided Design (CAD) Software, SolidWorks 2016. It employs an Arduino Pro Mini and a Bluetooth module. The proposed system runs with a 3.7 V, 850 mAh rechargeable lithium-polymer battery that allows long-term, continuous monitoring. Raw data are continuously and wirelessly plotted on a mobile phone app. The test was performed on 10 subjects under resting and exercising in a total period of 25 min. Achieved results were compared with the commercially available Braun Thermoscan, Original Thermopile, and Cosinuss One ear thermometers. It is also comprehended that such system will be useful in personalized medicine as wearable in-ear device with wireless connectivity.

Application of a registration method on magnetic resonance images to evaluate the displacement field of a human subject ear canal due to various earplug insertions

Earplugs are a usual way to protect workers subjected to noise exposure. However, the efficiency of these hearing protection devices is often affected by induced discomforts. A factor suspected to impact both acoustical and physiological comfort attributes of earplugs is the deformation they apply on the ear canal walls. As the geometry of both open and occluded ear canal is difficult to obtain, the ear canal deformation due to earplug insertion is not trivial to evaluate. Current medical imaging techniques and image post-processing methods are promising tools to investigate this deformation. In a previous study of the authors, an approach using registration methods on medical images had been proposed to estimate the ear canal displacement field induced by earplug insertion. This approach had been validated in the case of computed tomography scans of a human-like artificial ear occluded by a controlled-shape custom molded earplug. In the present study, this approach is used to evaluate the ear canal displacement of a human subject for various earplug insertions (foam, pre-molded and custom made) in the case of magnetic resonance images. The computed displacement field shows noteworthy differences between each earplug and gives information on how and where the occluded ear canal deforms.

Alternative Ear-Canal Measures Related to Absorbance

Several alternative ear-canal measures are similar to absorbance in their requirement for prior determination of a Thévenin-equivalent sound source. Examples are (1) sound intensity level, (2) forward pressure level, (3) time-domain ear-canal reflectance, and (4) cochlear reflectance. These four related measures are similar to absorbance in their utilization of wideband stimuli and their focus on recording ear-canal sound pressure. The related measures differ from absorbance in how the ear-canal pressure is analyzed and in the type of information that is extracted from the recorded response. Sound intensity level and forward pressure level have both been shown to be better as measures of sound level in the ear canal compared with sound pressure level because they reduced calibration errors due to standing waves in studies of behavioral thresholds and otoacoustic emissions. Time-domain ear-canal reflectance may be used to estimate ear-canal geometry and may have the potential to assess middle ear pathology. Cochlear reflectance reveals information about the inner ear that is similar to what is provided by other types of otoacoustic emissions, and may have theoretical advantages that strengthen its interpretation.

Development of an advanced hearing protection evaluation system

Acoustic Test Fixtures (ATFs) are practical and often necessary tools for testing Hearing Protection Devices (HPDs) especially with extremely loud impulsive and/or continuous noise, for which the use of live subjects might not be advisable. Although there have been various standardized and laboratory ATFs from past research, there still exists large uncertainty in the correlation between the attenuation results obtained from ATFs and those obtained from actual human subject tests, particularly for intraaural HPDs. It is suspected that one of the main factors contributing to the discrepancy may be insufficient fidelity in the circumaural/intraaural flesh and bone. Human subject testing was performed to obtain median parameters of ear canal geometry and eardrum reflectance, which are considered to be critical parameters for circumaural/intraaural HPD attenuation performance. This presentation discusses the research methodologies and design implementation of these important subsystems in this advanced Hearing Protection Evaluation System (HPES).

Accuracy of acoustic ear canal impedances: Finite element simulation of measurement methods using a coupling tube

Acoustic impedances measured at the entrance of the ear canal provide information on both the ear canal geometry and the terminating impedance at the eardrum, in principle. However, practical experience reveals that measured results in the audio frequency range up to 20 kHz are frequently not very accurate. Measurement methods successfully tested in artificial tubes with varying area functions often fail when applied to real ear canals. The origin of these errors is investigated in this paper. To avoid mixing of systematical and other errors, no real measurements are performed. Instead finite element simulations focusing on the coupling between a connecting tube and the ear canal are regarded without simulating a particular measuring method in detail. It turns out that realistic coupling between the connecting tube and the ear canal causes characteristic shifts of the frequencies of measured pressure minima and maxima. The errors in minima mainly depend on the extent of the area discontinuity arising at the interface; the errors in maxima are determined by the alignment of the tube with respect to the ear canal. In summary, impedance measurements using coupling tubes appear questionable beyond 3 kHz.

