Hearing Aid With Directional Microphone System

Abstract

A hearing aid is depicted which has a highly directional microphone system comprising means defining a front-to-back acoustic conduit through the hearing aid and a microphone assembly supported in acoustic isolation within the conduit in such a way as to define a sound passageway around the microphone assembly. The microphone assembly includes an acoustically sensitive diaphragm and associated electro-mechanical transduction means. A housing for the microphone assembly has a front orifice communicating exclusively with one side of the diaphragm which presents a predetermined acoustic impedance to sound waves of predetermined audible frequencies propagating in the passageway. The housing for the microphone assembly also has a rear orifice which communicates exclusively with the opposite side of the diaphragm. The rear orifice is effectively spaced from the front orifice by a predetermined acoustic path length which is such that sound waves, at least those in low and middle ranges of audible frequencies reaching opposite sides of the diaphragm through the front and rear orifices in the microphone housing establish a pressure gradient across the diaphragm. The rear orifice has a predetermined acoustic impedance greater than the acoustic impedance of the front orifice and of such magnitude as to produce a substantially cardioid directional characteristic for the hearing aid.

Description

BACKGROUND OF THE INVENTION

It is well known that the human auditory system, functioning normally, provides a high degree of directional discrimination in sound perception. This ability to discriminate enables a person to readily detect the direction from which a sound is emanating. However, persons wearing conventional omnidirectional hearing aid devices have great difficulty discerning with facility the direction of sound origin, especially under adverse conditions. For example, detecting the origin of a particular vocal sound in a room filled with background noise is commonly a difficult and annoying effort.

PRIOR ART

U. S. Pat. No. 3,458,668 -- Hassler discloses a directional hearing aid including two microphones (or a single microphone with two diaphragms) which are separately excited by front-emanating and rear-emanating sound waves. Separate excitation of the two diaphragms produces two electrical signals which are electrically combined in appropriate phase and amplitude to develop a resultant signal which, when transduced, allegedly renders the Hassler hearing aid directional.

A German Pat. No. 1,277,347 discloses a directional microphone for a hearing aid which is asserted to operate on a phase shift principle. This microphone has a single sound-responsive diaphragm communicating with the exterior of the microphone housing from one side of the diaphragm through a front orifice and from the opposite side of the diaphragm through a rear-directed bundle of very small capillary tubes. It is stated in this patent that directionality is imparted to the microphone by the tubes which act as transit delaying acoustical resistances.

Microphones of the type used in broadcasting and communications are described in texts such as "Microphones" by A. E. Robertson, New York Hayden Book Co., Inc., New York. See also U.S. Pat. No. 3,115,207.

OBJECTS OF THE INVENTION

It is a general object of this invention to provide a hearing aid device having a highly directional microphone system.

It is another object to provide a hearing aid having an improved microphone system which has a directional discrimination characteristic which is relatively unaffected by external obstruction or interference.

It is yet another object of this invention to provide a hearing aid having a directional microphone system which is significantly more compact and less expensive to manufacture than prior art directional microphone systems for hearing aids.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a sectional view of a hearing aid device embodying the principles of the invention;

FIG. 2 is an enlarged sectional view of a microphone system forming part of the FIG. 1 hearing aid;

FIGS. 3, 4 and 5 are sectional views of the system taken along lines 3--3, 4--4 and 5--5, respectively, in FIG. 2;

FIG. 6 is an isolated perspective view of a microphone assembly shown in FIGS. 1-5;

FIG. 7 is a frequency-versus-output curve depicting typical front and rear frequency response characteristics at 1000 Hz for the FIGS. 1-6 hearing aid;

FIG. 8 is a polar diagram illustrating the directional characteristic at 1000 Hz of the microphone system illustrated in FIG. 1-6; and

FIG. 9 is a side view, partially broken away, of a hearing aid embodying an alternative form of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a hearing aid 10 embodying the principles of this invention. The hearing aid 10 is illustrated as comprising a shell 12, which may be formed of injection-molded plastic, containing an improved microphone system 14 incorporating the teachings of this invention for detecting incident sound waves and transducing them to an electrical audio signal. The system 14 will be described in detail below. An amplifier 15 amplifies the audio signal from the microphone system 14. The transducer 16 converts the amplified audio signal into sound waves which are conducted through a tube 20 to an ear mold (not shown) for introduction into the auditory canal of the user. A manually operable volume control 18 is provided.

This invention concerns the improved microphone system 14. By this invention, a microphone system for hearing aids is provided which has a highly directional response characteristic, yet which is simple, compact, low cost and relatively insensitive to external physical interference or obstruction. According to this invention a hearing aid is provided which has a front-to-back, sound-isolating acoustic conduit in which is supported in acoustic isolation a microphone assembly. The microphone assembly has a housing having a front orifice communicating exclusively with one side of a pressure sensitive diaphragm inside the housing and a rear orifice communicating exclusively with the opposite side of the diaphragm. The rear orifice presents an acoustic impedance to sound waves at predetermined audible frequencies which is substantially greater than the acoustic impedance presented by the front orifice to the same sound waves. The front and rear orifices are effectively spatially separated by a predetermined acoustic path length which is such as to introduce a predetermind pressure gradient across the diaphragm as a result of the time difference for a sound wave to reach opposite sides of the diaphragm.

