(12) United States Patent
`Lichtblau
`
`USOO67.14654B2
`(10) Patent No.:
`US 6,714,654 B2
`(45) Date of Patent:
`Mar. 30, 2004
`
`(54) HEARING AID OPERATIVE TO CANCEL
`SOUNDS PROPAGATING THROUGH THE
`HEARING AID CASE
`
`(76) Inventor: series Epiphy, pack
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 122 days.
`
`(21) Appl. No.: 10/068,487
`1-1.
`(22) Filed:
`Feb. 6, 2002
`(65)
`Prior Publication Data
`
`US 2003/0147544A1 Aug. 7, 2003
`(51) Int. Cl. ................................................ H04R 25/00
`(52) U.S. Cl. ........................................ 381/317; 381/322
`(58) Field of Search ............................... 381,711, 712,
`381/72, 151, 312, 313, 317, 318, 328,331,
`FOR 127, FOR 128, FOR 130, FOR 140,
`380
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,622,440 A 11/1986 Slavin ....................... 381/68.1
`5,201,006 A * 4/1993 Weinrich ..................... 381/68
`5,390,254 A
`2/1995 Adelman ..................... 381/68
`5,402,496 A 3/1995 Soli et al. ..................... 381/94
`5,526,819 A 6/1996 Lonsbury-Martin
`et al. .......................... 128/746
`5,692,059 A 11/1997 Kruger .............
`... 381/151
`5,757,933 A
`5/1998 Preves et al. .............. 381/68.1
`
`5,933,506 A * 8/1999 Aoki et al. ................. 381/151
`6,068,589 A 5/2000 Neukermans ................ 600/25
`6,421,448 B1
`7/2002 Arndt et al. ................ 381/312
`2002/O1416O2 A1 10/2002 Nemirovski ................. 381/93
`2002/O143242 A1 10/2002 Nemirovski ................ 600/300
`2003/0007647 A1
`1/2003 Neilsen et al. ................ 381/60
`FOREIGN PATENT DOCUMENTS
`
`2/1990
`4/1990
`3/2001
`11/1993
`11/1994
`9/1996
`11/1997
`6/1998
`10/1998
`
`EP
`O 354 698 B1
`EP
`O 364 O37 B1
`EP
`O 782 371 B1
`WO
`WO 93/23942
`WO
`WO 94/25957
`WO
`WO 96/29009
`WO
`WO 97/44987
`WO
`WO 98/19498
`WO
`WO 98/43567
`* cited by examiner
`Primary Examiner–Curtis Kuntz
`Assistant Examiner-Brian Ensey
`(74) Attorney, Agent, or Firm Weingarten, Schurgin,
`Gagnebin & LebOvici LLP
`(57)
`ABSTRACT
`Apparatus for use in a hearing aid wherein a first micro
`phone is connected to the hearing aid case and exposed to
`free air and a Second microphone is connected to the hearing
`aid case and Sealed from free air. The audio inputs from the
`two microphones are applied to a subtractive circuit so as to
`cancel any audio Signals transmitted through the case of the
`hearing aid while passing audio signals received by the
`microphone exposed to free air.
