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Publication numberUS2950357 A
Publication typeGrant
Publication dateAug 23, 1960
Filing dateMay 1, 1956
Priority dateMay 1, 1956
Publication numberUS 2950357 A, US 2950357A, US-A-2950357, US2950357 A, US2950357A
InventorsRobert E Mitchell, Stanley K Webster
Original AssigneeRobert E Mitchell, Stanley K Webster
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electronic sound transmitting device
US 2950357 A
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

Aug 23, 1950 R. E. MITCHELL. r-:TAL

ELECTRONIC SOUND TRANSMITTING DEVICE Filed March l, 1956 2 Sheets-Sheet 1 MICROPHONE i- TO'AMPUHER 50 DETECTOR fs d' -l 6 Escrima l 55 Tum-:p`

QlRCUIT @To AMPLIFIER runen cmcurr INVENTORS. 3, 7 @06m a, www/S To AMPLmsa BY''o/vxllua, K, wxzballz/ THERHO- (OUPLE BOQY HEAT' ATTY.

Aug. 23, 1960 R. E. MITCHELL. ETAL 2,950,357

ELECTRONIC SOUND TRANSMITTING DEVICE;

2 Sheets-Sheet 2 Filed March l, 1956 BATT ERY 2,956,357 Patented ug. 23, 1960 ELECTRONIC SUND TRANSMITI'ING DEVICE Robert E. Mitchell, 3833 S. Oak Park Ave., Berwyn, Ill.,

and Stanley K. Webster, 229 E. Oneida Ave., Elmhurst,-1ll.

Filed May 1, 1956, Ser. No. 581,873

3 Claims. (Cl. 179-107) This invention relates, in general, to hearing aids and is concerned with the combination of an electronic hearing aid with a custom built ear mold in such a manner that the unit is self-contained and may be Worn entirely in the ear.

Otherwise stated, the invention is embodied in a hearing aid particularly characterized as being a self-contained unit whereby the entire electronic circuit and mechanism including a power source are incorporated in an ear mold requiring no wires or other connections running from the ear mold to remote parts of the body of the user.

More specifically stated, it is a general object and accomplishment of the invention to provide a novel hearing aid of unitary design wherein all of the elements of the electronic circuit, the power source and the microphone are placed yin an ear mold thereby eliminating the necessity of cords, plastic tubes and the like, which serve as connecting devices to units disposed remotely from the earmold on the users body as may be found in conventional hearing aids.

it is an important object :and accomplishment of the invention to provide a hearing aid of unitary design wherein the output transducer or receiver is mounted in the earmold in a unique manner so `as to dispose the receiver close to the tympanic membrane (eardrum) providing a tight acoustic seal between the output transducer and the entrance to the ear canal.

The disposition of the output transducer in close proximity to the ear canal provides distinct advantages not found in conventional hearing aids. Because the diaphragm of the receiver must transmit sound pressure waves through the air between the eardrum and the diaphragm, reduction vof the volume of air that must be moved drastically reduces the amount of acoustic power necessary from the receiver. Moreover, in accordance with well-known laws of acoustics, the high frequency response of the waves reaching the eardrum are increased almost inversely as the distance, and since the great majority of people with hard-of-hearing diculties have less acuity in the high frequencies, that Iis a distinct advantage over former methods, resulting in higher discrimination for speech.

lt is another important object and accomplishment of the invention to provide a hearing aid of unitary design and a new and unique circuit approach in the electronic amplifier in order to accomplish the results contemplated and enable all of the necessary circuit components to be placed in the earmold. Most present day healing aid circuits employing transistors require transformers as impedance matching devices to couple one transistor to the next. Transformers take up room, and limit frequency response in proportion to their physical size. Another method sometimes used in present day hearing aids is known as resistance capacitive coupling. This method also requires relatively large amount of space thereby making it almost impossible to use this method to build a complete useful circuit inside an earmold to provide a ted States atent unitary structure as contemplated by the present invention.

It is a further object and accomplishment of the invention to provide an electronic circuit for a hearing aid of unitary design wherein one transistor is directly connected to the next without any electrical components. By employing four transistors with two pair directly coupled, it `is possible to produce an amplier with high electrical gain and an excellent frequency response. The input voltage from the microphone is then capacitively coupled to the amplifier with a single tiny capacitor. The invention further contemplates the simpliiication of the electronic circuit and simplifies the components thereof with a three transistor circuit thereby producing an amplifier with excellent frequency response, and medium gain, suicient to provide enough acoustical gain and power for those with mild and medium hearing losses.

