US2363175A - Electrically and acoustically excited hearing aid - Google Patents

Description

Nov. 21, 1944. F. M GROSSMAN ELECTRICALLY AND ACOUSTICALLY EXCITED HEARING AID Filed Aug. 26, 1942 IN VEN TOR. mama/(K M. G/ OISMAA/ @fnyJ & My

Patented Nov. 21, 1944 ELECTRICALLY AND ACOUSTICALLY EXCITED HEARING AID Frederick M. Grossman, New York, N. Y. Application August 26, 1942, Serial No. 456,162 Claims. {'01. 179-107) This invention relates to electrically and acoustically excited hearing aids for individuals with impaired hearing function and it is working on the principle of air conduction, i. e. sound transmission through the air in the external ear canal.

Electrically excited hearing aids of this type are well known in the art. They comprise principally a microphone for picking up and transforming sound waves into corresponding electrical energy usually derived from a battery, an amplifier for this energy and a receiver for transforming again the electrical energy into acoustical energy. It is convenient to mount the receiver on a mold of plastic material individually shaped to lit the outer ear and projecting into the external ear canal; the mold is provided with a channel which at one end opens into the ear canal, while at its other end it opens toward the receiver.

As outlined in the paper entitled Some Problems Involved in the Fitting of Hearing Aids by Halstead and the inventor (N. Y. State Journal of Medicine, vol. 41, issue No. 4 of Feb. 15, 1941), hearing impairments can be classified in four classes which requires specific and difierent characteristics of the hearing aid.

This invention resides substantially on the following considerations and experience.

The resonance of the external ear canal is important for the perception of the high frequency partials contained in the speech sound spectlum. It is therefore necessary that the acoustical characteristic of the external ear canal be changed as little as possible by a hearing aid.

Low frequencies contained in the sound spectrum are chiefly responsible for the loudness sensation of speech sound. Approximately 95% of speech power is attached to frequencies below 1000 cycles in the spectrum of speech sound (cf. Dunn and White, Journal of the Acoustic Society of America, vol. XI, 1940, p. 278). Persons suffering from the variable type hearing loss, (cf. Steinberg and Gardner, Journal of the Acoustic Society of America, vol. DI, p. 11, 1937) have of necessity a narrower band of comfortable loudness sensation as compared to the intensity band oflered.

It is essential to equalize peaks of speech power with parts of lower speech power. This can be done efiectively by attenuating the low frequency content of speech sound.

It has further been observed that loud speech sounds which must contain low frequency partials of relatively higher intensity so as to be loud, produce with individuals with perceptive hearing loss the sensation of banging." Low frequency partials also mask the high ones and are chiefly responsible for the loudness sensation of speech sound. High frequency partials are, however, indispensable for the intelligibility of speech. The perception of high frequency partials is impaired in perceptive hearing loss so that the masking eflect is multiplied. Since the perception of high partials is reduced in individuals sufiering from perceptive hearing loss and the loudness sensation in those individuals is pathological inasmuch as the intensity band producing comfortable loudness sensation is narrower, the low frequency content of speech sounds transmitted to those individuals should be reduced.

It has therefore been suggested to translate to individuals sull'ering from perceptive hearing loss and impairments of similar eiiect, the speech sound through an electrical filter in such a manner that the low frequency partials of the sound are amplified to a lesser degree than the high frequency partials. Surprisingly it appeared that such arrangements did not have the expected result. The banging could not be prevented or reduced to a tolerable extent.

The inventor believes to have discovered the reason for the failure of this known arrangement. While it is possible to change the intensities of different frequencies in a speech sound by means of an elaborate electrical filter and thus to deliver to the receiver a changed sound spectrum which theoretically suits the conditions of the individual sufl'ering from perceptive hearing loss, the sound emanating from the receiver and passing the channel in the mold before discharging into the part of the ear canal left free by the mold, is again being changed in its qualities.