`
`57 Claims, 4 Drawing Sheets
`
`12
`
`
`
`48
`
`Exhibit 1010
`Page 01 of 12
`
`

`

`U.S. Patent
`U.S. Patent
`
`Mar. 30, 2004
`Mar. 30, 2004
`
`Sheet 1 of 4
`Sheet 1 of 4
`
`US 6,714,654 B2
`US 6,714,654 B2
`
`360
`
`ees
`d,
`f
`on {
`
`_d
`
`y
`
`FIG. I
`
`tT
`
`dy
`
`cf)320
`
`ns
`
`360
`360
`FIG. 2
`FIG. 2
`
`Exhibit 1010
`
`Page 02 of 12
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`Exhibit 1010
`Page 02 of 12
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`

`

`U.S. Patent
`
`Mar. 30, 2004
`
`Sheet 2 of 4
`
`US 6,714,654 B2
`
`460
`
`E
`
`
`
`560
`
`E
`
`N
`
`FIG. 3
`
`Kx 100x E
`
`(Kx 100x E)
`NOISE = Kx 73XN
`
`* G = 1
`DFF
`GcoMMON 01
`FIG. 4
`
`660
`
`E
`
`N
`
`K ?ky
`
`/N
`
`
`
`630
`GDIFF = 1
`634
`Nuf GcoMMON-01
`O
`
`FIG. 5
`
`640
`
`(KX 100XE)
`NOISEE KXN
`
`Exhibit 1010
`Page 03 of 12
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`

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`U.S. Patent
`U.S. Patent
`
`Mar. 30, 2004
`Mar. 30, 2004
`
`Sheet 3 of 4
`Sheet 3 of 4
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`US 6,714,654 B2
`US 6,714,654 B2
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`
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`
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`122
`
`Exhibit 1010
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`Page 04 of 12
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`Exhibit 1010
`Page 04 of 12
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`U.S. Patent
`U.S. Patent
`
`Mar. 30, 2004
`Mar. 30, 2004
`
`Sheet 4 of 4
`Sheet 4 of 4
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`US 6,714,654 B2
`US 6,714,654 B2
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`
`
`
`
`Exhibit 1010
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`Page 05 of 12
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`Exhibit 1010
`Page 05 of 12
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`

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`US 6,714,654 B2
`
`1
`HEARING AD OPERATIVE TO CANCEL
`SOUNDS PROPAGATING THROUGH THE
`HEARING AID CASE
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`N/A
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH OR DEVELOPMENT
`
`N/A
`
`2
`microphone, cancels out high level noise signals but does
`not cancel out the low level DPE signal. There is some
`“hint” of how the second “less sensitive' microphone works,
`since the patent states, “The output of 30B is of less
`sensitivity so that the frequency components of the DPE
`Signal are missing from the Signal applied to the minus input
`of the differential amplifier.” In other words, the less sensi
`tive microphone is designed to be “leSS Sensitive” at the
`Specific frequency that is being looked for in this patent; i.e.
`frequencies F1 and F2 are applied to the ear and the
`instrument looks for an "echo” at 2F1-F2. Also, the ratio of
`F2 to F1 is fixed. This patent clearly states that the typical
`test procedure varies F1 from 1,000 Hertz to 8,000 Hertz in
`steps of 100 Hertz. Therefore, the electronics must vary the
`frequency response of the Second microphone for each Step
`in the input frequencies.
`In claim 1, lines 21 to 29, the patent states: “Said eartip
`including a Second microphone means with a Second output
`lead connected thereto for generating a Second electrical
`Signal on Said Second output lead which is proportional to
`Said other body noises but is not Substantially proportional
`to distortion product emission tone generated in the ear of
`said human being at the frequency of 2F1-F2.” This claim
`clearly States that the Second microphone must be very
`frequency Selective Since it is specifically not Substantially
`Sensitive to a specific frequency. Again, Since the patent
`requires that the frequencies F1 and F2 be varied over a wide
`range (typically 1 kHz to 8 kHz in 100 Hertz steps), this
`means that the frequency Selectivity of the Second micro
`phone must also be varied accordingly.
`In U.S. Pat. No. 6,068,589, two or more microphones are
`used, all exposed to free air, but placed in different positions
`on the body (skull) So that noise generated from a relatively
`large distance is coupled almost equally to both micro
`phones and canceled out, wherein noise generated nearby is
`Selectively coupled much greater to the closer of the two
`microphones and less Selectively to further microphone.
`Therefore, the “nearby' audio signal is amplified much more
`than those audio signals generated at a much greater dis
`tance. This improves the Signal to noise ratio Since the local
`Signal is considered the “desirable Signal” as compared to
`the noise generated at a distance. In addition, by locating the
`microphones on opposite sides of the Skull, it is possible to
`increase the “directivity of the hearing aide. Again, all of
`these microphones must be exposed to all of the audio
`Signals transmitted through the air and there is no mention
`of using multiple microphones to cancel noise directly
`coupled to the hearing aid case and generated within the
`body.
`U.S. Pat. No. 4,622,440 also indicates the use of two
`microphones that are physically Separated and exposed to
`the air in order to “hear the audio signals transmitted
`through the air. AS in the previous patents, this patent
`Subtracts the audio signals generated in each microphone in
`order to cancel noise generated at a “distance' that is large
`compared to the physical space between the two micro
`phones. If this “distance' is relatively large compared to the
`physical Separation of the two microphones, than the Signals
`received by the two microphones will be substantially equal
`in amplitude and phase and may be canceled using a
`differential amplifier.