If both microphone assembly orifices produced no impedance or equal impedance to the impinging sound waves, the resultant directional response would show two maxima 180.degree. apart and two minima 180.degree. apart and spaced 90.degree. from each maxima. When the rear orifice has greater impedance to sound waves than the front orifice, the frontal response lobe magnitude exceeds that of the back lobe. As the rear orifice acoustic impedance further increases the rear lobe sensitivity decreases and the minima each rotate toward the rear lobe. There is a range of acoustic impedances for which the rear lobe becomes vanishingly small and coincident with each minima, resulting in a cardioid shaped response characteristic.

It is thought that the acoustic admittance of the rear orifice and a portion of the acoustic admittance of the front orifice of identical magnitude may be thought of as acting to establish a pressure gradient microphone which would be alone capable of producing the 8-shaped two lobe frequency response pattern which characterized pressure gradient microphones. The remaining portion of the acoustic admittance of the front orifice may be thought of as acting as a conventional single opening pressure-type microphone alone capable of producing an omnidirectional frequency response characteristic. By causing the pressure gradient and pressure microphone response characteristics to be of equal magnitude but opposite sign, the response characteristic characterizing certain directional microphones in the broadcast and communications field.

The microphone system 14 according to this invention comprises a microphone assembly 22 and acoustic conduit means defining a front-to-back sound conduit 21, here shown as being defined by a microphone casing 24. Support means for supporting the microphone assembly 22 in the conduit 21 are illustrated as comprising a visco-elastic boot 26 around the assembly 22. The boot 26 serves a number of functions: (1) it acoustically isolates the microphone assembly 22 from the casing 24; (2) it acts to space the microphone assembly 22 from the casing 24 so as to define an interstitial passageway 23 for sound waves to travel around the microphone assembly 22; and (3) it defines openings effective to conduct sound waves through the passageway 23 without substantial impedance thereof.

The boot 26 is here shown as including a jacket 36 and a number of feet 38, here shown as being eight in number, one at each corner of the microphone assembly 22. The feet 38 provide an eight point cushioning suspension for the microphone assembly 22 and also act as spacing elements for spacing the boot jacket 36 away from the inner surface of the microphone casing 24 so as to define sound-transmissive openings to allow the conduction of audible sound waves through the passageway 23 without substantial impedance thereof.

The microphone casing 24 is here shown as comprising a thin metal can, formed for example by an electroforming process, for substantially enclosing the microphone assembly 22. The microphone casing 24 has front and rear openings 28, 30 disposed to admit sound waves to the microphone assembly 22 without significant impedance thereof. A pair of conventional grilles 32, 34 cover the openings 28, 30.

In accordance with one aspect of this invention the sound passageway 23 around the microphone assembly is such that a sound wave, at least in low and middle ranges of audible frequencies, reaches opposite sides of the diaphragm with a phase difference. A microphone assembly as shown at 22 may in actual construction have a typical back-to-front dimension of 5/16 inch. A path length of 5/16 inch will introduce a phase delay in a 1000 Hz sound wave at sea level (having a wavelength of approximately 13.6 inches) of approximately 8.0.degree. .

The microphone assembly 22 will now be discussed in detail. The microphone assembly 22 is illustrated schematically as having a diaphragm 40 coupled to a transducer 42 for translating mechanical vibrations acoustically induced in the diaghragm 40 into corresponding electrical signals. The electrical signals are supplied to transducer 16 which converts the electrical signals into acoustic signals for delivery to the auditory canal of the user.

In accordance with the principles of this invention, the microphone assembly 22 includes a housing 44 for enclosing the diaphragm 40 and transducer 42. The housing 44 is illustrated as having a generally box-like configuration including front and rear panels 46, 48.

The front panel 46 has formed therein a relatively large front orifice 50 which communicates with the lower side of the diaphragm 40. The front orifice 50 is of a size sufficiently great to pass to the diaphragm relatively unimpeded sound waves of audible frequencies propagating in the passageway 23. In a commerically available hearing aid implementing this invention, the front orifice 50 is round and has a diameter in the order of 0.043 inch. With a front orifice 50 having the described dimension, audible frequencies in the range normally capable of being received and amplified by hearing aids are passed with an acoustic impedance which is relatively low.

In accordance with this invention the housing rear panel 48 defines a rear orifice 52 which communicates with the top side of the diaphragm 40 and which is of a relatively small size relative to the size of the front orifice 50. The rear orifice 52 is here shown as having a generally slit-like configuration. In the above-mentioned commerical aid, the rear orifice has a length dimension of approximately 0.050 inch and a width dimension of approximately 0.0015 inch.

Throughout this specification, the terms "low," "middle," or "high" as used to describe frequency ranges of audible sounds are intended to be considered with reference to the normal operating range of hearing aid devices, typically 100-4000 Hz.

