Acoustic feedback stabilization system particularly suited for hearing aids

[57] ABSTRACT

Linearity is increased and distortion reduced in a miniature acoustic amplifying system by providing an acoustically communicating passage between the microphone and speaker diaphragms to define a negative acoustic feedback loop for the amplifying system. The negative acoustic feedback is feasible because of the miniature arrangement itself of the microphone and speaker wherein the distance of the acoustic passage can be made a small fraction of the wave length of the highest frequency in a given frequency range to be amplified. An adjustment is provided for changing the acoustic coupling between the microphone and speaker diaphragms to thereby adjust the negative feedback loop. This adjustment can also serve as a volume control and the entire system is highly suitable for use in hearing aids wherein miniature microphones and speakers are utilized in close physical proximity.

Claims, 3 Drawing Figures

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PATENTED OCT 21973

3,763,333

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FIG. I

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•030 JOO .300 Too 3Ό0 ' 10*0 30.0 100 0 (LOG SCALE)

FIG. 3

3,763,333

ACOUSTIC FEEDBACK STABILIZATION SYSTEM PARTICULARLY SUITED FOR HEARING AIDS

This invention relates to stabilization techniques in amplifying systems and more particularly to a novel method and apparatus for increasing linearity and reducing distortion in miniature type amplifying systems such as used in hearing aids.

BACKGROUND OF THE INVENTION

Present day miniature amplifying systems such as utilized in hearing aid devices incorporate a sub-miniature microphone usually acoustically coupled to a pick-up horn through a fine section of tubing. The electrical output from the microphone is amplified through an electronic circuit and the amplified signal in turn drives a receiver or speaker acoustically coupled to an earpiece through another fine section of tubing. In certain types of hearing aids all of the components are packaged in a relatively small enclosure adapted to fit in or adjacent to a person’s ear.

Typically, the microphone introduces distortion of the order of 5 percent. The distortion in the electronic amplifier or amplifiers may range between 1 percent and 10 percent depending upon the quality of the circuit and the amount of negative feedback utilized in the amplifier. Because of the relatively small size of the receiver or speaker, approximately 10 percent additional distortion is introduced.

From the foregoing, it will be evident that a substantial improvement in overall performance with respect to distortion as well as linearity would be highly desirable. Limitations are placed on the amount of electronic sophistication that can be introduced not only because of the desirability of maintaining a small package particularly in the case of a hearing aid but also from an economic standpoint.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The present invention contemplates a novel method and means for accomplishing negative feedback around the entire amplifier chain including the microphone and speaker for increasing linearity and minimizing distortion. This type of feedback control operates directly on the acoustical input into the microphone from the acoustical output by means of an acoustically communicating passage between the responsive diaphragms for the microphone and speaker. The concept of acoustical feedback only becomes feasible when the coupling between the diaphragms takes place over a distance a small fraction of the signal wave length at the highest frequency of the frequency range to be amplified. Such conditions prevail in miniature amplifying systems such as utilized in hearing aids.

In addition to the method step of providing an acoustically communicating passage between the responsive diaphragms of the microphone and speaker, the degree of acoustic coupling between the diaphragms can also be varied by, in effect, varying the physcial crosssection of the passage itself. This variation not only permits adjustment of the negative acoustic feedback loop but also serves as a volume control.

In an actual system including a microphone, amplifier and speaker, the electronic negative feedback loop around the amplifier has its response adjusted to compensate for the response of the microphone and speaker which responses are relatively poor, to thereby control the overall amplifier gain roll-off characteristics and permit stable closed loop operation with a substantial amount of the negative acoustic feedback taking place.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be had by now referring to the accompanying drawings in which:

FIG. 1 is a highly diagramatical showing of a miniature type amplifying system useful in explaining the basic principles of the present invention;

FIG. 2 is a partial cross-section of the manner in which the system of FIG. 1 might be packaged in a typical miniature type device such as a hearing aid; and,

FIG. 3 is a series of curves constituting mathematical representations of frequency responses useful in explaining the manner in which improved performance is obtained in accord with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 there is schematically depicted an enclosure 10 defining an interior microphone acoustic chamber 11 receiving an acoustic input through a horn 12. A microphone 13 transduces acoustic signals into electrical signals which pass through an amplifier 14 to a receiver or transducer 15 converting the electrical signals back into acoustic energy within a speaker acoustic chamber 16. These sound signals may be passed out through a horn 17. The chambers 11 and 16 have a common wall defined by a rigid partition 18, the other wall portion being defined by a microphone diaphragm 19 and speaker diaphragm 20 schematically shown as mechanically connected to the microphone 13 and receiver 15 respectively.

The partition 18 of the enclosure 10 includes an acoustically communicating passage 21 between the chambers or diaphragms 19 and 20, the length of the communicating passage or distance between the diaphragm being substantially less than the wave length of the highest frequency in a contemplated signal frequency range to be amplified.

Means are provided in the form of a threaded member 22 for attenuating the acoustic coupling through the passage 21 in a controlled manner. Essentially, the cross-sectional area of the passage is varied by threading the member 22.