`Audio signals generated much closer to the two micro
`phones will be coupled greater to one microphone than the
`other and will not be completely canceled. In this type of
`circuit, both microphones are exposed to air in order to
`cancel noise generated at a distance. This type of circuit can
`also be used to enhance the “directivity” of a microphone.
`
`BACKGROUND OF THE INVENTION
`The purpose of this invention is to discriminate between
`audio signals propagated through the air and noise Signals
`which are propagated through the plastic Shell of the hearing
`aid. Most of the noise transmitted through the plastic case of
`the hearing aid is created within the body itself, i.e. chewing,
`talking, Walking, etc.
`The present invention utilizes two microphones, both
`coupled to the hearing aid case, wherein one microphone is
`exposed to Sounds propagating through the air and the
`Second microphone is purposely Sealed from Sounds propa
`gating through the air.
`In a typical hearing aid, one or more microphones convert
`Sound transmitted through the air to an electrical Signal. A
`Signal processing System (amplifiers, filters, digital filtering,
`etc.) processes the Signal, and a loudspeaker converts the
`processed electrical Signal back to an audio signal. The
`hearing aid apparatus is enclosed in a plastic case that is
`placed In The Ear (ITE) canal or Behind The Ear (BTH).
`The microphone(s) is(are) attached to the plastic case and
`the case is acoustically coupled to the bone and flesh of the
`wearer. Therefore, any Sounds that originate in the skull are
`conducted to the microphone(s) and amplified and echoed
`back to the ear.
`There have been many inventions for hearing aids
`wherein two or more microphones are used to cancel ambi
`ent noise. Both microphones “hear the same audio signal
`that is propagating through the air. The noise cancellation
`technique assumes that the noise Source is much further
`distant from the two microphones as compared to the local
`or desired audio signal. AS long as the noise is generated at
`a distance which is large compared to the distance between
`the two microphones, the Signals received at each micro
`phone will be almost equal in amplitude and phase. Then a
`“difference device', for example, a differential amplifier, can
`Subtract the two microphone signals and effectively cancel
`the ambient noise. In addition, two or more microphones
`have been used with a differential amplifier to develop a
`directional microphone for use on a hearing aid. However, in
`all cases, all of the microphones were exposed to the audio
`Signals propagating through the air in order to produce the
`desired effect.
`In U.S. Pat. No. 5,526,819, two microphones are used to
`attempt to cancel out high amplitude “body noises” (Such as
`a cough) from the very low level desired (DPE) audio
`information. However, in this patent, both microphones are
`exposed to the same audio Signals in the air. One of the
`microphones is designed to not respond to the DPE signals
`that are of a fixed and known frequency in this test. It is also
`“necessary” that the microphones be different. In addition,
`the patent does not give any information as to how the
`“second microphone, which is less “sensitive” than the first
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`Exhibit 1010
`Page 06 of 12
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`US 6,714,654 B2
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`3
`U.S. Pat. No. 5,757,933 is almost identical to U.S. Pat.
`No. 4,622,440 except that this patent includes the use of a
`Switch to either add the Signals of the two microphones
`together or to Subtract the Signals from the two microphones.
`By varying the phase and gain of one microphone versus the
`other, the microphones can be made Somewhat "direc
`tional'. Both microphones must be exposed to the air.
`U.S. Pat. No. 5,402,496 again discusses the use of two
`microphones, both exposed to hear the audio signals trans
`mitted through the air. The output of one microphone passes
`through a “first filter'. To quote: “The first filter generates a
`reference signal by Selectively passing an audio spectrum of
`the input signal which primarily contains the undesired
`component.” This Statement alone Says that one must first
`“know” what the “undesired component” (i.e. noise) is,
`which is not useful in situations in which it is not possible
`to “know’ in advance what the noise is. In addition, in this
`patent, both microphones are not identical.
`Also, according to this invention, only one microphone is
`actually required. The output from this microphone is passed
`through a “first filter as well as directly to the remainder of
`the Signal processing electronics. There appears to be no
`explicit mention of a “second microphone in the claims.
`EP 0782371 B1 refers to the design of a microphone for
`use in hearing aids, particularly for use in “in-the-ear” (ITE)
`hearing aids. This patent clearly defines a technique for
`"mechanically mounting a microphone in a hearing aid So
`that the sudden motion of the body does not pass from the
`body to the hearing aid Shell and then to the hearing aid
`microphone. If this Sudden motion is coupled to the
`microphone, the audio Signal is amplified by the hearing aid
`electronics and passed into the ear.
