I. Field of the Invention
The present invention relates to an ear-worn device that is comprised of a soft yet solid elastomer corpus for use in custom in-the-ear hearing products. The degree of stiffness of this soft-solid material preferably ranges from negligible to forty points, Durometer Hardness, Shore A. Specifically, the present invention relates to a system and method for producing a custom soft yet solid elastomer hearing product yielding the user greater comfort and superior acoustic performance. Additionally, this product will provide solutions to a population previously unsuccessfully fit by traditional custom in-the-ear technology. By the nature of its soft design this product will better accommodate dynamic changes in the external ear canal, that is, jaw movement, tortuous ear canals, and individuals with severe hearing loss.
The field of mass communication is also addressed for future applications of this invention, such as an ear-worn digital telephone or two-way radio system.
II. Description of the Related Art
The Hearing Health Industry has enjoyed major advancements in electronic design. The on-going miniaturization of electronics has brought the industry from a table worn unit, using vacuum tubes in the nineteen twenties, to a wearable body worn unit in the late nineteen thirties. With the introduction of transistors in the nineteen fifties, behind-the-ear (BTE) hearing aids became possible. As integrated circuits were developed the custom worn in-the-ear hearing aid became a reality. Further miniaturization has brought the industry to the latest completely in the canal (CIC) models, which are virtually invisible.
Because of demands driven by market forces more powerful than the hearing industry, electronic components have reached a new level of micro-miniaturization. As a consequence, the electronic components used in hearing aids have increased signal processing capabilities requiring very limited space. The development of programmable hearing aids, using either analog or digital processing, has lessened the need for custom electronic design at the manufacturing level, and has allowed the clinician to more directly shape the electro-acoustic response via programmable software. That is, it is no longer necessary for the device to be returned to the manufacturer for hardware changes to arrive at the desired electro-acoustic response.
In direct contrast to electronic advances in the industry, little or no advancement has been realized in shell lab technology. Since the late nineteen sixties, when the custom in-the-ear hearing aid was developed, the materials and the construction techniques have remained virtually unchanged. This industry has taken the dental industry's approach as a means of construction of custom hearing aid design. This device, commonly called a "shell", is slush molded with dental acrylic which has a ninety point Durometer Hardness Shore D. Using a material this hard in the human ear brings to fore the issues of comfort and acoustic performance especially with the ever deeper placement of the hearing instrument into the ear canal. When the acrylic shell design was introduced, hearing instruments were worn in a relatively forgiving cartilaginous portion of the ear canal. Continued micro-miniaturization of electronic components, combined with increased consumer demand for a cosmetically acceptable device, has shifted the placement of the hearing aid into the bony portion of the external auditory canal. This bony canal is extremely sensitive and intolerant of an acrylic shell that is over sized or extended beyond the second anatomical canal bend. Rigid acrylic that does not compress must pivot in reaction to jaw or head movement, thereby changing the angle of attack of the receiver yielding a distorted acoustic response. In addition the pivot action causes displacement of the entire device creating unwanted acoustic feedback. This has necessitated shell modifications reducing the scientific integrity of the original dental technology. Current shell design is thereby reduced to more of a craft than a science. The need for a more compliant shell material has been recognized by both industry and clinical professionals.
As of today, a few manufacturers have attempted all-soft shells as an alternative to acrylic shells, and wearers did report greater comfort and better sound quality. Unfortunately, while rigid acrylic does not lose dimensional stability, soft vinyl materials shrink, turn yellow, and become hard after a relatively short period of wear (the replacement of vinyl material used for BTE earmolds, for example, is recommended on at least a yearly basis). Polyurethane has proven to provide a better acoustic seal than polyvinyl, but has an even shorter wear life (approximately three months). Silicones have a long wear life but are difficult to bond to plastics, a necessary process for the construction of custom hearing instruments. Furthermore, silicone is difficult to modify when the dimensional structure requires alteration for proper fit. To date, then, acrylic has proven to be the only material with long term structural integrity. The fact remains, however, that the entire ear is a dynamic acoustic environment ill-served by a rigid material such as acrylic.
As aforementioned, the current trend for hearing aid placement is to position the instrument into the bony portion of the ear canal extending three millimeters medially from the second directional bend, commonly referred to as "deep insertion technology". Anatomically, the ear canal is defined as the area extending from the concha to the tympanic membrane. It is important to note that the structure of this canal consists of elastic cartilage laterally, and porous bone medially. The cartilaginous portion constitutes the outer one third of the ear canal. The medial one-third of the ear canal is osseous or bony and is oriented forward and downward making it slightly concave as compared to the more cylindrical cartilaginous portion. The average canal is approximately twenty-five millimeters in length but is as much as six millimeters longer on the anteroinferior wall of the osseous canal. The osseous canal, measuring only two-tenths of a millimeter (0.2 mm) in thickness, is much thinner than the cartilaginous canal, measuring five-tenths to one millimeter (0.5 to 1 mm) in thickness. The difference in thickness directly corresponds to the presence of apocrine (cerumenous) and sebaceous glands found only in the fibrocartilaginous area of the canal. Thus, this thin-skinned poorly-lined area of the bony canal is extremely sensitive to any hard foreign body, such as an acrylic hearing instrument.