The amplifier 14 itself includes an electronic feedback loop indicated by the block N adjusted to provide a frequency response characteristic which compensates for the combined response characteristics of the microphone 13 ‘and receiver 15 and their associated diaphragms 19 and 20. This compensation permits a stable closed loop operation of the amplifying system with a substantial amount of negative acoustic feedback taking place through the passage 21.

Referring to FIG. 2, there is illustrated a practical packaging for the various components described in FIG. 1. In FIG. 2 there is shown an enclosure 23 dimensioned and shaped to fit behind a person’s ear, although the shape can be such as to fit within the ear. As shown, the enclosure includes a microphone 24 and associated diaphragm communicating with an input acoustic passage 25. The output of the microphone passes into a housing 26 to a suitable amplifier circuit from which amplified signals are passed to a receiver 27. The hous

3,763,333 ing 26 may incorporate a battery or other equivalent power supply means. The receiver 27 and its associated diaphragm connect with an output acoustic passage 28 through fine tubing 29 to ear piece 30.

The portion of the enclosure 23 defining an acoustically communicating passage is shown at 31 extending transversely between the input and output acoustic passages 25 and 28 respectively. As in the discussion of FIG. 1, the length of the acoustically communicating passage from the speaker diaphragm, through passage 31 to the microphone diaphragm is a small fraction preferably less than a quarter wave length of the highest frequency in the contemplated frequency range of signals to be amplified.

Adjustment of the acoustic coupling through the passage 31 is achieved by a threaded member 32 extending partially into a side of the passage as shown. This member 32 is accessible from the exterior of the enclosure 23 so that the effective cross-section and thus acoustic coupling through the passage 31 can be adjusted.

While the theoretical principles involved in the acoustic feedback approach described above are relatively straight forward, realization of a practical system is not obvious or simple because of the mechanical and electromechanical constraints in the acoustic coupling passage and the electrical circuits and mechanical characteristics of the microphone, receiver and associated diaphragms.

As will be appreciated from the previous description, phase shift due to acoustic lag can be minimized by making the dimensions of the acoustically communicating passage very small in comparison with the wave length of the highest frequency in the range under consideration, that is, where the open loop gain is in excess of zero db. In the case of a hearing aid, the upper frequency limit is normally about five KHz. The wave length corresponding to this frequency is close to two and a half inches and in the case of miniature hearing aids, there is no problem in dimensioning the distance between the diaphragms through the passage to a small fraction of this wave length.

On the other hand, phase shift due to electromechanical resonances is critical because the frequency response characteristics of both the microphone and speaker usually roll-off at —12db/octave at the upper end of the design frequency range. This roll-off characteristic usually results from design trade offs aimed at maximizing microphone sensitivity and receiver efficiency, especially in sub-miniature designs. The combined roll-off of —24db/octave must be compensated to yield an overall roll-off for the system of —6db/octave which should extend well above the usable frequency range.

The range over which phase compensation must be accomplished depends on the desired feedback ratio which in turn depends on the desired ratio of inherent open loop distortion to closed loop distortion requirements.

By way of specific example such as for the microphone, amplifier, receiver and acoustical passage characteristics of the embodiment shown in FIG. 2, assume the following conditions:

1. L_a = 20db, where L is the combined microphone and speaker acoustic input to acoustic output loss.

2. D_o = 15 percent, where D„ is the maximum open loop distortion.

3. D_c = 1.5 percent, where D_c is the desired maximum closed loop distortion.

4. G_c = 30db, where G_r is the closed loop acoustic gain.

5. Range of frequency of signal F is from 0.3 KHz to 5 KHz.

Under the foregoing conditions, certain design parameters may be established as follows:

If G_o is the open loop gain and M is the feedback ratio, which should be approximately 20db for a ten to one reduction in distortion, then:

G_o = G_r = M and the electronic amplifier gain is: G_o + L_a = 70db.

A closed loop stabilized amplifier is thus required including the necessary shaping networks.

The open loop frequency response must be compensated for —6db/octave roll-off starting at 0.5 KHz.

Referring now to FIG. 3, there are diagramed the loop response characteristics. With reference to the bottom dashed curve A, there is represented, approximately, the combined average frequency response of the microphone and receiver.

The top dotted line curve B represents the necessary closed loop response of the electrical amplifier circuit after adjustment of the electronic negative feedback to compensate for the response of the microphone and receiver or speaker shown in dashed curve A.

The dash-dot curve C represents the resultant system open loop response; that is, the sum of the curves A and B. This frequency response characteristic is thus tailored to ensure stability.

The sold curve D represents the complete closed loop response for the entire amplifier system when the negative acoustic feedback is applied and it will be noted that the response is substantially flat from 0.030 KHz to 5 KHz.

The negative acoustic feedback technique thus reduces substantially distortion and erratic response variations normally contributed by the microphone and receiver and associated diaphragms. In addition, an increase in linearity is provided. With respect to improved linearity, it is to be understood that by increasing the linearity it is then possible to more easily maintain a desired amplitude contouring.

While the invention has been described with respect to hearing aids, it will be evident that the principles are applicable to any acoustic amplifier system wherein the microphone and receiver are physically disposed in close proximity such as might be the case in earphones used for high fidelity sound reproduction or in industrial applications where it is desired to add cordless amplification or response shaping to counteract ambient noise conditions or enhance sensitivity of hearing in particular frequency zones.