`The patent describes in great detail the mechanical mount
`ing of Such a microphone. This patent does not discuss nor
`hint of the application of a Second microphone and elec
`tronics to cancel mechanical vibrations received by both
`microphones.
`WO 97/44987 describes a hearing aid system wherein one
`microphone is connected to an amplifier, battery and loud
`Speaker (microactuator) and placed in a hermetically Sealed
`case. The patent document also Suggests the use of an
`acoustic array of individual microphones arranged in a
`horizontal row acroSS the electronics module. This array of
`microphones is connected to electronics that creates a
`“directional microphone which may enhance the Sounds of
`interest while concurrently reducing noise.
`In this patent there is no attempt to reduce noise which is
`directly coupled to the electronic “module” from being
`amplified and fed back to the ear. There is no mention of
`using one microphone that is Sealed from audio signals
`propagated through the air and another microphone that is
`purposely exposed to audio signals propagated through the
`
`EP 0364037 B1 teaches the use of two microphones,
`both exposed to audio signals propagated through the air,
`wherein one microphone is Specifically designed to be
`Sensitive to audio signals that originate close to it and the
`Second microphone to be sensitive to audio signals which
`originate at a relatively large distance from the microphone.
`The outputs of the two microphones are electronically
`Subtracted from each other in order to Suppress hearing aid
`oscillation caused by audio feedback from the loudspeaker
`to the microphone.
`In this patent the two microphones “must be of different
`design in order for one microphone to be only Sensitive to
`sound “close to it”. The patent does not discuss the problem
`
`4
`of audio signals coupled directly to the microphones via the
`hearing aid case. In addition, both the microphones in this
`patent must be exposed to the air.
`WO 94/25959 describes a microphone and electronic
`module wherein the microphone is placed in the ear and
`Sensitive only to Sounds passing from the inside of the ear to
`the outside of the ear (the opposite direction from the normal
`hearing aid). These "outbound” audio signals are amplified
`and electrically transmitted to a remote receiver. Noise
`reduction and cancellation features are described in this
`invention. This patent document requires that the inner ear
`be sealed from the air external to the ear and that one
`microphone be placed into this Sealed chamber in order to be
`Subjected to audio Signals originating from the inner ear. A
`Second microphone is placed external to the ear to be
`Sensitive to noise originating outside of the ear. The Signals
`from these two microphones are electronically processed to
`Subtract the noise external to the ear from the same type of
`noise generated from inside the ear.
`This patent clearly describes the use of a “structural
`configuration of the earpiece and gasket around extension
`section 147 (that) substantially eliminates audio vibrations
`conducted by the bone and tissue of the earpiece wearer.'
`"Such Sound gasketing avoids audio vibration pickup of
`Sound transmitted by the canal wall. Furthermore, the gasket
`composition dampens any audio Vibration pickup of Sound
`through the gasket material touching the canal inner wall.”
`EP 0 354 698 B1 specifically concerns hearing aids and
`their assembly and is especially concerned with the long-felt
`need to avoid the amplification of noise caused by vibrations
`of either the casing or the components of the hearing aid.
`This patent attempts to Solve this problem by making the
`hearing aid case out of a new plastic type material described
`as “a Viscoelastic layer adhering the transducer to the
`casing”. This Solution is totally mechanical and the patent
`Specifically States that ". .
`. Some hearing aids include
`electronic devices to filter out noise. Not only are electronic
`devices quite expensive, but they also can take up valuable
`Space.” Thus, this patent attempts to Solve the problem of
`Vibrations coupled to the hearing aid case by mechanical
`CS.
`WO 96/29009 describes a configuration of two micro
`phones and electronics wherein one microphone is posi
`tioned to Sense body Sounds of a patient when the transducer
`(microphone) is placed against the patient's skin and the
`Second microphone is positioned to Sense noise in the
`external environment close to the first microphone. In this
`document, the two microphones are purposely "acoustically
`and mechanically isolated from each other”. This is the
`classical case wherein two microphones are used to Subtract
`one Source of noise from another. However, in this case the
`external noise is normally very much larger than the “inter
`nal noise’ generated by the body. Therefore, there must be
`very Specific electronic signal processing to Subtract the very
`large external noise from the very Small internal noise and
`not end up with a very large (negative phase) external noise.
