Abstract:
A hearing aid comprises a microphone that receives incident sound waves from one or more sources external to the hearing aid, and converts the sound waves into electronic signals; a circuit that amplifies the electronic signals; a receiver that converts the amplified electronic signals into amplified sound waves; and a tuned resonant cavity between the microphone and the at least one external sound source. At least one parameter of the tuned resonant cavity is selected to modify the frequency response of the incident sound waves that are received by the microphone. In particular, the geometry of one or more openings through which sound waves enter the chamber, the geometry of the chamber itself, and/or the geometry of one or more openings through which sound waves exit the chamber, are selected to condition the incident sound waves by modifying the frequency response of the audio signal prior to the signal being received at the microphone.

Description:
RELATED APPLICATIONS 
     This application is related to a co-pending U.S. Utility application Ser. No. 11/343,969, filed Jan. 30, 2005 entitled “Hearing Aid Circuit With Integrated Switch and Battery,” in the name of Walter P. Sjursen, Michael DeSalvo, Hassan A. Mohamed, Paul J. Mulhauser, and Karl D. Kirk III. This application is also related to a co-pending U.S. Design Pat. No. 29/253,043, filed Jan. 30, 2006 entitled “Hearing Aid,” in the name of Walter P. Sjursen, Michael DeSalvo, Hassan A. Mohamed, Paul J. Mulhauser and Karl D. Kirk, III. The entire teachings of the above applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     A hearing aid, in general, comprises a housing or ear mold which contains a receiver, a microphone, electronic circuitry connecting the receiver and the microphone, and a battery for operating the electronic circuitry. The housing is an ear mold which fits into the ear canal of the user. 
     In a conventional hearing aid, the microphone converts incident sound waves into an analog electrical signal which is then processed to filter out unwanted noise etc., amplified, and coupled to a receiver or speaker which converts the electrical signal back to sound waves. The electrical signal processor may be an analog processor which operates directly upon an analog electrical signal. Alternatively, the analog signal may be converted to a digital signal and processed by a digital signal processor (DSP). Typically, the signal processing circuitry is designed to provide a particular frequency response in order to compensate for the type of hearing loss suffered by the user. For example, one common type of hearing impairment is the difficulty in hearing soft sounds at high audible frequencies. Thus, it is common for the signal processing scheme of a hearing aid to increase the gain of these high-frequency sounds relative to lower frequency sounds. 
     There has been a growing need for small, reliable, easy to use low-cost hearing aids. In particular, it would be desirable to be able to provide a low-cost hearing aid design that could meet the needs of the vast majority of users experiencing age related hearing loss. 
     One approach to meet these goals has been the development of low-cost, mass-produced hearing aids, including disposable hearing aids. The disposable hearing aid is of a structure that is so inexpensive to manufacture that it is possible to merely replace the whole hearing aid, rather than just the battery, when the battery runs out. Thus, the life of a disposable hearing aid is dependent on the life of the battery. Examples of disposable hearing aids are described in, for example, U.S. Pat. No. 5,881,159 to Aceti et al., U.S. Pat. No. 6,058,198 to Aceti et al., U.S. Pat. No. 6,473,511 to Aceti et al., and U.S. Pat. No. 6,865,279 to Leedom, and in U.S. patent application Ser. No. 09/804,978 to Leedom et al., and Ser. No. 10/688,099 to Leedom et al., the entire teachings of which are incorporated herein by reference. 
     A limiting factor on the development of high-quality, inexpensive hearing aids is that many of the component parts of these devices, such as the signal processing circuitry, remain relatively expensive. Thus, there is a need to further reduce the cost of hearing aids. 
     SUMMARY OF THE INVENTION 
     A hearing aid comprises a microphone that receives incident sound waves from one or more sources external to the hearing aid, and converts the sound waves into electronic signals; a circuit that amplifies the electronic signals; a receiver that converts the amplified electronic signals into amplified sound waves; and a tuned resonant cavity between the microphone and the at least one external sound source. At least one parameter of the tuned resonant cavity is selected to modify the frequency response of the incident sound waves that are received by the microphone. In particular, the geometry of one or more openings through which sound waves enter the chamber, the geometry of the chamber itself, and/or the geometry of one or more openings through which sound waves exit the chamber, are selected to condition the incident sound waves by modifying the frequency response of the audio signal prior to the signal being received at the microphone. 