Extreme advantages are claimed by the device contemplated by this invention with respect to it being a genuine contribution to better hearing, impossible by former methods of approach. Contemporary hearing aids are made in such a manner that the microphone is worn on some part of the head or body, separate from the actual ear. Studies made by the Bell Telephone Laboratories, the Harvard Acoustical Laboratories, and others, prove conclusively that the sound energy must be collected at the pina of the ear for best hearing. This is due to bafiie effects of the body and head, and dramatic differences in the frequency response. Speech intelligibility, psychological comfort of the user are obtained only when the microphone is placed exactly in the entrance to the ear canal. Moreover, nature constructed the pina of the ear to collect sounds, and unless the hearing aid is worn completely enclosed in the ear of the user, as contemplated by the present invention, the function of the pina is inoperative.

People wearing a single hearing aid or two hearing aids with the microphones placed on some other part of the body, describe a real sensation of unbalance, confusion, and unreality, which completely disappears when two hearing aids, of the nature herein prescribed, are used.

Accordingly, it is another important object and accomplishment of the invention to provide a hearing aid of unitary design and in which true orientation of the direction of sound is possible thereby avoiding sensations of unbalance, confusion and unreality on the part of the user.

While the hearing aid of the present invention contemplates the use of a battery as a power source, it is to be understood, that some other source of power may be used such as, for example, a thermal device, a sun battery, detection of electro-magnetic waves, or some other means of power could be used without changing the basic concept of the present invention.

lt is further pointed out that while the microphone is used to collect sound energy, it might be easily replaced and changed by some other detection device, such as, for example, a tuned circuit to pick up radio programs, or detection devices for other types of stimuli. The transducer shown and contemplated by this invention is for the purpose of giving a sound stimuli to the eardrum. It could be replaced by a device to give other types of response, such as an electrical stimuli to page a doctor in a hospital paging system, or to transmit orders and information to military forces or to stagehands and cameramen on television programs thereby eliminating wire connection which usually restricts the movement of the workmen.

The invention further contemplates vthe provision of an earmold for the hearing aid of the present invention, said 3 Y earmold being capableA of being conveniently molded of a plastic composition to provide economies of manufacture, and more important, to appeal to the aesthetic senses of the user from theV design standpoint.

. An ancillary object and accomplishment of the'invention is to provide a hearing aid of unitary design and which is. adapted to Vbe economically manufactured and which is so designed as to permit the manufacture and assembly thereofin accordance with present-day large scale mass production methods of construction and assembly.

Additional objects, features and advantages of the in- Ventionvdisclosedherein will be apparent to persons skilled in the art after Ythe construction and operation are understood from. the within description. Y It is preferredto accomplish the various objects of this invention and to practice `the same in substantially the manner as more fully described herein and as more particularly pointed out in the appended claims.

A preferred embodiment of the invention is illustrated in the accompanying drawings which form a part hereof and wherein:

Fig. l is a perspective view of the hearing aid embodying the-features of the present invention, this View showing the complete hearing aid and including all of its component parts and which hearing aid may be disposed completely in the ear of the user;

Fig. 2 is a side sectional view of the hearing aid contemplated in Fig.` l, this View being taken substantially on the plane of the line 2-2 in Fig. 1 and showing the relative disposition of the component parts inside the earmold; Fig. 3 is a top elevational View of the hearing aid illustrated in Figs. l and 2 and being taken substantially on the plane of the line 3--3 in Fi". 2;

Y Figjl is a wiring diagram Vof a three transistor circuit producing an amplifier with an excellent frequency response, 'and mediumgain, suicient toV produce enoughV acoustical gain and power for those of mild and medium hearing losses; Y

Fig. 5 is a Wiring diagram illustrating a four transistor ircuitwrith two transistors directly coupled to make two pairsand each pair coupled by a single capacitor to produce an amplifier with high electrical gain, and excellent 'frequencyresponse without the use of a capacitator for each transistor and is suihcient to produce acoustical gain and power for those with very high hearing losses;

Fig. 6 is a. wiringrcircuit. showing a tuned circuit for the detection of electro-magnetic waves, this circuit being a substitutefor the battery illustrated in Figs. 4 and 5 thereby to provide another means for a source of'power;

Fig. 7 illustrates a wiring circuit employing a thermal couple which may be used as a substitute for the battery illustrated in the diagram in Figs. 4 and 5 thereby providing another means as a source of power; and

Fig. 8 is a wiring diagram incorporating in the wiring circuit shown in Figs. 4 and 5 a tuning circuit whereby the hearing aid contemplated by this invention will pick up radio programsthereby to provide in an earmold not only a hearing aid but a small radio adapted to be completely inserted in the ear of the user.