`Since the level of the external noise is not known relative
`to the internal noise, this document requires the use of a
`“digital Signal processor' coupled to the first and Second
`microphones to process the audio signals to produce an
`output Signal that is indicative of the very Small body Sounds
`as compared to the very large external noise. How the
`“digital Signal processor actually determines how to Sub
`tract the very large external noise from the very Small
`internal noise is not described.
`WO 98/43567 describes a hearing aid noise cancellation
`System wherein the hearing aid Specifically does not com
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`Exhibit 1010
`Page 07 of 12
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`US 6,714,654 B2
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`S
`pletely "plug the ear'; that is, the hearing aid has a “vent'
`So that Sound can propagate directly to the inner ear without
`passing through the electronics of the hearing aid. In this
`type of hearing aid, the ear hears the Sum of Sounds received
`directly through the “vent” and also via the electronic
`microphone and loudspeaker of the hearing aid. In this
`document there is only one microphone. This microphone
`and the loudspeaker are both acoustically coupled to the
`inner ear via a hollow tube. Any Sound that reaches the inner
`ear via the vent is (1) directly heard by the inner ear and (2)
`picked up by the microphone, processed, and fed back to the
`loudspeaker to cancel unwanted “noise' signals. The pro
`cess of active noise cancellation is well known (see U.S. Pat.
`No. 4,473,906). As discussed in the WO 96/29009 patent
`document above, the WO 98/43567 patent document must
`“predetermine” what the noise is in order to cancel out this
`noise. This is the basic problem of all hearing aids designed
`to improve the intelligibility of Speech Signals. This patent
`does show any specific method of Signal processing to
`improve the intelligibility of Speech Signals.
`WO 98/19498 relates to the design of an ear muffler to
`reduce ambient noise from entering the ear and protect the
`ear against damage. This patent does not use any micro
`phones or electronics.
`WO 93/23942 describes the development of an “ear
`mounted microphone' and Speaker that does not require
`entry of any physical Structure within the ear canal. In this
`patent document, "Acoustical isolation means is coupled
`between the Speaker element and the housing for Selectively
`isolating undesirable frequencies that might interfere with
`Sensitivity of audio pick up at the microphone .
`. . ' This
`patent document does not use two microphones to acousti
`cally isolate the speaker from the housing using electrical
`means. It clearly uses mechanical means to isolate the
`Speaker from the housing. Also, in this document, the
`Speaker directS its Sound “outside' of the ear and not into the
`ear. In the exact reverse from hearing aids, this invention
`places the microphone inside the ear and the Speaker on the
`outside of the ear. The document also claims to have noise
`reduction from the case to the microphone because the
`Sound coupled from the housing to the vibration rings and
`then to the microphone Somehow have equal and opposite
`phase relationships when they arrive at the microphone.
`There is, however, no explanation of this conclusion. The
`document also States: ". . . it is necessary to acoustically
`isolate the Speaker element 13 from the casing. In the present
`invention, this is accomplished by tuning the insulation
`means 50 similar to operation of a band pass filter within
`radio circuitry. Specifically, the Speaker element 13 is
`mounted to the end housing 36 through a nonvibrational ring
`50 which extends around the Speaker element and operates
`to isolate the housing as Sound insulation means from the
`speaker element.” The “nonvibrational ring,” however, is
`not described or explained.
`SUMMARY OF THE INVENTION
`The present invention relates to a hearing aid that
`addresses the problem of Sounds created internally. In the
`present hearing aid, at least two microphones are used, one
`attached to the plastic case of the hearing aid and isolated
`from free air, and a Second microphone, also attached to the
`plastic case, but exposed to free air. Only the Second
`microphone, open to the air, converts audio information
`conducted through free air from an external audio Source to
`electrical signals. Both microphones convert audio signals
`that are conducted via the plastic case to electrical Signals.
`The two microphones are connected to a difference device
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`Such as a differential amplifier that causes cancellation of the
`Signals that are conducted through the plastic case. Because
`the path length of the audio signals via the plastic case is
`very short, the amplitude and phase of the audio signals
`received by both microphones is nearly equal. Therefore,
`high cancellation of any audio signals present in the plastic
`case can be achieved. The gain and phase of one microphone
`Versus the Second microphone could be adjusted to enhance
`cancellation.