A hearing aid having a microphone system 14 as illustrated in the drawings and as described above has been found to have a substantially cardioid-shaped directional response characteristic. FIG. 7 illustrates the frequency response characteristic of hearing aid 10 in the forward and rear directions, curve 54 representing the frequency response of the hearing aid 10 in the forward direction and curve 56 illustrating the frequency response of the hearing aid in the rear direction.

FIG. 8 illustrates a polar diagram of the response characteristic at 1000 Hz of a hearing aid offered commerically by the assignee of this invention. The hearing aid contains a microphone system 14 as described above and as illustrated in FIGS. 1-6. The polar diagram shows a response in the frontal direction of approximately 32 decibels and a response in the rear direction of approximately 11 decibels, a front-to-back discrimination of 21 decibels. A front-to-back discrimination of 20 decibels has been found to be very adequate to give the user of a hearing aid good directional discrimination of sounds. As stated, the FIG. 8 diagram represents the response characteristic of the FIGS. 1-6 hearing aid at one frequency only, namely 1000 Hz. The diagram would be different for other sound frequencies.

The invention is not limited to the particular details of construction of the embodiment depicted, and other modifications and applications are contemplated. Certain changes may be made in the above-described apparatus without departing from the true spirit in scope of the invention herein involved. For example, the microphone casing 24, here shown and described as comprising a metal can, may be eliminated and the enclosure formed thereby provided by a modification of the hearing aid shell, as shown in FIG. 9. In FIG. 9, a casing 58 for a microphone assembly 60 is formed integrally with a protective plastic shell 62 for the hearing aid. The casing 58 may have generally the same configuations and functions as the casing 24 in the FIGS. 1-6 embodiment. Other configurations, locations and dimensions of the front and rear orifices in the microphone housing may be provided consistent with the principles of this invention. It is intended, therefore, that the subject matter of the above depiction shall be interpreted as illustrative and not in a limiting sense and that the appended claims cover all such modifications as may fall within the true spirit and scope of the invention.

Claims

We claim:

1. In a hearing aid, a highly directional microphone system comprising:

acoustic conduit means defining a front-to-back sound conduit through the hearing aid;

a microphone assembly including a housing and diaphragm means coupled to transducer means for translating mechanical vibrations acoustically induced in said diaphragm means into corresponding electrical signals; and

support means for supporting said microphone assembly in said conduit such that said assembly is substantially isolated from stray mechanical energy in the hearing aid, but such as to define a passageway for the conduction of sound waves of audible frequencies around said microphone assembly without substantial impedance thereof,

said housing for said microphone assembly having a front orifice which communicates exclusively with one side of said diaphragm means and which presents a predetermined acoustic impedance to sound waves, at least those in low and middle ranges of audible frequencies, propagating in said passageway,

said housing for said microphone assembly also having a rear orifice which communicates exclusively with the opposite side of said diaphragm means, which is effectively spaced from said front orifice by a predetermined acoustic path length which is such that sound waves, at least those in said low and middle ranges of audible frequencies reaching opposite sides of said diaphragm means through said front and rear orifices establish a pressure gradient across said diaphragm means, and which has a predetermined acoustic impedance greater than said acoustic impedance of said front orifice and of such magnitude as to produce a highly directional characteristic for the hearing aid.

2. The apparatus defined by claim 1 wherein said rear orifice is a slit.

3. In a behind-the-ear hearing aid having an electroacoustic transducer, a highly directional microphone system comprising:

a microphone assembly having a box-like housing with front and rear wall panels, said assembly also including diaphragm means coupled to transducer means for translating mechanical vibrations acoustically induced in said diaphragm means into corresponding electrical signals;

microphone casing means substantially surrounding said microphone assembly except for front and rear openings disposed to admit sound waves to said microphone assembly; and

visco-elastic cushioning means positioned between said housing and said casing means for isolating said microphone assembly from stray mechanical energy in said casing means and for spacing said microphone assembly from said casing means, to define an interstitial sound-transmissive passageway around said microphone assembly,

said front panel of said housing for said microphone assembly having a relatively large front orifice which communicates exclusively with one side of said diaphragm means and which presents a predetermined first acoustic impedance to sound waves, at least those in low and middle ranges of audible frequencies propagating in said passageway,

said rear panel of said housing for said microphone assembly having a slit-like rear orifice which communicates exclusively with the opposite side of said diaphragm means, which is effectively spaced from said front orifice by a predetermined acoustic path length which is such that sound waves, at least those in said frequency ranges, reaching opposite sides of said diaphragm means, through said front and rear orifices establish a pressure gradient across said diaphragm means, and which rear orifice has a predetermined acoustic impedance greater than said acoustic impedance of said front orifice and of such magnitude as to produce a highly directional characteristic for the hearing aid.

4. The apparatus defined by claim 3 wherein said rear orifice is a slit having a smaller dimension of approximately 0.0015 inch.

5. The apparatus defined by claim 3 wherein said rear orifice has length and width dimensions of approximately 0.050 and 0.0015 inch, respectively.

6. The system defined by claim 5 wherein said casing means is a thin metal can.

7. In a hearing aid having a protective shell, the system defined by claim 5 wherein said casing is plastic and is integral with said shell.