The drawings are to be understood to be more or less of a schematic character for the purpose of illustrating or disclosing a typical or preferred form of the improvements contemplated herein, and in the drawings like reference charactersidentify the'same parts in the several views.

Inthe exemplary embodiment of the invention depicted in Figs. 1, l2 and 3,'the hearing aid with which this invention is particularly concerned is designated in its entirety by the numeral 20, and comprises, in general, an earmold 21 formed of rubber and plastic according to the ear contour of the user, an electronic circuit 22, disposed within the contines of the earmold 21, and a cover cap 23 containing the iadditional components of said electronic circuit 22.' Y

l Particular attention is invited to Figs. l and 2 wherein it can be seen that the earmold 21 is formed of an im- 4 pression (not shown) of the ear of the user to dene the shape and contour substantially as shown and to define a canal projecting `from a body portion 31 which is hollow and deiining a chamber 32 adapted to receive the component parts of the electronic circuit 22 to be hereinafter described in detail, saidrbody defining a cup-like shape with the upper regions thereof terminating in a anged rim133 surrounding the periphery of the body, and said canal 30 having a through bore 35 dening a sound channel. Y y Y Itis important to "note that the earmold shell can be made of hard plastic or rubber or may be -a combination of soft plastic or soft rubber'with hard plastic or hard rubber. When the shell 21 is formed of soft plastic or rubber, a hard rubber or plastic inner liner is used to prevent collapsing of the earmold. It is to be understood that any material which has the characteristics of the materials hereinbefore mentioned kcould be used. The im- Y portant requirement of the material is-that it must create tion 31 of the earmold 21 may be formed of hard plastic a complete seal of the ear to prevent feed back.

The sound channel 35conveys and directs sound into the ear canal (not-shown) and toward the eardrum (not shown) of the user, said sound canal being molded at the same time as the body 31'of the earmold 21 or drilled after the earmold is made.

VThe earmold should be made as thin as possible to accommodate the component electronic parts of the electronic circuit 22. The unit should also be as light in weight as possible to prevent ear fatigue.

It is of particular note in some cases that the body poror rubber while the canal 30 may be formed of soft plastic thereby providing rigidity to the body portion and yet affording a certain amount of resiliency to the canal 30.

Thus, it can be seen that by a 'combination of the intri-V cate skills of the plastics industry,the dental arts, the earmold laboratories, and the chemist, it is possible to control the colors of the finished device so that the skin is matched, thusallowing the device to blend inconspicuously into the 'color of the individual user.

Having thus described the earmold 2.1, attention is invited to Figs. V2 and 5 wherein there is illustrated the relative disposition of the component parts of the electronic circuit 22. Y It is important to note that this invention contemplates a newand unique circuit approach in the electronic antpliier in order to accomplish the results and enable all the necessary circuit components toi be placed in the earmold 21. The majority of contemporary circuits employ transistors which require transformers as impedance matching devices Vto couple onetransistor to the next. Transformers take up room, and limit frequency response in proportion to their physical size. Another method sometimes used in contemporary designs is known as resistance capacitive coupling. VThis Y method also requiresa relatively large amount of space, thereby making it impractical to build a complete, useful circuit inside an earmold.Y Y Y* In general, it may be Vstated tha-t this invention contemplates a method'of directly connecting one transistor to the next without anyk electrical components. By employing `four transistors with two pairs directly coupled, it is possible to produce an amplier with high electrical gain, and excellent frequency response. As clearly shown in Fig. 5, the input Voltagefromthe microphone is then directly coupled to the amplier.