`
`DESCRIPTION OF THE DRAWINGS
`The invention will be more fully understood from the
`following detailed description taken in conjunction with the
`accompanying drawings in which:
`FIG. 1 is a Schematic view of an audio Signal generated
`at a physical location centered between two microphones,
`FIG. 2 is a Schematic view of an audio Signal generated
`at a physical location farthest from one microphone relative
`to the other microphone;
`FIG. 3 is a Schematic diagram for determining a desired
`audio output level for a one-microphone hearing aid;
`FIG. 4 is a schematic diagram of the audio output level for
`a two-microphone hearing aid in which both microphones
`receive external audio signals through free air;
`FIG. 5 is a schematic diagram of the audio output level for
`a two-microphone hearing aid of the present invention in
`which one microphone is Sealed from external audio signals
`through free air;
`FIG. 6 is an exploded view of a hearing aid according to
`the present invention;
`FIG. 7 is an exploded view of a further embodiment of a
`hearing aid according to the present invention;
`FIG. 8 is a schematic diagram of a further embodiment of
`the circuitry of the present invention; and
`FIG. 9 is a schematic diagram of a further embodiment of
`the present invention in which two microphones receive
`audio signals through free air and a third microphone is
`Sealed from external audio signals through fee air.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`To aid in understanding the present invention, the case of
`two external microphones is distinguished from the present
`invention in which an internal and an external microphone
`are used. When two external microphones are used in a
`“Subtractive” mode through the use of, for example, a
`differential amplifier, the “net” audio output from these
`microphones is radically reduced compared to that of a
`Single microphone. This reduction is due to the necessarily
`close Spacing of the two microphones in a miniature device
`Such as a hearing aid. For example, the Spacing between two
`microphones may be 0.5 inches.
`When an audio signal is generated at a distance of 3 feet
`from the ear, the amplitude of the Signal received at the two
`microphones So closely spaced is virtually identical. Under
`ideal conditions, if the audio signal is generated at a physical
`location centered between the two microphones, there is a
`total cancellation of the audio signal via a differential
`amplifier. Referring to FIG. 1, two microphones 320,330 are
`placed a distance d apart and an audio Source 360 is located
`at a distance d from each of two microphones and equi
`distant between each microphone. In this instance, the
`distances d and d are equal. Thus, the amplitude and phase
`of the audio Signal are identical at each microphone. When
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`Exhibit 1010
`Page 08 of 12
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`7
`the microphone signals are Subtracted from each other via a
`differential amplifier, the net audio signal is Zero.
`The conversion gain from the audio level to the electrical
`level for a one-microphone System can be defined as equal
`to 1. Thus, for a two-microphone system used in the differ
`ential mode, the conversion gain from the audio level to the
`electrical level is not fixed, but is determined by the angle of
`the audio signal relative to the two microphones, the dis
`tance of the audio Signal from the two microphones, and the
`distance between the two microphones. FIG. 2 illustrates the
`location of greatest audio to electrical conversion for the
`two-microphone System, wherein the Signals from the two
`microphones are Subtracted from each other using a differ
`ential amplifier. The typical conversion gain from audio to
`electrical for a two-microphone System is significantly leSS
`than for a simple one-microphone System. The typical gain
`of a one-microphone System compared to a two-microphone
`System can be calculated as follows:
`A simplified audio level can be given by:
`
`V=Kid,
`
`where
`V=Voltage out of microphone X;
`K=microphone Sensitivity constant whereby the Voltage
`output of a microphone is proportional to the audio
`level input; and
`d=distance between microphone to audio Source. This
`equation is relatively accurate when the distance d is
`much greater than di. Thus, for the case in which the
`two microphones are 0.5 inch apart and the audio
`Source is 36 inches from microphone 330:
`
`=36.5/0.5
`
`Accordingly, the conversion gain from a one-microphone
`System is approximately 73 times greater than the conver
`Sion gain from a two-microphone differential System. This
`conclusion means that the audio amplifier gain for a two
`microphone System must be at least 73 times greater than
`that required for the one-microphone System to obtain the
`Same audio level input to the ear.
`The desired audio output level for a hearing aid can be
`determined from a one-microphone System, illustrated in
`FIG. 3. The single microphone 420 has a conversion gain of
`K (audio input to electrical output) followed by an audio
`amplifier 425 with a gain of 100. The signal from the
`amplifier drives a loudspeaker 440 located within the hear
`ing aid. The relative audio output level is 100KxE, where E
`represents an external audio signal from a Source 460. This
`output level Sets the audio level required for normal hearing.
