The present invention relates generally to transducers for converting audio signals to audible mechanical vibrations, and more particularly to hearing devices with improved energy efficiency, sound fidelity, and inconspicuous wear.
The external acoustic means (ear canal) is generally narrow and contoured as shown in the coronal view in FIG. 1. The ear canal 10 is approximately 27 mm in length from the canal aperture 17 to the center of the tympanic membrane 19 (eardrum). The lateral part (away from the tympanic membrane) of the ear canal, a cartilaginous region 11, is relatively soft due to the underlying cartilaginous tissue. The cartilaginous region 11 of the ear canal 10 deforms and moves in response to the mandibular (jaw) motions, which occur during taking, yawning, eating, etc. The medial (towards the tympanic membrane) part, a bony region 13 proximal to the tympanic membrane, is rigid due to the underlying bony tissue. The skin 14 in the bony region 13 is thin (relative to the skin 16 in the cartilaginous region) and is more sensitive to touch or pressure. There is a characteristic bend 15 that roughly occurs at the bony-cartilaginous junction 19 (referred to herein as the bony junction), which separates the cartilaginous 11 and the bony 13 regions. The magnitude of this bend varies among individuals.
Hair 5 and debris 4 in the ear canal are primarily present in the cartilaginous region 1. Physiologic debris includes cerumen (earwax), sweat, exfoliated skin and hair, and oils produced by the various glands underneath the skin in the cartilaginous region. Non-physiologic debris consists primarily of environment particles that enter the ear canal. Canal debris is naturally extruded to the outside of the ear by the process of lateral epithelial cell migration that begins on the eardrum and extends the length of the ear canal (see. e.g., Ballachanda, The Human ear Canal, Singular Publishing, 1995, pp. 78, 195). There is no cerumen production or hair in the bony part of the ear canal.
The ear canal 10 terminates medially with the tympanic membrane 18 which has a characteristic conical depression at its center—known as the umbo 20. Laterally and external to the ear canal is the concha cavity 2 and the auricle 3, both also cartilaginous. The junction between the concha cavity 2 and the cartilaginous part 11 of the ear canal at the aperture 17 is also defined by a characteristic bend 12 known as the first bend of the ear canal.
The tympanic membrane is medially connected to the handle of the malleus ossicle 21 (FIG. 2) which is connected to the incus 22, stapes 23 and ligaments and muscles (not shown) within the middle ear cavity 25. The tympanic membrane 18 and associated middle ear ossicles 21, 22 and 23 are extremely sensitive to pressure waves which are imperceptible by even the most delicate receptors of skin.
Hearing loss affects a substantial percentage of the population, and is of several types. Hearing loss occurs naturally with aging, beginning with the higher frequencies (4000 Hz and above) and increasingly spreads to lower frequencies. Conductive losses attributable to obstruction of the transmission of mechanical vibrations in the middle ear or the tympanic membrane also effect the hearing. It is customary, of course, to fit individuals who suffer from hearing loss with hearing aid devices if they cannot be treated with medication or surgery.
In general, conventional hearing aids rely primarily on air-conduction transducers to produce amplified acoustic pressure waves which are transmitted to the tympanic membrane through the air between the transducer and the tympanic membrane. These transducers, also referred to as receivers or speakers, are used in various audio devices including telephones, and other communication devices. Recent advances in miniaturization have led to new types of hearing aids that fit deeply in the ear canal, with receivers close to the tympanic membrane. Such devices are largely inconspicuous, and thereby tend to alleviate the social stigma and vanity concerns associated with wearing a visible hearing aid, which are considered a significant obstacle to hearing aid use among the hearing impaired population. Nevertheless, a number of fundamental limitations remain in hearing devices that utilize air-conduction based technology, including problems of (1) frequency, daily device handling, (2) acoustic feedback, (3) ear canal occlusion, and (4) low sound fidelity.
The problem of frequent conventional device handling relates to frequent insertion and removal from the ear canal. Conventional hearing aids are typically removed daily to relieve the ear canal from contact pressure. The requirement of frequent handling, particularly with miniature hearing devices, poses a serious challenge to potential users who suffer physical impairment beyond hearing loss because of age or disorders, such as arthritis, tremors, or other neurologic problems. Frequent hearing air removal is also required to replace the battery. For miniature canal devices (the term “canal devices” refers to miniature hearing devices that are primarily fitted in the ear canal, and includes the industry-recognized “In-The-Canal” (ITC) devices and “Completely-In-the-Canal” (CIC) devices), typical battery lifetimes range from few days to two weeks. The need for frequent battery replacement is attributable in large part to the magnitude of energy consumption by conventional air-conduction receivers (speakers).