As it has been shown by Stewart (Acoustics, by Stewart and Lindsay, 1930, p. 159) any constriction in a channel through which sound waves proceed, results in attenuation of higher frequencies, while lower frequencies pass such channel with unchanged intensity. Consequently the sound emanating from the receiver and which contains low frequencies attenuated in intensity by the electrical filter, are now subject to attenuation of the higher frequencies while proceeding through the conduit composed of the narrow channel in the mold, which forms a constriction, and the far wider free part of the ear canal. Thus the per se favorable filtration of the speech sound by the electrical filter is substantially neutralised by the filtration to the opposite effect in the acoustical low pass filter formed by the constricted channel in the mold followed by the far larger part of the external ear canal.

It is therefore an object of the invention to provide the acoustic line from an electrically controlled sound generator of an air conduction hearing aid to the ear, acoustically operating means for filtering the sound translated along that line in such a manner that it becomes intelligible and comfortable for individuals suffering from perceptive hearing loss and similar ailments.

It is still another object of the invention to provide an air conduction hearing aid with an adjustable acoustical filter which renders intelligible the sound proceeding from the sound generator of the hearing aid to the ear of an individual suffering from perceptive hearing loss and similar disabilities, and permits adjustment of the frequency response characteristic of the sound even without the compulsory use of electrical filters.

It is still another object of the invention to provide the acoustic line for an electrically controlled sound generator of an air conduction hearing aid to the ear with acoustically operating means for filtering the sound translated along said line and to translate this sound to the car without considerable change in the acoustic characteristics of the external ear canal.

These and other objects of the invention will be more clearly understood when the specification proceeds with reference to the drawing in which a few embodiments of the invention are shown by way of exemplification. Conventional elements of electrically controlled air conduction hearing aids are omitted in the drawing for sake of simplification, for instance the microphone, the source of electrical energy, the amplifier, means for regulating the amplifier (or set of amplifiers) and heating their cathodes and feed ing their plate circuit, and conductors between them. Furthermore, while an electromagnetically controlled membrane or diaphragm is shown as sound generator in the receiver, it should be understood that any other sound generator, such as of the crystal type can be used to the same effect.

Referring to the drawing, Fig. 1 shows in perspective view and with parts in cross section a hearing aid according to the invention fitted to the ear; Fig. 2 on a larger scale a cross section with parts in elevation through the hearing aid: Figs. 3 and 4 cross sections along line 3-3 in Fig. 2 in d fferent relative positions of two elements of the hearing aid; Fig. 5 a perspective view of an element of the hearing aid attached to a part of the receiver; Fig. 6 a perspective view of another element of the hearing aid attached to an ear piece; and Fig. 7 a perspective view of a modification of the element shown in Fig. 5.

Referring to Fig. 1, a receiver ID of an ear con duction hearing aid of conventional structure is shown, provided with conductors H leading to the other conventional elements of such a hearing aid. An acoustical and preferably adjustable high pass filter I 2 is associated andsoundtightly connected at one end with receiver I0, and at the other end with an ear piece or mold i3 which is fitted into the external ear canal It. Any other suitable fitting of the free end of the filter l2 to the ear can be used.

Referring now to Figs. 2 to 6. it will be appreciated that the receiver in comprises a membrane 76 or diaphragm l5 held or clamped between adiacent surfaces of the casing or any other element of the receiver, and its cover l6.

An element of an acoustical high pass filter is conveniently connected with cover it or made integral with it, as shown. It consists of a tubular member l1 provided with a number, one as a minimum, of holes IQ of equal or difierent diameters. The tubular member I! slightly tapers on its inside towards cover l6. Another tubular member I! snugly fits with its tapered outside into tubular member I! and is cut out at 28. in this exemplification over about half its circumference. It is connected with the ear piece 13 in any suitable way or made integral therewith as shown. While the telescoping tubular members H and I9 may be made of metal and connected. respectively with cover l6 and earpiece l3 in any suitable way, member I9 if it be made integral with ear piece l3 consists of the same material as the ear piece, e. g. of moldable plastic material.

A circumferential groove 20 is provided near the free end of tubular member l9 into which the inner edge of an annular spring element 2| snaps when member I9 is assembled with member 11. The conical portion of member 2| is slotted radially so as to permit that snap action. Thereby the tubular members I! and iii are kept permanently in their relative position, as shown, permitting relative rotation to one another.