`FIG. 4 illustrates a system with two microphones 520, 530
`using a differential amplifier 523 with a gain of one and a
`
`US 6,714,654 B2
`
`8
`common mode rejection ratio of 100. Thus, the differential
`amplifier passes one percent (0.01) of the audio signal if
`both audio inputs are equal and has a gain of one (1) for
`audio signals from only one microphone. (A differential
`amplifier with a rejection ratio of 100 (40 dB rejection) is
`about the best that can be done using matching resistors with
`a tolerance of 1%.) In FIG. 4, E represents an external audio
`Signal from a Source 560 and N represents a noise Signal
`generated within the wearer's body and coupled to both
`microphones attached to the case of the hearing aid.
`Using identical microphones 520, 530 and assuming that
`the audio Source is located 36 inches from the microphones,
`the conversion gain of the two-microphone System is at least
`73 times less than the one-microphone System. Thus, to
`arrive at a final audio level of 100KxE, a further amplifier
`527 with a gain of 73 is added in series with the first
`amplifier 523 and the amplifier 525 with a gain of 100.
`Referring to FIG. 5, in the present invention, a first
`microphone 620 is used in accordance with the above
`discussion. A Second microphone 630 is attached to the case
`of the hearing aid and Sealed from the air, illustrated
`schematically in FIG. 5 by placing the microphone in a box
`634. Because the audio signal E from a source 660 is heard
`only by the first microphone 620, this microphone has a
`conversion gain of K. Because the second microphone 630
`is Sealed from the air, it has a conversion gain of 0 for Sounds
`that are transmitted through the air, but a conversion gain of
`K for Sounds transmitted through the case of the hearing aid.
`Thus, for Sounds transmitted through the air, the conversion
`gain of the first microphone through the differential ampli
`fier 623 is also K. Therefore, the audio amplifier 625 has a
`gain of 100 to be equal to the one-microphone case.
`When noise is generated within the wearer's body, it is
`coupled directly to the case of the hearing aid. If the Second
`microphone 630 were not sealed, as illustrated in FIG. 4, the
`noise source N would be coupled to both microphones
`equally and would be passed through the differential ampli
`fier with a common mode rejection of 100 (gain of 0.01).
`Therefore, the noise output of the differential amplifier
`would be 0.01N. This is multiplied by 73 and by 100 to
`result in a noise output of 73N.
`When the Second microphone is Sealed, as in the present
`invention, the noise N is also coupled to both microphones
`equally and passed through the differential amplifier.
`Therefore, the output of the differential amplifier is again
`equal to 0.01 N. This output is amplified by a gain of 100 for
`a net output noise of 1N. Therefore, for noise coupled
`directly to the shell of the hearing aid, that is, noise gener
`ated within the body, the noise generated in the ear is 73
`times less when the Second microphone is Sealed from the
`a.
`A hearing aid 10 according to the present invention is
`illustrated schematically in FIG. 6. The hearing aid shown is
`an in-the-ear (ITE) type hearing aid and includes a case 12
`having a shell 14 and a faceplate 16 attached to a distal end
`18 of the shell 14. The shell and faceplate may be formed,
`for example, of molded plastic. The faceplate is attached to
`the shell in any Suitable manner, as known in the art.
`A first external microphone 20 is in electrical communi
`cation with an electronicS package 22 by, for example, a
`twisted pair of wires 24. The external microphone is
`attached to an outer or distally facing Side 26 of the faceplate
`16 of the case. The external microphone 20 is able to pick
`up Sounds propagating through free air, that is, Sounds
`propagating through air external to the ear, as well as Sounds
`propagating through the case 12.
`A Second internal microphone 30 is also in electrical
`communication with the electronics package 22 by, for
`
`1O
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`Exhibit 1010
`Page 09 of 12
`
`

`

`9
`example, a twisted pair of wires 32. The internal microphone
`is enclosed in a further enclosure 34 that seals the internal
`microphone from Sounds propagating through free air. Thus,
`the internal microphone is only able to pick up Sounds
`propagating through the hearing aid shell 14 and the enclo
`Sure 34. The internal and external microphones are prefer
`ably identical Such that the responses of both microphones
`to audio inputs have Substantially the same audio to elec
`trical conversion characteristics. The electronics package 22
`contains Subtractive circuitry for eliminating noises propa