The Measurement of 2D Curvature of In-Vivo Human Ear Canal

Problem To non-invasively measure the 2D curvature of human ear canal and produce the earmold by the non-invasive 3D ear impression. Methods The images of external ear were scanned by high-resolution computed tomography (HRCT). The resolution for each slice was 512⋉512 pixels. The pixel size was 0.188⋉0.188mm and the slice thickness was 0.625mm. The boundary between tympanic membrane and external auditory meatus was enhanced by image processing. Additionally, 3D model of ear canal was reconstructed by 2D images. The length and angle of first and second bends of canal were measured based on the 3D model. 2D curvature of first and second bends of canal was then computed by sine and cosine laws. Results The volume of ear canal was 862.0 cubic mm. The angle and curvature of superior wall of first bend at axial view were 121.5 degrees and 0.0685; of inferior wall of first bend were 246 degrees and −0.1102; of superior wall of second bend were 227.8 degrees and −0.0332; of inferior wall of second bend were 143.1 degrees and 0.0130 respectively. 2D curvature of superior and inferior wall of first and second bends was diagrammed. Conclusion The 2D curvature of ear canal at first and second bends could be measured and produce the ear impression non-invasively. The geometry of canal changed by tumors is a common syndrome in ear disease. Therefore, the geometry of ear canal can be tracked after the otoplasty. Significance The 3D geometry of canal can help physicians diagnose the syndrome of external canal before otoplasty. Moreover, the hearing aid earmold can be made by non-invasive ear impression instead of invasive ear impression.

The sound field in life‐size replicas of human ear canals occluded by a hearing aid

In addition to the longitudinal sound pressure distributions that form in the human ear canal, large transverse variations can arise in the vicinity of an occluding hearing aid. These effects are being studied, numerically and experimentally, making use of life‐size ear canal replicas. Using digital representations of real ear canal geometries [Stinson and Lawton, J. Acoust. Soc. Am. 85, 2492‐2503 (1989)], a polyjet fabrication process forms replicas with a spatial accuracy of better than 0.1 mm. A hearing aid test fixture, with vent, receiver, and an inner microphone, occludes the replica canal and provides the acoustical input. The sound field inside the canal is measured using a 0.2 mm o.d. probe microphone. In parallel, the interior sound field is calculated using a boundary element method, using the same ear canal geometry as the replicas and accounting for the acoustical boundary conditions presented by the eardrum and the hearing aid. In the current series of ear canal replicas, the eardrum is rigid. Measurements and calculations are in good agreement. Large transverse variations of sound pressure level, as much as 20 dB at 8 kHz, are observed across the inner face of the hearing aid, particularly near the receiver and vent ports.

Comparison of an analytic horn equation approach and a boundary element method for the calculation of sound fields in the human ear canal

The sound field inside a model human ear canal has been computed, to show both longitudinal variations along the canal length and transverse variations through cross-sectional slices. Two methods of computation were used. A modified horn equation approach parametrizes the sound field with a single coordinate, the position along a curved center axis-this approach can accommodate the curvature and varying cross-sectional area of the ear canal but cannot compute transverse variations of the sound field. A boundary element method (BEM) was also implemented to compute the full three-dimensional sound field. Over 2000 triangular mesh elements were used to represent the ear canal geometry. For a plane piston source at the entrance plane, the pressure along the curved center axis predicted by the two methods is in good agreement, for frequencies up to 15 kHz, for four different ear canals. The BEM approach, though, reveals spatial variations of sound pressure within each canal cross section. These variations are small below 4 kHz, but increase with frequency, reaching 1.5 dB at 8 kHz and 4.5 dB at 15 kHz. For source configurations that are more realistic than a simple piston, large transverse variations in sound pressure are anticipated in the vicinity of the source.

Sound propagation in straight, conical, exponential and real external ear canal

Mathematical and numerical modeling of the noise attenuation of hearing protectors (HP) is a powerful tool for optimizing the HP design. The human ear canal has a complex geometry and therefore can be modeled by numerical methods. Below the cut-off frequency, plane wave propagation can be modeled by simple geometric. In this paper straight, conical, exponential, and real ear canal geometry are considered. The Webster equation is used to model the sound propagation inside these canals. In this paper a comparison of sound field propagation obtained from a plane wave model of the simple geometries and the finite elements model of the real ear are presented.

The effects of variability in ear canal geometry and middle ear impedance on noise exposure level

If a group of industrial workers is uniformly exposed to a particular noise level, each person develops a different amount of hearing loss. The reasons for individual differences in susceptibility to noise are not entirely clear, but might relate, in part, to variability in ear canal acoustics and middle ear impedance. This variability alters the sound field to eardrum SPL transformation, effectively changing the magnitude of the exposure, and potentially producing differences in hearing loss. To study these effects, a computer simulation was created from an electrical analog model of the middle ear and a network representation of the ear canal. The model predicts how variation in ear canal geometry (length and diameter) and middle ear load impedance influences the eardrum SPL, both for individuals wearing hearing protector devices (‘‘leaky’’ earplugs) and for the unprotected ear. The model also estimates the amount of attenuation offered by the earplug and the amount of conductive hearing loss created by changes in the middle ear. Therefore, it was possible to evaluate whether changes in exposure levels would likely result in hearing loss, given the amount of protection offered by either earplug attenuation or conductive hearing loss.