The problem of acoustic feedback occurs when a portion of the sound output, typically from a receiver (speaker), leaks to the input of the hearing system such as a microphone of a hearing aid. Such leakage often causes a sustained oscillation which is manifested by “whistling” or “squealing”. Acoustic feedback is not only annoying to hearing aid users but also interferes with their speech communication. Feedback is a common occurrence in conventional hearing aids since the output of the device (acoustic) is in the same form of energy as the input of the device (also acoustic). Feedback is typically alleviated by occluding (sealing) the ear canal tightly with the hearing device. Whichever acoustic sealing method is used, ear canal occlusion causes an array of occlusion-related side effects.
Occlusion related problems include discomfort, irritation and even pain; moisture building-up in the occluded ear canal; cerumen impaction; and acoustic occlusion effect, Discomfort, irritation and pain may occur from canal abrasion caused by frequent insertion and removal of a tightly fitted hearing device. Moisture build-up in the occluded ear canal can cause infection in the ear canal as well as damage to the hearing device within. To reduce possible damaging effects of anal moisture, it is recommended that hearing devices be removed daily.
Another important problem is cerumen impaction (i.e., blockage of the ear canal by earwax) which occurs when ear wax is pushed deeper in the ear canal by the frequent insertion of a hearing device. Cerumen can also build up on the receiver of the hearing device, thereby causing frequent malfunction.
The occlusion effect is a common acoustic problem caused by the occluding hearing device, manifested by the perception of a person's own-voice (“self-voice”) being loud and unnatural compared to that with the open ear canal. This phenomenon is sometimes referred to as the “barrel effect” since it resembles the experience of talking into a barrel.
Low or inadequate sound fidelity is often experienced with air-conduction receivers (speakers), particularly in hearing aid applications where the frequency response is limited to about 5000 Hz.
Considering the state of the art in alternative hearing device technology, hearing devices employing transducers that are not based on air-conduction are well known in the art. The rationale is that when no acoustic output is present in such devices, oscillatory feedback is usually reduced and in most cases eliminated. Distortion and frequency response characteristics are also potentially improved.
For example, vibratory middle ear implants attempt to circumvent some of the above-cited limitations by vibrating directly any of the ossicular (middle ear bones) or cochlear structures. Vibratory transducers and hearing devices for middle ear implant are disclosed in numerous patents, e.g., U.S. Pat. Nos. (U.S. Pat. Nos.) 3,594,514 to Wingrove, 3,870,832 to Fredrickson, 3,882,285 to Nunley et al., 5,015,224 to Maniglia, and 5,554,096 and 5,456,654 to Ball. The transducer technology employed includes piezoelectric and electromagnetic elements, which provide electrical output via an electrical wire connection to the transducer. Disadvantages of middle ear implants include the cost and risk involved in the surgical procedure, and the additional surgery that may be required to repair device malfunctions or to replace an implanted battery.
Several other hearing systems that are less invasive have been proposed and are known in the art. Magnetic transducers which are surgically implanted or surgically attached to the tympanic membrane are disclosed in a number of patents, e.g., U.S. Pat. Nos. 4,840,178 and 5,220,918 to Heide et al., 4,817,607 to Tatge et al., 4,606,329, 4,776,322 and 5,015,225 to Hough et al., 4,957,478 to Maniglia, 5,163,957 to Sade et al., and 5,338,287 to Miller et al. These transducers typically employ high energy-product magnets which vibrate in response to a radiant electromagnetic signal, representative of acoustic signals. The electromagnetic signal is typically radiated by a coil positioned in the external ear canal (e.g., 44 of FIG. 1 in the Manigila '478 patent, and 28 of FIG. 1 in the Tatge '607 patent). Similarly, a primary disadvantage of this type of device is the cost and risk of surgery performed on the delicate vibratory structures of the ear.
Among others of the less invasive approaches to hearing systems are those proposed in U.S. Pat. Nos. 5,259,032 to Perkins et al., and 5,425,104 to Shennib. In each of these disclosures, a magnet transducer is attached non-surgically to the exterior side of the tympanic membrane, and transducer receives radiant electromagnetic signals from a device in the ear canal (FIG. 4 of the Perkins et al '032 patent), or from an externally positioned coil (FIGS. 1A and 1B of the Schennib '104 patent).