     In one aspect, the tuned resonant cavity acts much like a passive acoustical filter. The geometry of the cavity is designed to provide a desired frequency response in the incident audio signal. For instance, the geometry of the cavity can result in certain audio frequencies, or ranges of frequencies, being amplified or attenuated relative to the other frequencies, resulting in a conditioned or filtered audio signal being received at the microphone. The physical characteristics of the tuned resonant cavity can be represented as an electronic circuit, specifically an RLC circuit, and the frequency response of the sound waves in the cavity can be analyzed by reference to the frequency response of the corresponding electrical circuit representation of the cavity. 
     In general, the incident sound waves that are conditioned by the resonant cavity can comprise sound waves having frequencies between 1 and 10 kHz, and more specifically between 5 and 7 kHz. The parameters of the resonant cavity that can be selected to condition the incident sound waves include, for example, the number of entrance holes into the cavity, the cross-sectional area of the entrance hole(s), and the shape of the entrance hole(s). In addition, the number of exit holes from the cavity, the cross-sectional area of the exit hole(s) and the shape of the exit hole(s) can also be selected. Other parameters of the resonant cavity that can be selected include the volume of the cavity, the shape of the cavity, and the materials of the cavity. 
     An advantage of the present hearing aid is that the tuned resonant cavity can be designed to provide a frequency response(s) that help compensate for various types of hearing loss. For instance, the tuned resonant cavity can be designed to increase the gain at higher frequencies relative to lower frequencies in the incident sound signal, since one common type of hearing impairment is a difficulty in hearing low-volume, high-frequency sounds. The tuned resonant cavity can help reduce the cost of the hearing aid, since the passive conditioning of the incident sound waves afforded by the tuned resonant cavity minimizes the requirements of the signal processing electronics. Thus, smaller, less complex, and/or less expensive circuitry can be employed. In addition, because the tuned resonant cavity is a passive component that does not consume any electrical power, the power requirements of the hearing aid are reduced. This is particularly important in the context of a disposable hearing aid, since lower power requirements translate to a longer useful life for the hearing aid. 
     In certain embodiments, the tuned resonant cavity is an integral component of the hearing aid. For example, the cavity can comprise a portion of a hearing aid shell that contains the hearing aid electronic components (e.g., the microphone, battery, circuitry, and receiver). The hearing aid shell can comprise a face plate having one or more openings for sound waves. The microphone can be mounted generally parallel and spaced away from the face plate within the hearing aid shell, and the tuned resonant cavity can comprise the interior volume of the shell between the face plate and the microphone. Preferably, the tuned resonant cavity is substantially acoustically isolated from the rest of the hearing aid shell. For instance, the microphone can be sealed into the interior of the shell (by a gasket or o-ring, for example), so that sound is contained in the tuned resonant cavity. If necessary, a small opening can be provided to permit air to travel behind the microphone into the interior of the hearing aid shell (for example, to provide oxygen for an air-activated battery). However, the acoustical impedance of this opening is preferably sufficiently high to substantially prevent audible sound waves from exiting the cavity through the opening. 
     In other embodiments, the tuned resonant cavity comprises a component that is mounted to or within the hearing aid. The cavity can be separately manufactured for incorporation within the hearing aid. An advantage of this is that the geometry of the cavity can generally be more precisely controlled than in the case where the resonant cavity is formed from the hearing aid shell. In certain embodiments, the tuned resonant cavity can comprise a conduit that is mounted in front of the microphone. The conduit can have any practical size and shape. It can have a cross-section that is substantially circular, elliptical, triangular, rectangular, or irregularly-shaped. Preferably, a first end of the conduit contacts a surface of the microphone. The cross-sectional area of the first end of the conduit is preferably approximately equal to the area of the microphone diaphragm, and the first end of the conduit can be substantially aligned with the diaphragm. Preferably, the interface between the first end of the conduit and the microphone diaphragm is substantially sealed, so that the sound waves in the conduit are directed into the microphone diaphragm. 
     In another aspect, the first end of the conduit comprises a flange that extends radially from the conduit. The conduit can comprise part of a switching mechanism for modifying an operating state of the hearing aid. In one embodiment, at least one switch trace for the switching mechanism is mounted directly or indirectly on the flange of the conduit. For example, a circuit board can be mounted on the flange of the conduit, and at least one switch trace can be located on the circuit board. Preferably, the circuit board comprises a flexible circuit board that is mounted on the flange. The flange supports the flexible circuit board, and functions as a stiffener for the circuit board. 
     In another aspect, the conduit functions as a shaft such that a component of the switching mechanism rotates around the conduit. Preferably, the switching mechanism comprises a rotary switch that is rotatable around the conduit. The rotary switch can comprise one or more electrical contacts that engage with the switch trace(s) as the rotary switch is rotated around the conduit. 