Exacerbating the issue of placement of a hard foreign body into the ear canal is the ear canal's dynamic nature. It is geometrically altered by the tempro-mandibular action of the mandible and by changes in head position. This causes elliptical elongation (widening) of the ear canal. These alterations in canal shape vary widely not only from person to person, but also from ear to ear within a person. This canal motion makes it impossible to achieve a comfortable, true acoustic seal with hard acrylic material. When the instrument is displaced by motion, a leakage of sound pressure occurs. This leakage, described by Victoreen as "peripheral leakage", creates an open loop between the receiver and the microphone and relates directly to an electroacoustic distortion commonly known as feedback. This peripheral acoustic leakage (PAL) is a complex resonator made up of many transient resonant cavities. These cavities are transient because they change with jaw motion as a function of time resulting in impedance changes in the ear canal that destroys even the best electroacoustic performance, digital or analog.
The properties of hard acrylic have obvious limitations. These limitations require modification to the hard shell exterior to accommodate anatomical variants and the dynamic nature of the ear canal. The shell must be buffed and polished until comfort is acceptable without significantly compromising acoustic performance. The peripheral acoustic leakage caused by these modifications lead to acoustic feedback (whistling) before sufficient amplification is reached to adequately address the wearer's hearing loss. Additionally, this acoustic leakage causes annoying low frequency sounds to be inadvertently amplified by means of a Helmhotz resonator. Patients commonly report this sensation as "My voice is hollow" or "My head sounds like it is in a barrel."
Furthermore, the hollow shell used in today's hearing aid designs creates internal acoustic feedback pathways, which are unique to each device and ear, causing countless electronic modifications to "tweak" the product to a compromised performance or a "pseudo-perfection." In the industry's efforts to facilitate the fine-tuning of these acoustic anomalies, programmable devices were developed. The intent was to reduce the degree of compromise, but by virtue of their improved frequency spectrum the number of high frequency feedback pathways were increased. As a result the industry still falls well short of an audiological optimum.
As we trace the evolution of earmold technology from the early seventies, it is apparent that venting systems, earmold plumbing, acoustic feedback and occlusion effect have been the subjects of extensive research. Excellent studies by Lybarger, Pascoe, Skinner, Sung, Cox, Killion and others have demonstrated the benefits of venting systems and earmold plumbing. In venting studies by Curran, peripheral acoustic leakage (PAL) was categorized as a venting system called a "slit" vent. The paper points out that both high and low frequency components of the spectrum were affected as opposed to the traditional low frequency roll off which normally occurs with parallel venting. A more recent study by Tecca reiterated the earlier findings of Lybarger in that ITE venting were dramatically affected by the diameter and length of the vent. As we continue with deeper placement of instruments, this knowledge of PAL and venting becomes more critical to a successful fit.
Some references of interest are discussed below. These references are all incorporated herein by reference.
U.S. Pat. No. 4,870,688 to Voroba, Barry, et al.
Voroba describes a patient selected mass produced, non-custom molded form fitting shell with a malleable covering having a hook and twist which in theory precisely conforms to the patient's own ear.
U.S. Pat. No. 4,880,076 to Ahlberg, Carl, et al.
Ahlberg discloses a user-disposable foam sleeve comprising a soft polymeric retarded recovery foam that can be compressed to be freely inserted into the patient's ear and then allowed to expand until secure in the ear canal.
Other patents that may be of interest include the following:
U.S. Pat. No. 5,002,151 to Oliveira, Robert J., et al.; PA1 U.S. Pat. No. 4,607,720 HEARING AID; PA1 U.S. Pat. No. 4,375,016 VENTED EAR TIP FOR HEARING AID AND ADAPTER COUPLER THEREFOR; PA1 U.S. Pat. Nos.: 5,500,902; 4,051,330; 5,430,801; 5,185,802; 5,201,007; 4,937,876; 4,811,402; 4,716,985; 5,659,621; 5,530,763; 5,068,902; and 5,259,032. PA1 1. to provide an instrument that, by virtue of its softness, will not migrate out of the external ear canal with jaw excursion. PA1 2. to provide an instrument that, by virtue of its softness, will remain acoustically sealed with jaw excursion. PA1 3. to provide an instrument that, by virtue of its softness, will not exert pressure on the bony portion of the external ear canal with jaw excursion. PA1 4. to provide an instrument that, by virtue of its softness, will ensure correct receiver orientation during jaw excursion. PA1 5. to provide an instrument that, by virtue of its solidity, will protect the embedded electronic components. PA1 6. to provide an instrument that, by virtue of its solidity, will eliminate the internal air conducted feedback pathway from the receiver to the microphone. PA1 7. to provide an instrument that, by virtue of the solidity, will eliminate the need to tube-mount the receiver, thereby preventing displacement of the receiver within the hearing instrument.
Also of interest are published Japanese patent application no. JA61-238198 and Staab, Wayne J. and Barry Finlay, "A fitting rationale for deep fitting canal hearing instruments", HEARING INSTRUMENTS, Vol. 42, No. 1, 1991, pp. 7-10, 48.