It is possible to further simplify the Vnumber of components with the three transistor circuit illustrated in Fig. 4Y and produce an amplifier with excellent frequency response and medium gain, suiiicient to provide enough acoustical gain and power for those with mild and medium hearing losses. It Visrobvious that it would be possible to house two transistorbeads 'in `a single housing having them directly coupled internally. Y

In the electronic circuit 22 as illustrated in Fig. 2,

all of the circuit elements are mounted on a sheet of melamine 40 which makes the device practical from a mass production point of view, `and permits repairs to be made, if necessary, and gives structural strength to the device. Melamine has been chosen because of its excellent dielectric, mechanical, electrical and low moisture absorption properties. It is obvious that other materials could be used for this purpose.

The four transistor electronic circuit comprises transistors 41, 42, 43 and 44 suitably mounted on the sheet melamine 4i), an output transducer (receiver) 45, -a microphone 46 disposed in the cap 23, resistors 47, 48, 49, 5@ and r 51 also suitably mounted on said melamine sheet 40, a condenser 52, a volume control 55 disposed in said cap 23, a power source or battery 56 mounted in a movable segment 57 of the cap 23, and suitable circuit wiring as at 58 electronically connecting said components according to the diagram illustrated in Fig. 5. interposed in said circuit is an oi-on switch 60 which will be hereinafter further described in detail.

As illustrated in Fig. 4, the three transistor electronic circuit comprises transistors 70, 71 and 72, `an output transducer (receiver) 73, a microphone 74, resistors 75, 76, 77, 78 and 79, a condenser 34, a volume control 80, a power source or lbattery 81, and suitable circuit wiring as at S2, electronically connecting said components according to the diagram illustrated in Fig. 4. interposed in said circuit is an ofi-on switch S3, which will be hereinafter further described in detail.

`ln audio-ampliers such as described herein, a limiting factor in the amount of gain possible without acoustic feed back (or whistle) is the proximity of the microphone 46 fto the receiver 45. It has been found in these devices that part of the feed back is caused by magnetic coupling resulting in transformer action. The relatively high current in the' coil of the receiver 45 sets up magnetic lines of force which cut across the coil turns in the microphone 46, and induce voltages which are then amplified by the gain of the system. A mu-metal shield is used around the coil of the receiver 45, or the coil of the microphone 46, .or between the two coils-or a combination of any or all of these shields. Other materials having magnetic shielding properties could be used and the coils could be oriented in such a manner as to minimize or eliminate coupling. One or both coils could be movable allowing for iinal adjustment for cancelling this coupling, or a coil wound in opposite direction with the coil in the microphone 46 or the receiver 45 `to cause the eifect of cancellation of induced voltages.

Attention is invited to the output transducer (receiver) 45 illustrated in Fig; 2, lwhich comprises a housing 90 having depending therefrom an ear 91 through which sound Waves are transmitted into the sound canal 35 for conveyance to the eardrum (not shown) of the user, said ear being adapted for removable locking engagement -in a through bore 94 of a locking ring 95 which is xedly impregnated into the body 31 of the earrnold 21 at the upper regions of the sound canal 35.

The output transducer 45 can be easily removed from its operative position as shown in Fig. 2 by merely applying a slight force against the housing 90 which will cause the ear 91 to be removed from its locking engagement in the through bore 94 and, thereafter, the output transducer may be removed from the body 31 of the earmold 21 for repair or replacement. In assembly, the transducer may be inserted in its operative position by merely exerting a slight downward force thereupon to force the ear 91 into the through bore 94 whereupon the transducer will be removably locked into its proper operative position as shown in Fig. 2.

It is important to note the manner in which the output transducer 45 is held into the earmold 21 by the employment of the locking ring 95 which may be made of metal or plastic and is permanently mounted in the earmold in close proximity to the ear canal 35. This permits the receiver 45 to be snapped and held rigidly in position as hereinbefore described and enables tight acoustic seal between the receiver 45 and the entrance to the ear canal 35. Certain advantages are provided when the output transducer is positioned closer to the tympanic membrane (eardrum).

Among these advantages are:

(l) Because the diaphragm of the receiver must transmit sound pressure waves through the air lbetween the' eardrum and the diaphragm, reduction of the volume of air that must be moved drastically reduces the amount of acoustic power necessary from the receiver.