A major disadvantage with all of the above electromagnetic hearing systems is the inefficiency associated with transducing radiant electromagnetic energy into magnet vibrations, attributable to the relatively small portion of radiant electromagnetic energy produced by the coil that reaches the magnet. As is known in the art of electromagnetics, the efficiency of such coupling is inversely proportional to the distance between the driving coil and the magnet transducer. This and other limitations of such devices render the various modes of radiant electromagnetic transconduction impractical for most hearing aid applications.
A potentially more energy efficient transducer and hearing system is disclosed in U.S. Pat. No. 5,624,376 to Ball et al. In a non-invasive embodiment of the transducer disclosed in FIG. 19a of the Ball et al '376 patent, a floating mass transducer 100 is attached non-surgically to the exterior side of the tympanic membrane via an attachment membrane 502. The transducer 100 may be directly connected (not shown, but disclosed at col. 16, line 62) to a hearing device 506 via electrical wires 24. The “floating mass transducer” (FIG. 3), incorporates a magnet 42 (floating mass) and a coil 14 within a housing 10. The transducer 100 is free to vibrate within the housing 10 in response to the electrical signal via wires 24. The inertial forces of the vibrating magnet cause the housing to vibrate and subsequently vibrate the attached tympanic membrane and ossicles. According to the Ball et al '376 patent, vibrational forces are maximized by optimizing the mass of the magnet assembly relative to the combined mass of coil and housing, and the energy product of the permanent magnet. Since the transducer receives electrical energy directly from the hearing device via a wire, energy loss is reduced and the device is potentially more energy efficient than air-conduction or radiant electromagnetic hearing systems. However, a major disadvantage of the floating mass transducer is the weight of the transducer assembly being positioned directly on the tympanic membrane.
Another alternative to air-conduction hearing devices is disclosed in U.S. Pat. Nos. 4,628,907 and 4,756,312 to Epley. The Epley '907 patent describes a canal hearing device with an electromechanical transducer part directly contacting the tympanic membrane (FIG. 1), the contact element 38 being secured to the tympanic membrane by clip means for attachment to malleus bone (claim 1). The devices are not only invasive as disclosed, but also pose a considerable risk to the delicate structures of the tympanic membrane from inadvertent movement of the hearing device, which may occur, for example, simply by normal jaw motion.
Many of these prior art devices are either energy inefficient or occlusive to the ear canal which render them impractical for extended wear. As used in the present application, extended wear use means continuous placement and operation of a hearing device within the ear canal for at least two months.
Leysieffer in U.S. Pat. No. 5,833,626 describes a non-invasive hearing testing method involving vibrating the tympanic membrane via a rod placed within the ear canal. Leysieffer is primarily concerned with providing, temporarily, audiological test signal to the test ear while minimizing audibility by the contralateral ear which is not being examined. Clearly, Leysieffer's invention is not concerned with hearing devices and particularly devices for extended wear within the ear canal.
Shennib et al., in the aforementioned co-pending '486 application, describes a canal hearing device having a thin elongated vibrational assembly which directly contacts the tympanic membrane causing audible vibrations. The canal hearing device of that invention uses strain relief methods for minimizing static pressures on the tympanic membrane by the coupled vibrational assembly. However, device movements within the ear canal or changes in the atmospheric pressure affecting the position of the tympanic membrane, can cause considerable variations in the dynamic coupling and therefore the perceived sound. The effect of dynamic coupling due to changes in the static coupling is highly undesirable since it necessitates readjustment of the electroacoustic parameters (i.e., volume, frequency response, etc.) whenever changes in the static coupling occur.
A key goal of the present invention is to provide is to provide efficient sound conduction by vibrating the tympanic membrane directly and consistently regardless of the exact position of the canal hearing device with respect to tympanic membrane.
Another goal of the present invention is to position a vibration force transducer within the ear canal at a distance from the tympanic membrane thus minimize the mass loading effect on the tympanic membrane.
An other goal of the present invention is to offer an inconspicuous and non-occlusive energy efficient hearing device suitable for extended wear within the ear canal.
Extended wear as used in this specification and appended claims is defined as continuous placement and use of the hearing device within the ear canal without need for removal for at least about two months.