     The second end of the conduit receives incident sound waves, and can extend partially or completely through a face plate of the hearing aid. 
     In another aspect of the invention, a hearing aid comprises a plurality of tuned resonant cavities between the microphone and the at least one external sound source. In certain embodiments, at least two tuned resonant cavities are arranged in series. In yet further embodiments, at least two tuned resonant cavities are arranged in parallel. A series of tuned resonant cavities can be used, for example, to provide a notch filter to selectively modify various frequency bands over a frequency spectrum of interest. 
     A method for manufacturing a hearing aid comprises providing a microphone that receives incident sound waves from one or more sources external to the hearing aid, and converts the sound waves into electronic signals; providing a circuit that amplifies the electronic signals; providing a receiver that converts the amplified electronic signals into amplified sound waves; selecting parameters for a tuned resonant cavity to modify the frequency response of the incident sound waves; and providing a tuned resonant cavity comprising the selected parameters between the microphone and the at least one external sound source. The selected parameters include at least one of the geometry of one or more openings through which sound waves enter the cavity, the geometry of the cavity, and the geometry of one or more openings through which sound waves exit the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a side cross-sectional view of one embodiment of a hearing aid with tuned resonant cavity; 
         FIG. 2A  is an electronic circuit representation of the tuned resonant cavity of  FIG. 1 ; 
         FIG. 2B  is an electrical circuit representation showing the impedances of an air vent between two acoustical cavities in a hearing aid; 
         FIG. 3  is a cross-sectional front-view of a sealing gasket and air hole; 
         FIG. 4  is a side cross-sectional view of the hearing aid microphone of  FIG. 3 ; 
         FIG. 5  is a cross-sectional side view of a tuned resonant cavity according to one aspect of the invention; 
         FIG. 6  is a cross-sectional side view of the tuned resonant cavity of  FIG. 5  incorporated in a switching mechanism; and 
         FIG. 7  is a front cross-sectional view of the tuned resonant cavity and switching mechanism of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     A hearing aid  10  having a tuned resonant cavity  12  is shown in  FIG. 1 . As shown in this figure, the hearing aid  10  includes a hearing aid shell  11  that contains various hearing aid components, such as a microphone  20  and battery  30 , as is known in the art. The hearing aid shell  11  includes a face plate  17  located at the front end of the device. The face plate  17  includes a pair of openings  15 ,  16  that allow air and sound into the tuned resonant cavity  12 . The resonant cavity  12  is tuned by selecting one or more parameters of the cavity, including the number, shapes and sizes of the openings  15 ,  16 , and the shape and volume of the chamber  12 . Additionally, the cavity can be tuned by selecting the number, shapes and sizes of any opening(s) between the chamber  12  and the microphone  20 . By adjusting these parameters, the cavity  12  can effectively “tune” the incident sound wave signal, P 1 , to provide a desired frequency response characteristic prior to the acoustic signal being received at microphone  20 . In this respect, the tuned resonant cavity acts much like a passive acoustical filter. 
     In one aspect, the physical characteristics of the tuned resonant cavity  12  can be represented as an electronic circuit, specifically an RLC circuit (a circuit with resistors, inductors and capacitors), and the frequency response of the sound waves in the cavity can be analyzed by reference to the frequency response of the corresponding electrical circuit representation of the cavity. The RLC circuit analogy has been known in the field of acoustical engineering as a useful model for designing acoustically-tuned structures. The present inventors have discovered that this model is particularly advantageous for designing an acoustically tuned microphone cavity for a hearing aid. 
     In prior hearing aids, signal processing is performed electronically by the hearing aid electronic circuitry, after the incident acoustic signal has been transformed into an electrical signal by the hearing aid microphone. In the present hearing aid, at least a portion of the signal processing can be performed acoustically through the use of a tuned resonant cavity in front of the microphone. An example of the signal processing capability of the tuned resonant cavity is illustrated in the electronic circuit representation of the cavity shown in  FIG. 2A . In this diagram, the tuned resonant cavity is represented by  12 . The incident sound wave signal P 1  can be represented as an input voltage signal. The pair of openings  15 ,  16  to the cavity can be represented by a pair of parallel impedences. The volume of the cavity, V 1 , can be represented as a capacitor. The acoustic signal received at microphone  20  can be represented as an output voltage signal. 