By turning tube I! relative to tube l9 into a position as shown in Fig. 3, all the holes or orifices l8 are communicating both with the interior of the tubular members as well as with the surrounding air.

By turning those members into the position shown in Fig. 4, for instance one of the orifices i8 is covered by the solid portion of tubular member i9.

It will be observed that in other relative positions of the two tubular members any desired number of orifices i8 can be covered or left open and thereby become effective.

With increasing number of side branch orifices in series, the degree of attenuation and range of attenuated low frequencies is increased; with a smaller number of orifices the attenuation decreases gradually with increasing frequency.

By arrangement of more than one orifice in parallel, i. e. in one plane substantially vertical to the axis of the conduit, the attenuating effect can be increased while the band width of low frequencies so affected remains substantially the same.

It is a further condition of the filtering action of a conduit provided with side branch orifices that their diameter or dimension of a slot measured in the direction of the axis of the conduit. is considerably smaller than a wave length to be attenuated, and if orifices are arranged in series. that their distance measured in the direction of the axis of the conduit be small compared with the wave length to-be attenuated.

Applying these per se known acoustical laws to the conduit formed by the tubular members l1, l9 and the side branch orifices l8 provided therein in a plane substantially vertical to the axis of the conduit, it is clear that the diameter of holes IE, or if there be provided elongated slots instead, the largest dimension of such slots in the direction of the axis of the conduit should be reasmaller than a wave length to be attenuated. Ex-

pe'riments show that holes ll of a diameter of about A to of a millimeter gave satisfactory results, though larger diameters can be used under observation of the laws of Stewart.

Also the wall thickness of member I! should be such that the orifices I! are reasonably short and the desired effect is invariably obtained, and therefore advantageously between about V to 2 /2 mm. Too thick a wall might becloud the proper operation of the side branch orifices.

The total axial length of the conduit formed by the telescoping tubular members II, It should also be made as short as possible in order to eliminate the effect of phase differences, and for the further reason that thereby the hearing aid is rendered less conspicuous.

The ear piece II is made as short as possible in axial direction and projects as little as possible into the external ear canal l4, compatible with firm seating of the ear piece in the ear. Care should also be taken that the inner diameter of the mouth 29, Figs. 2 and 6, of the ear piece is as large as possible and approximates that of the external ear canal, so as to avoid constrictions in the acoustical line from the sound generator, e. g. membrane ii to theear drum. such con strictions, if large enough, could detrimentally affeet the fidelity of the sound translated along it, since constrictions in an acoustical line or conduit render it a low pass filter. By making the cross sectional area of the conduit formed by members l1, l9 and of the mouth piece or mold I; approximately the same as the cross sectional area of the external ear canal, detrimental distortions or low pass filter effects are avoided. By making the ear piece as short as'possible, it occupies an acoustically almost negligible part only of the external ear canal whereby the resonance characteristics of the latter which are essential for true translation of the sound are essentially retained.

By adjusting, i. a. turning the tubular members i1, is relative to one another. any desired number of side branch orifices 18 or portion of one orifice can be made operative or eliminated, and thereby any desired degree of attenuation of low frequencies and consequently any desired degree of comfort and intelligibility of the translated sound offered to the individual fitted with this hearing air can be realized. Such adjustment will be made preferably by the expert who fits the hearing aid to the ail ng individual; the latter may also bring about slight adjustments to accommodato himself to particular conditions of loudness of sound to be perceived, which also depend on and change with the architectural acoustics of the room in which the individual stays.

In the event that in addition to degree of attenuation also the width of a low frequency band is intended to be influenced. two (or more) rows of orifices I8, 28, Fig. 7, may be provided, and member i9 and its out out window 28 shaped so that its solid portion covers or its window uncovers the same or different numbers of orifices in each row in the diflerent adjusted relative positions of member is to member II.

It will be understood that window It may be provided in member I! and the orifice or orifices II (and 26) in member l9.

It will also be understood that instead of a. membrane 15, electrically controlled by electromagnet 21, any other electrically controlled sound generator can be provided, such as of the crystal type.

