Jonathan P. Fay

The discordant eardrum

At frequencies above 3 kHz, the tympanic membrane vibrates chaotically. By having many resonances, the eardrum can transmit the broadest possible bandwidth of sound with optimal sensitivity. In essence, the eardrum works best through discord. The eardrums success as an instrument of hearing can be directly explained through a combination of its shape, angular placement, and composition. The eardrum has a conical asymmetrical shape, lies at a steep angle with respect to the ear canal, and has organized radial and circumferential collagen fiber layers that provide the scaffolding. Understanding the role of each feature in hearing transduction will help direct future surgical reconstructions, lead to improved microphone and loudspeaker designs, and provide a basis for understanding the different tympanic membrane structures across species. To analyze the significance of each anatomical feature, a computer simulation of the ear canal, eardrum, and ossicles was developed. It is shown that a cone-shaped eardrum can transfer more force to the ossicles than a flat eardrum, especially at high frequencies. The tilted eardrum within the ear canal allows it to have a larger area for the same canal size, which increases sound transmission to the cochlea. The asymmetric eardrum with collagen fibers achieves optimal transmission at high frequencies by creating a multitude of deliberately mistuned resonances. The resonances are summed at the malleus attachment to produce a smooth transfer of pressure across all frequencies. In each case, the peculiar properties of the eardrum are directly responsible for the optimal sensitivity of this discordant drum.

The EarLens System: New Sound Transduction Methods

The hypothesis is tested that an open-canal hearing device, with a microphone in the ear canal, can be designed to provide amplification over a wide bandwidth and without acoustic feedback. In the design under consideration, a transducer consisting of a thin silicone platform with an embedded magnet is placed directly on the tympanic membrane. Sound picked up by a microphone in the ear canal, including sound-localization cues thought to be useful for speech perception in noisy environments, is processed and amplified, and then used to drive a coil near the tympanic-membrane transducer. The perception of sound results from the vibration of the transducer in response the electromagnetic field produced by the coil. Sixteen subjects (ranging from normal-hearing to moderately hearing-impaired) wore this transducer for up to a 10-month period, and were monitored for any adverse reactions. Three key functional characteristics were measured: (1) the maximum equivalent pressure output (MEPO) of the transducer; (2) the feedback gain margin (GM), which describes the maximum allowable gain before feedback occurs; and (3) the tympanic-membrane damping effect (D(TM)), which describes the change in hearing level due to placement of the transducer on the eardrum. Results indicate that the tympanic-membrane transducer remains in place and is well tolerated. The system can produce sufficient output to reach threshold for those with as much as 60 dBHL of hearing impairment for up to 8 kHz in 86% of the study population, and up to 11.2 kHz in 50% of the population. The feedback gain margin is on average 30 dB except at the ear-canal resonance frequencies of 3 and 9 kHz, where the average was reduced to 12 dB and 23 dB, respectively. The average value of D(TM) is close to 0 dB everywhere except in the 2-4 kHz range, where it peaks at 8dB. A new alternative system that uses photonic energy to transmit both the signal and power to a photodiode and micro-actuator on an EarLens platform is also described.

Preliminary evaluation of a light-based contact hearing device for the hearing impaired.

Objective To assess the safety, stability, and performance of the broad-spectrum, light-based contact hearing device (CHD) on listeners with hearing impairment. Study Design Feasibility study. Setting Single-site research and development facility. Participants Thirteen participants with symmetric mild-to-severe sensorineural hearing impairment had the CHD placed bilaterally. Intervention A custom-molded light-activated tympanic contact actuator (TCA) was placed into each ear by a physician, where it stayed in contact with the umbo and a portion of the medial wall of the ear canal for 4 months. Each CHD was calibrated and programmed to provide appropriate broad-spectrum amplification. Main Outcome Measures Safety was determined through routine otologic examinations. Aided and pre-TCA-insertion unaided audiometric thresholds (functional gain), maximum gain before feedback, tympanic membrane damping, Reception Threshold for Sentences (RTS), and Abbreviated Profile of Hearing Aid Benefit (APHAB) measurements were made to characterize system performance as well as the benefits of amplification via the CHD. Results The TCAs remained on participants’ ears for an average total of 122 days, without causing signs of inflammation or infection, and there were no serious device-related adverse events. Measured average maximum output of 90 to 110 dB SPL in the range of 0.25 to 10 kHz, average maximum gain before feedback of 40 dB, and functional gain through 10 kHz show extended-bandwidth broad-spectrum output and gain. RTS results showed significant aided improvements of up to 2.8 dB, and APHAB results showed clinically significant aided benefits in 92% of participants (11/12). Conclusion The safety, stability, and performance demonstrated in this initial 4-month study suggest that the CHD may offer a feasible way of providing broad-spectrum amplification appropriate to treat listeners with mild-to-severe hearing impairment.

Inflation of Rolled Tubes

Extensive plans are in progress for large structures in space applications, as discussed in the 2nd National Space Inflatables Workshop (1998). The consensus is that much of this will be possible only with inflatable structural elements. There are considerable challenges in the development of such devices as reliable tools. Analytical simulation can play a significant role in speeding the development and increasing the reliability. However, the analysis of an inflated tube undergoing large displacements is prohibitive for a direct calculation using thin shell theory. An evaluation of the approximate techniques in use for inflatables is given by Jenkins (1991). A comprehensive study of nonlinear shell theory is given by Libai and Simmonds (1998). However, a convenient method, both sufficiently accurate and numerically efficient, for dealing with the deployment of membranes and complex structures has not seemed to be available.