(2) In accordance with the Well-known laws of acoustics, the high frequency response of the waves reaching the eardrum is lincreased almost inversely as the distance because the great majority of people with hard of hearing difculty have less acuity in the high frequencies; this is a distinct advantage over former methods, resulting in higher discrimination for speech.

It is to be understood that the shape and mechanical configuration of the receiver 45 as shown could vary in almost any manner. For example, the receiver might be made in the shape of a cylinder of very small diameter, and relatively longer length so that it could be recessed even further into the ear canal 35 and occupy less space in the earmold cavity 32.

The microphone 46 is a self-contained unit disposed in a housing 98 which is press tted into a hollow cavity 99 disposed in the cap 23, said microphone 46 being adapted to receive sound waves through an aperture 100 in the cap 23, said sound waves being picked up by the microphone 46 and conveyed via the electronic circuit 22 and amplified thereby to the output transducer (receiver) 45 whereupon the sound waves are transmitted into the sound canal 35 for conveyance to the eardrum (not shown) of the user.

The resistors 47-50 as illustrated in the wiring diagram depicted in Fig. 5, and the resistors 75-79 as depicted in Fig. 4 are of conventional design and are as respectively marked in these diagrams. Accordingly, these resistors will not be further described herein.

As may thus be seen in Fig. 2, the volume control 55 is press fitted linto a through bore in the cap 23 and comprises an adjustment screw which may be located by inserting a conventional screwdriver in theslot 107 disposed in the upper end portions of the adjustment screw 106.

It is preferred to use the battery 56 as a source of power to operate the electronic circuit 22. The battery cell is disposed in a housing 110, disposed in a cavity 111 in the movable segment 57 of the cap 23. One electrical side of the battery is in frictional contact with a metallic plate 112 disposed on a cover plate 113 upon which the cap 23 is disposed. The other electrical side of the battery is connected by suitable wiring as at 114 to the switch 60.

A feature of the invention resides in the movable segment 57 which comprises a pair of pins 115, 116 which project from the movable segment into the body of the cap 23 and act as guides for the movable segment 57 when moved outwardly from the cap to the position shown in dotted -lines in Figs. 2 and 3. Movement of the movable segment 57 to its outer position permits the battery to be conveniently removed from its normal operative position in the cavity 111.

Attention is invited to Figs. 2 and 3 wherein the off-on switch 60 is disclosed and which comprises a spring plate carried by the cap 23 and sandwiched between the said cap and the movable segment 57. The extreme outer ends 121, 122 of which are arranged respectively to frictionally engage the guide pins 115, 116 which are provided with 'slots (not shown) into which the ends 121 and 122 may fall to provide a locking arrangement to hold the movable segment 57 in its operative position as shown in full lines in Figs. 2 and 3. The guide pins 115,

vspring plate 120 only when the movable segment 57.is

in its operative position shownin` full lines, thereby making an electrical connection throughsaid spring plate 120 to Wiring 132 and through Vthe eleotronicgcircuit 22, When the movable segment 57 is moved outwardly from its. normal operative position, it is obvious that the stud 131 will not contact the spring plate 120 and therefore the electricalconnection between these elements will be broken and the hearing aid will be shut off.

It is obvious lthat when ythe hearing aid is not in use, the operator will pullV the movable segment S7 out from its normal position aY slight distance sufficient to break the contact between the stud 131 and the spring plate 120. When the instrument lis to be used, the userV will push the movable segment 57 to its proper position whereupon the exterme ends of the spring plate 120 will fall into the slots in the guide pins 115 and 116, thereby to maintain the movable segment in its proper. operative position. Y Y VIt is ponited out that the battery 56 as shown in this device has as its Vfunction merely to provide power forV the 1arnprlitier, and it is to be understood that some other source of power such as thermo device illustrated in Fig. 7, detection of electro-magnetic waves, or some other means of power would not change the basic concept of l'this invention. It is important to note that only one milliampere current is necessary as a power source for the hearing aid contemplated herein.