     In the exemplary embodiment shown in  FIGS. 1 and 2A , by adjusting the size of the air inlet hole(s) of the cavity, and the volume of the cavity (i.e. the impedence and capacitance, respectively, of the acoustic circuit), the frequency response of the output signal can be modified. 
     The air openings  15 ,  16  form an acoustical mass-resistance element where the acoustical mass or inertance (inductance in the equivalent electrical circuit) and resistance (resistance in the equivalent electrical circuit) increase with the length of the opening (i.e., thickness of the faceplate) and decrease as the area of the opening increases. The relationships are not linear and the resistance generally is also dependent on the frequency. 
     For small acoustical cavities, the acoustical impedance (compliance) is represented by a capacitance in the equivalent electrical circuit. The capacitance is proportional to the volume of the cavity, V 1 . 
     By increasing the volume of the cavity, the capacitance is increased and the resonant frequency is decreased. By increasing the area of the openings  15 ,  16 , the inductance and resistance are decreased and the resonant frequency is increased. By adjusting the size and number of openings and the volume of the cavity, one can adjust the resonant frequency as well as the amount of high-frequency boost relative to the low frequencies. 
     In the hearing aid of  FIG. 1 , a sealing member  19  occupies the volume of the hearing aid between the microphone  20  and the interior of the shell  11 . The sealing member isolates the tuned resonant cavity  12  from the remaining volume of the hearing aid  14 . Cavity  12  can be “tuned,” for example, by modifying the size of the cavity  12 . This can be accomplished in any practical manner, such as by adjusting the size and/or location of the sealing member  19 , filling a portion of the cavity  12  with a potting substance (such as an epoxy), or by adjusting the spacing between microphone  20  and face plate  17 . Similarly, the cavity  12  can be easily tuned by adjusting the size and/or number of air inlet hole(s)  15 ,  16 . By way of example, reducing the volume of the cavity  12  and increasing the size of the air inlet holes  15 ,  16 , the resonant frequency of the cavity is increased, and the high-frequency sounds can be boosted relative to the low frequency sounds. 
     In general, the incident sound waves that are conditioned by the resonant cavity can comprise sound waves having frequencies between 1 kHz and 10 kHz, and more specifically between 5 kHz and 7 kHz. The cavity can be tuned to any reasonable frequency in the audio range for humans and pets, generally between 20 Hz and about 70 kHz, though typically tuning the cavity to frequencies between 1 kHz and 10 kHz is most beneficial for hearing aids. The tuned resonant cavity of the present invention is advantageously able to “pre-condition” the incident acoustic signal using passive, acoustical means. Preferably, this passive “pre-conditioning” is used in conjunction with conventional electronic signal processing of the hearing aid circuitry. In preferred embodiments, the passive “pre-conditioning” works in conjunction with the signal processing scheme of the hearing aid circuitry, and can therefore lessen at least a portion of the processing requirements of the circuitry. This can help reduce the cost of the signal processing circuitry, and can also reduce the power requirements of the hearing aid. 
     Turning now to  FIGS. 3 and 4 , yet another aspect of the invention is illustrated.  FIG. 3  shows a cross-section of a hearing aid according to one embodiment of the invention. As shown in this figure, the hearing aid microphone  20  is located within the hearing aid shell  28 . As in the embodiment of  FIG. 1 , a sealing member  203 , such as a gasket or o-ring, concentrically surrounds the periphery of the microphone  20 , and substantially completely fills the area of the hearing aid between the outer periphery of the microphone and the interior surface of the hearing aid shell  28 . The sealing member  203  surrounding the microphone  20  is shown in the side view of  FIG. 4 . 
     In certain embodiments, it is necessary to provide air to the volume  14  of the hearing aid behind the microphone  20 . For instance, where the hearing aid uses an air-activated battery  30 , it is required that a certain amount of air is able to exit the resonant cavity  12  and pass behind the microphone  20  to reach the battery  30 . 
     As shown in  FIGS. 3 and 4 , an air vent  201  is provided to allow air to pass behind the sealing member  203  to the interior portion  206  of the hearing aid housing, which includes the battery  30 . In the embodiment illustrated in  FIG. 3 , the air vent  201  comprises a slot that is molded into the interior of the hearing aid shell  28 . In this embodiment, the slot is about 8-mils wide and about 12-mils deep. Other shapes and sizes for the vent could also be used. In addition, multiple vents could be employed. The vent could also comprise a passageway through the sealing member  203 . For example, as shown in  FIG. 4 , a hollow tube  207  (such as a hypodermic needle) could be inserted through the sealing member  203  to provide a vent passageway. In addition, the sealing member  203  itself could comprise an air-permeable material to provide the venting of air to the interior of the hearing aid. 
