U.S. patent number 3,763,333 [Application Number 05/274,607] was granted by the patent office on 1973-10-02 for acoustic feedback stabilization system particularly suited for hearing aids.
This patent grant is currently assigned to Ambitex Company. Invention is credited to Abraham Lichowsky.
United States Patent |
3,763,333 |
Lichowsky |
October 2, 1973 |
ACOUSTIC FEEDBACK STABILIZATION SYSTEM PARTICULARLY SUITED FOR
HEARING AIDS
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.
Inventors: |
Lichowsky; Abraham (Los
Angeles, CA) |
Assignee: |
Ambitex Company (Santa Monica,
CA)
|
Family
ID: |
23048911 |
Appl.
No.: |
05/274,607 |
Filed: |
July 24, 1972 |
Current U.S.
Class: |
381/318; 381/121;
381/93 |
Current CPC
Class: |
H04R
25/453 (20130101); H04R 3/002 (20130101) |
Current International
Class: |
H04R
25/00 (20060101); H04R 3/00 (20060101); H04r
025/00 () |
Field of
Search: |
;179/107,1F,1FS |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Blakeslee; Ralph D.
Claims
What is claimed is:
1. A method of increasing linearity and reducing signal distortion
over a given frequency range in a miniature acoustic amplifying
system comprised of a microphone, amplifier and speaker, including
the steps of:
a. providing an acoustically communicating passage between the
responsive diaphragms for the microphone and speaker of length less
than one quarter of the signal wave length of the highest frequency
in said range; and
b. varying the degree of acoustic coupling between the diaphragms
to provide an adjustable negative acoustic feedback loop for said
amplifying system.
2. The method of claim 1, including the step of providing an
electronic negative feedback loop around said amplifier having its
response adjusted to compensate for the response of the microphone
and the speaker to thereby control the overall amplifier gain
roll-off characteristics and permit stable closed-loop operation
with a substantial amount of said negative acoustic feedback taking
place.
3. A miniature acoustic amplifying system having a microphone,
amplifier and speaker and including:
a. an enclosure for the microphone and speaker diaphragms, a
portion of said enclosure defining an acoustically communicating
passage between the diaphragms of length substantially less than
the wave length of the highest frequency in a contemplated signal
frequency range to be amplified to thereby provide acoustic
coupling between the microphone and speaker; and
b. means in said passage for attenuating the acoustic coupling
through said passage in a controlled manner whereby a negative
acoustic feedback loop is provided for said amplifying system to
increase linearity and reduce distortion.
4. A system according to claim 3, in which said amplifier includes
an electronic negative feedback loop adjusted to provide a
frequency response characteristic which compensates for the
response characteristics of the microphone and speaker to permit
stable closed loop operation with a substantial amount of said
negative acoustic feedback taking place.
5. A system according to claim 3, in which said enclosure is
dimensioned and shaped to fit a person's ear, and includes an input
acoustic passage communicating with said microphone and an output
acoustic passage communicating with said speaker, said acoustically
communicating passage extending between the input and output
passages; and in which said means in said passage for attenuating
the acoustic coupling includes a threaded member extending
partially into a side of the passage and accessible from the
exterior of said enclosure for threaded adjustment whereby the
effective cross-sectional area of the passage is adjustable by the
degree of threading of the member into the passage, so that a
hearing aid is provided in which the volume can be adjusted by said
member.
Description
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
cross-section 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 housing 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.sub.a = 20db, where L is the combined microphone and speaker
acoustic input to acoustic output loss.
2. D.sub.o = 15 percent, where D.sub.o is the maximum open loop
distortion.
3. D.sub.c = 1.5 percent, where D.sub.c is the desired maximum
closed loop distortion.
4. G.sub.c = 30db, where G.sub.c 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.sub.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.sub.o = G.sub.c = M and the electronic amplifier gain is:
G.sub.o + L.sub.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.
* * * * *