It is further pointed out that while a microphone as at 46 is used to collect sound energy, it might'easly be changed and replaced by some other detection device, such as a tuned circuit as illustrated in Fig. 6 to pick up radio program-s, or detection devices such as illustrated in Fig. 8 for other types of stimuli. The transducer shown is for the purpose of giving a sound stimuli to the eardrum. It could be replaced by a device to give other types of response such as electrical stimuli to page a doctor in a hospital paging system, or to transmit orders 'and information to military forces. n

From the foregoing disclosure, it can be seen that we have provided a hearing aid which efficiently fullls the objects hereinbefore set forth and provides numerous advantages which may be summarized as follows:

( l) Structurally simple, efficient and durable;

(2) Economical to manufactureand readily adaptable to mass production manufacturing; and

(3) The provision of a hearing aid particularly characterized as being a self-contained unit whereby the entire electronic circuit and mechanism including a power source are incorporated in an earmoldrequiringno wire or other connections running from the earmold to remote parts of the body of the user.

While we have illustrated a preferred embodiment of our invention, many modifications may be made without departing from the spirit of the invention, and we do not wish to be limited to the precisedetails of construction set forth but desire to avail ourselves .of all changes Awithin Vthe scope of the appended claims.

We claim:V

1. An electronic hearing aid comprising: an earmold said circuit, a second cap portion slidably attached to said earmoldv and comprising a source of power for said circuit and v'a switch .for connecting said source of power to said circuit for energizing itjY saidinicrophone being disposedY in Ysaid firstV capV portion Yand said'electronic circuit comprising said transistors beingdisposed in the hollowV body portion of said earmold, the complete assembly of which comprises a unitary structure adapted to fit into the ear of the user. p

Y 2.`An electronic Vhearing aid comprising `an year mold having a ,hollowY .body portion and a stem portion .defining asound canal formed integrallytherewith, a tirst cap prtionattached to said .ear mold, a microphone disposed within said cap, a receiver fixedly mounted within said ear mold adjacent the inner end of the sound canal, a transistor amplifier circuit mounted within said ear mold and interconnecting said microphone with said receiver, a battery for energizing said amplifier circuit, a second cap portion slidably attached to` said vear mold, said second cap portion comprising one contact of an electrical switch and a housing -for said battery and means defining a second contact connectible with said first contact .for thereby connecting said battery to said amplifier circuit.

3. An electronic hearing aid comprising an ear mold having a hollow body portion and a stem portion formed integrally therewith, said stem portion defining a sound canal, a receiver mounted on the interior of said ear mold and forming an acoustic seal on the inner end of the sound canal, said receiver being adapted to transmit sound vibrations into said sound canal, a microphone, a transistorized ampli-fier circuit interconnecting said micro'- phone with said receiver, a lfirst cap formed with a cylindrical recess for containing said microphone and attached to said ear mold, means defining .a first contact of an electrical switch on an edge of said cap, a second cap formed with a cylindrical'recess and slidably attached to said ear mold, means defining a second contact of an electrical switch on an edge of said second cap `and adapted to make electrical contact with said first contact, and a battery disposed -within the cylindrical recess of said second cap and adapted to supply electrical energy to said amplifier circuit upon closing said contacts.

References Cited in the le of this patent UNITED STATES PATENTS 2,319,627 Perlman May 18, 1943 2,430,229 Kelsey Nov. 4, 1947 2,461,344 Olson Feb. 8, 1949 2,485,405 Olney et al. Oct. 18, 1949 2,487,038 Baum Nov. 8, 1949 2,544,027 King Mar. 6, 1951 2,613,282 Scaife Oct. 7, 1952 2,765,373 Smith Oct. 2, 1956 '2,775,652 Stutz Dec. 25, 1956 2,777,057 Pankove Jan. 8, 1957 2,787,670 Rowland Apr. 2, 1957 2,824,177 Tado Feb. 18, 1958 2,830,132 Borg Apr. 8, 1958 2,856,466 Gustafson et al Oct. 14, 1958 FOREIGN PATENTS 922,889 Germany Jan. 27, 1955 1,089,681 France Mar. 2l, 1955 OTHER REFERENCES Audio High Fidelity, pp. 38 and 39, publication Radio Electronics, September 1953, title Build this Transistor Hearing Aid.

Radio Market, an insert to Electro-Technic, vol. 41, issue F l0, dated October 9, 1954, published in Wurzberg, Germany.

TransistorV Radio Uses No Power Supply by William H. Grace, Jr., Radio Electronics, April 1955, Apage 96.

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Classifications
U.S. Classification381/323, 381/120, 381/328
International ClassificationH04R25/00
Cooperative ClassificationH04R25/554
European ClassificationH04R25/55D