The outstanding advantages of the invention consist in its simplicity, negligible weight and inexpensiveness, doing away with complicated and comparatively heavy and expensive electric filters. If electrical filters are used for any particular reason, they can be simple and of reduced effectiveness. The acoustical hish pass filter used by the invention can more easily be adjusted than an electrical filter, particularly by the user. Any suitable degree of attenuation and band width of attenuated low frequencies can be obtained. With any number of orifices arranged in one or two rows always smooth curves representative of the degree of attenuation dependent on frequency, Fig. 8, are realized and undesirable cut-oils are eflectively avoided, while electrical filters often produce such cut-oil's. The acoustical line or conduit formed by the acoustical filter and channel of the ear piece can be made short and straight, its cross sectional areas can be made uniform over its entire length and almost the same as of the external ear canal, whereby most efilcient sound translation is secured and distortion of the resonance characteristic of the external ear canal is avoided.

It should be understood that my invention is not limited to the exemplification referred to herein, but to be derived in its broadest aspect from the appended claims. The reference in some of the latter to "side apertures or "side orifices should be understood in the light of the foregoing specification to define apertures or orifices, including slots, the dimensions of which (particularly measured in the direction of the axis of the sound conducting conduit or channel with which they communicate) are reasonably small and in any event considerably smaller than the length of a sound wave to be attenuated, in order to establish an acoustical high pass filter.

What I claim is:

1. An air conduction hearing aid including an electrically controlled soimd generator and hollow means associated therewith so .that sound generated by said generator is propagated through the interior of said hollow means the free end of which is adapted to be fitted into an external ear canal. characterized by the novel feature that said hollow means are provided between said generator and end with a number, one as a minimum, of side apertures establishing open communication between the interior of said hollow means and the surrounding atmosphere and constituting with said interior an acoustical high pass filter, and that additional means are provided for adjusting the effective area of said apertures.

2. An air conduction hearing aid including an electrically controlled sound generator and hollow means associated therewith so that sound generated by said generator is propagated through the interior of said hollow means the free end of which is adapted to be fitted into an external ear canal, characterized by the novel feature that said hollow means are provided with a number, one as a minimum, of side apertures establishing over the shortest way and essentially perpendicular to the axis of said hollow means, open communication between the interior of said hollow means and the surrounding atmosphere and constituting with said interior an acoustical high pass filter; that the maximum extension of any such aperture measured co-axially with said interior does not exceed about two and one-half millimeters; and that additional means are operatively connected with said hollow means for ad justing the eifective area of said apertures.

3. An air conduction hearing aid including an electrically controlled sound generator and hollow means associated therewith so that sound generated by said generator is propagated through the interior of said hollow means the free end of which is adapted to be fitted into an external ear canal, characterized by the novel feature that said hollow means are provided between said generator and end with a number, one as a minimum, of Side apertures establishing open communication between the interior of said hollow means and the surrounding atmosphere and constituting with said interior an acoustical high pass filter; that the cross sectional area of the interior of said hollow means from said generator to said free end is essentially constant and approximates the cross sectional area of an external ear canal to which the hearing aid is to be fitted; and that additional means are provided for adjusting the effective over-all area of said side apertures.

4. An air conduction hearing aid, substantially comprising, in combination, an electrically controlled sound generator exemplified by an electromagnetically controlled membrane, two telescoped tubular members adjustable one relative to the other, at least one of said members associated with said generator so that sound produced by the latter is translated through the inner one of said members, one of said members provided with a number, one as a minimum, of side apertures the maximum extension of any such aperture measured co-axiall'y with said member not exceeding about two and one-half millimeters, the other one of said members provided with one or more passages arranged so that they can be aligned with one or more of said apertures in the other member upon relative adiustment of said members, and an ear piece associated with the free end of at least one of said members, said ear piece provided with a channel communicating with the interior of the izmer one of said members.

5. A hearing aid as described in claim 4, in which the cross sectional areas of the interior of the inner one of said tubes and of said channel approximate the cross sectional area of an external ear canal to which the hearing aid is to be fitted.

FREDERICK M. GROSSMAN.

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