Bending and symmetric pinching of pressurized tubes

Abstract Two experiments quantified the forces necessary for large deformation of an inflated cylindrical tube made of a material with a high elastic modulus. In the first experiment, the end force required to maintain a buckled cylinder at a given kink angle was determined. In the second experiment, the lateral force required to pinch the membrane symmetrically between two flat blades was measured. An approximate theory is used, based on the observation that during deformation the membrane conserves its initial zero Gaussian curvature in regions free of wrinkling. The novel feature is a simple approximation for the cross-sectional shape. This permits the volume of the deformed cylinder to be quickly calculated. For walls that have negligible extensional and bending energy, the potential energy consists of only the pressure multiplied by the volume and the work of the prescribed load. Minimization of this potential energy yields results for the indentation and buckling problems that are in reasonable agreement with the experimental measurements. For small displacements in the blade pinching experiment, the volume approximation overestimates the force. It is found that a local solution analogous to the Hertzian contact problem provides a better approximation. For the kinked tube with end loading, an interesting feature is a decrease in the load when the fold from one side contacts the opposite side of the tube. The calculations indicate that a minimum potential energy exists with the fold straight. For slightly larger kink angles, however, the fold buckles out of the plane of symmetry. The moment at the single kink, due to the end loads, remains between bounds from the analysis of a pressurized elastic tube with nonpositive stresses.

Forces for Rolling and Asymmetric Pinching of Pressurized Cylindrical Tubes

The space application of inflatable structures makes desirable a better understanding of how membranes inflate from tightly packed arrangements. Two experiments determined the static forces and moments induced by internal pressure in a partially deployed membrane. The first experiment examined the forces at a fold or crease line. The second characterized the torque on a rolled-up membrane. In both experiments regions of nearly constant curvature dominated the deformed tube, which is consistent with general considerations of thin-shell behavior. This observation permits a simple approximation for the calculation of the volume of the deformed configuration. The volume can then be used in a potential energy formulation. The results from this formulation agree with the experimental measurements. Therefore, this formulation shows promise for simulating the inflation process with minimal computational effort. Nomenclature A = cross-sectional area of membrane a = semimajor axis of elliptical section of membrane E = Youngs modulus of Mylar (measured value), 855 MPa F = downward force on the blade h - gap height between blade and base board hb = vertical distance from the bottom edge of the poly vinyl chloride (PVC) pipe to the roll, 14 mm h i = contact height on left side of blade hi = contact height on right side of blade L = horizontal distance between the end of the PVC pipe and the center of the axle or blade L i = axial distance between the centers of curvature of the top and bottom cross cylinders L2 = axial distance from the center of the top cross cylinder to the axle

The EarLens Photonic Transducer: Extended Bandwidth

Objective: Light pulses transmitted through the external auditory canal are captured by a photodetector on a tympanic membrane contact transducer creating a current, which activates a micro-motor. The hypothesis that an extended bandwidth (10 kHz) contact transducer residing on the tympanic membrane will improve speech understanding in noise is tested. Method: Reception thresholds of sentences (RTS) were measured with target speech at -45° and two talkers at +45°, simulating a photonic transducer on (1) normal and (2) hearing-impaired subjects. HINT sentences and masking speech materials were recorded at 24-kHz bandwidth and low-pass filtered at 4, 6, 8, and 10 kHz. Results: For normal hearing subjects (n = 12), the mean RTS was -17.4, -19.1, -19.3, and -20.7 dB for 4, 6, 8, and 10 kHz respectively. For hearing impaired subjects (n = 12), the RTS unaided full bandwidth was -10.0 dB and -10.0, -11.9, -12.2, and -12.4 dB for aided 4, 6, 8, and 10 kHz respectively. The results indicate significant improvements of 3.3 dB (P < .001) in normal hearing and 2.4 dB (P = .02) for hearing impaired subjects for the 10 kHz vs the 4 kHz bandwidths. Initial measurements with the IDE-approved photonic hearing device are consistent with the simulation study. Conclusion: The results suggest that acoustic cues above 4 kHz in a hearing device can greatly enhance the ability to hear target speech in noisy environments. This opens up possibilities for a unique hearing system using only light to transmit sound to vibrate the tympano-ossicular system.

Cat Eardrum Response Mechanics

The function of the eardrum and middle ear is to resolve the acoustic impedance mismatch between the air of the outside world and the fluid of the inner ear. Without an impedance matching device, very little acoustic energy would be absorbed by the inner ear and hearing would be severely limited. Although the role of the middle ear is clear, it remains a mystery how the eardrum accomplishes this task over the audible frequency range. In the present work. a computer simulation of the cat eardrum was constructed. For the first time, the vibrations of the eardrum were fully coupled to the acoustics of the ear canal and the dynamics of the middle ear bones. The fibrous microstructure of the eardrum was taken into account. This model appears to be valid up to 20 kHz, which represents a significant improvement over previous modelling efforts.

Computational study of cat eardrum mechanics

The function of the eardrum and the middle ear is to resolve the acoustic impedance mismatch between the air of the outside world and the fluid of the inner ear. Without an impedance matching device, very little acoustic energy would be absorbed by the inner ear and hearing would be severely limited. It remains a mystery how the eardrum accomplishes this over the audible frequency range. A computer simulation of the cat eardrum was constructed. The vibrations of the eardrum were fully coupled to the acoustics of the ear canal and the dynamics of the middle ear bones. The eardrum’s fibrous microstructure was taken into account. This model appears to be valid up to 20 kHz. This represents a significant improvement over previous modeling efforts. With this model, the following questions surrounding the evolution and design of the eardrum have been addressed: (1) Why does the eardrum have its distinctive conical and toroidal shape? (2) What is the significance of its highly organized fibrous structure? (3) Wh...