     Preferably, the sealing member and vent arrangement provide an acoustic seal, so that audible sound waves are substantially prevented from entering the interior portion  206  of the hearing aid housing, while a sufficient quantity of air is able to pass through the air vent  201  to provide the necessary oxygen for the battery. In essence, the air vent is configured to provide a relatively high-impedance to audio frequency sound waves, but a relatively low impedance to the diffusion of oxygen to the battery. Said another way, the vent represents a low impedance to very low frequency signals including dc (direct current). However, the air vent does not substantially affect the frequency response of higher frequency signals in the audio range (e.g. 1-10 kHz). The sealing member and vent arrangement are thus air permeable, but substantially impermeable to sound waves in the audible frequencies.  FIG. 2B  is the electrical circuit representation showing the impedances of an air vent ( 201  or  207 ) between two acoustical cavities  205 ,  206 , such as shown in  FIGS. 3 and 4 . 
     Turning now to  FIG. 5 , a cross-sectional side view of a tuned resonant cavity  52  according to one aspect of the invention is shown. In this embodiment, the tuned resonant cavity  52  comprises a separate, optionally one-piece component that can be mounted to or within the hearing aid shell. The cavity can be separately manufactured for incorporation within the hearing aid during hearing aid assembly. An advantage of this design is that the geometry of the cavity can generally be more precisely controlled than in the case where the resonant cavity is formed from the hearing aid shell. For example, the tuned resonant cavity of this embodiment may benefit from comparatively tighter manufacturing tolerances, and may be easier to manufacture than an integrally-formed cavity such as shown in  FIG. 1 . 
     In general, the tuned resonant cavity  52  of this embodiment comprises a conduit  54  that may be mounted in front of a hearing aid microphone. The conduit can have any practical size and shape. It can have a cross-section that is substantially circular, elliptical, triangular, rectangular, or irregularly-shaped, for example. The conduit has a first end  56  and a second end  58 . Preferably, the first end  56  contacts a surface of the microphone  20 , as shown in  FIG. 6 . The cross-sectional area of the first end of the conduit is preferably approximately equal to the area of the microphone diaphragm  23 , and the first end of the conduit can be substantially aligned with the diaphragm  23 . Preferably, the interface between the first end  56  of the conduit  54  and the microphone diaphragm  23  is substantially sealed, so that the sound waves in the conduit are directed into the microphone diaphragm  23 . 
     In another aspect, the first end  56  of the conduit  54  comprises a flange  60  that extends radially from the conduit  54 . The flange  60  can help mount the cavity  54  to the microphone  20 . In a preferred embodiment, the cavity  54  can be incorporated into a switching mechanism for the hearing aid. Exemplary embodiments of a hearing aid switching mechanism are described in co-pending U.S. Utility application Ser. No. 11/343,969, filed Jan. 30, 2005 entitled “Hearing Aid Circuit With Integrated Switch and Battery,” in the name of Walter P. Sjursen, Michael DeSalvo, Hassan Mohamed, Paul J. Mulhauser, and Karl D. Kirk III, the entire teachings of which have been incorporated herein by reference. In one embodiment of a switching mechanism, at least one switch trace  71  for the switching mechanism is mounted directly or indirectly on the flange  60  of the conduit. In one preferred embodiment, a circuit board  65  is mounted on the flange  60 , and at least one switch trace  71  can be located on the circuit board. The circuit board  65  can comprise a flexible circuit board, and the flange can function as a stiffener for the flexible circuit board. The flexible circuit board  65  with switch traces  71  mounted on the flange  60  is more clearly illustrated in the front cross-sectional view of  FIG. 7 . 
     In another aspect, the conduit  54  of the cavity  52  functions as a shaft such that a rotary switch  73  is rotatable around the conduit  54 . The rotary switch  73  can comprise one or more electrical contacts  74  that engage with the switch traces  71  as the rotary switch  73  is rotated around the conduit. The rotary switch  73  includes a mechanism, such as a tab or protrusion (not shown) that extends through the face plate  17 , that permits the user to selectively rotate the contacts  74  into and out of engagement with the switch traces  71 , thereby altering the operating state of the hearing aid. 
     The second end  58  of the conduit  54  comprises one or more openings for introducing incident sound waves into the cavity  54 . Preferably, the second end  58  extends partially or completely through the face plate  17  of the hearing aid. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.