Accounting for possible differences in population susceptibility to noise caused by variations in ear canal geometry

Analysis of large population audiometric data bases, where gender and race have been considered, have consistently yielded significant differences in the indicated hearing thresholds for these subgroups. For whatever reason, females have consistently indicated better hearing than males and blacks have consistently indicated better hearing than whites. White males typically have the worst hearing, white females and black males similar hearing patterns, and black females the most sensitive hearing of all these groups. Therefore, it was deemed appropriate to now try to take into consideration model-predicted eardrum sound pressure level differences (as transferred from the same equivalent diffuse field values) based on known ear canal geometry differences. The findings from this reanalysis of the data bases previously analyzed will be presented.

A method for determining three-dimensional vibration in the ear

In the classical concept of the middle ear function the malleus rotates around a fixed axis which implies that at small amplitudes of vibration its displacement is essentially one dimensional. As a consequence malleus vibrations have been measured previously along a single viewing axis. As a first step in the study of the complete malleus motion we determined the three dimensional components at a single point (umbo) of the manubrium. To define 3-D motion it is in principle necessary to measure the vibrations from widely different observation angles. The viewing angles are limited however in our case by the ear canal geometry to about + / − 15°. In order to resolve the 3-D components under these conditions it is necessary to measure the vibration components with high accuracy. Amplitude and phase of the umbo vibrations were measured with a heterodyne interferometer over a wide frequency range (100 Hz to 20 kHz). The system included a two axis goniometer with the axes of rotation positioned at the focal plane of the interferometer objective lens. It was therefore possible to change the viewing angle in small increments around two orthogonal axes while keeping the same point in focus. From a redundant set of measurements the three orthogonal components of vibration were calculated by least squares fitting. The vector sum of the three components gives the three dimensional motion of the observed point. The vibration of the point on the umbo was found not to follow a straight line but an elliptical path instead. The shape of the ellipse and the inclination of the plane of the ellips with respect to the stationary malleus position changed with frequency. These observations are consistent with our earlier findings that the mode of malleus vibration changes with frequency [Decraemer et al. (1991) Hear. Res. 54, 305–318].

Quantifying ear-canal geometry with multiple computer-assisted tomographic scans

Several audiological tests require knowledge of the sound-pressure spectrum at the eardrum. However, microphone readings are typically made at another, more-accessible position in the auditory canal. Recordings are then "adjusted" to the plane of the eardrum via mathematical models of the ear canal and eardrum. As bandwidths of audiological instruments have increased, ear-canal models have, by necessity, become more precise geometrically. Reported herein is a noninvasive procedure for acquiring geometry of the ear canal in fine detail. The method employs a computer-assisted tomographic (CAT) scanner in two steps to make radiographic images of parasagittal cross sections at uniform intervals along the lateral length of the canal. Accuracy was evaluated by comparing areas of cross sections appearing in radiographic images of a cadaver ear canal to cross sectional areas of corresponding michrotome slices of an injection mold of the same canal. Percent differences between these two areas had a mean value of 9.65% for 26 different cross sections of the one ear canal studied. Ear canal volume estimated from the CAT images was 6.12% different from the estimated volume of the injection mold: an improvement over the reported 39% maximum error of conventional acoustic volume measurements.

Occluded-ear simulator with variable acoustic properties.

Ear simulators were designed to replicate acoustical characteristics of the average adult ear. Due to variability of ear-canal geometry and eardrum impedance among individuals, the possibility of any one person exhibiting such "average" characteristics--especially if that person is a child and/or has a conductive pathology--is remote. Thus, ear simulators have been of only peripheral value when prescribing a hearing aid (a high output impedance device) to fit the acoustical requirements of a particular patient. Reported herein is development of a programmable artificial ear (PAE) that can account for individual differences in ear-canal geometry and eardrum impedance. It consists of a 2.0-cc coupler, microphone, amplifier, computer, PAE code, and a computer card and/or software for digitization and Fourier transformation. Required input data includes ear-canal dimensions, eardrum impedance, and output impedance of the hearing aid being tested. Sound-pressure recordings produced in the 2.0-cc coupler by the hearing aid are adjusted by the computer to what they would have been had the recordings been made at the eardrum of a particular patient wearing the same hearing aid. Good agreement was observed between experiment and theory for one test case involving a totally occluding miniature earphone.