U.S. patent number 3,665,122 [Application Number 04/878,194] was granted by the patent office on 1972-05-23 for hearing aid construction utilizing a vented transducer compartment for reducing feedback.
This patent grant is currently assigned to Beltone Electronics Corporation. Invention is credited to Erwin M. Weiss.
United States Patent |
3,665,122 |
Weiss |
May 23, 1972 |
HEARING AID CONSTRUCTION UTILIZING A VENTED TRANSDUCER COMPARTMENT
FOR REDUCING FEEDBACK
Abstract
A hearing aid construction in which the rigid hearing aid case
is divided into at least two closely spaced compartments with a
microphone mounted in one compartment and a output transducer
mounted in the second compartment and further comprising means for
venting acoustical pressure waves within the second compartment to
substantially reduce the acoustical coupling between the output
transducer and the microphone positioned nearby within the rigid
case.
Inventors: |
Weiss; Erwin M. (Chicago,
IL) |
Assignee: |
Beltone Electronics Corporation
(N/A)
|
Family
ID: |
25371565 |
Appl.
No.: |
04/878,194 |
Filed: |
November 19, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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573917 |
Aug 22, 1966 |
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Current U.S.
Class: |
381/326;
381/327 |
Current CPC
Class: |
G02C
11/06 (20130101); H04R 25/552 (20130101); H04R
25/456 (20130101) |
Current International
Class: |
G02C
11/00 (20060101); G02C 11/06 (20060101); H04r
025/00 (); G02c 011/06 () |
Field of
Search: |
;179/17R,17BC,17S,17H |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Parent Case Text
This is a continuation-in-part of application, Ser. No. 573,917,
filed Aug. 22, 1966 and now abandoned, and relates generally to
arrangements for amplifying sound and, more particularly, to an
improved hearing aid construction.
Claims
What is claimed is:
1. In a bone conduction hearing aid of the type comprising a
compact rigid case having at least one internal partition for
dividing the case interior into at least two compartments, a
microphone mounted in a first compartment, a bone conduction
receiver mounted in a second compartment, said microphone and said
bone conduction receiver being positioned in close proximity to
each other within said case adjacent one of the ears of the wearer,
an electrical signal amplifier having an input connected to said
microphone and an output connected to said receiver, and venting
means for venting acoustical pressure waves within said second
compartment to substantially reduce acoustical coupling between
said bone conduction receiver and said microphone within said case,
said venting means comprising an elongated opening in said case
communicating at one end with the air outside the case and at the
other end with the interior of said second compartment.
Description
BACKGROUND OF THE INVENTION
With the advent of miniaturized electronic components, it has
become possible to construct extremely compact hearing aid units
which may be mounted within a single temple member of eyeglasses or
supported in a casing behind one ear. More recently, hearing aid
units which are sufficiently small to be worn within an ear have
been constructed.
Both the sound sensitive microphone and the output transducer
(termed the "receiver") are commonly mounted in close proximity
within the compact hearing aid housing. The likelihood of
undesirable acoustical and mechanical coupling between these two
transducers is greatly increased by their close proximity. As a
consequence, the amount of electronic amplification employed must
be reduced to prevent self-oscillation due to feedback. Mechanical
vibrations may be transmitted from the receiver to the microphone
by the vibration of the rigid hearing aid case. In addition, sonic
pressure waves may reach the microphone through the surrounding
air.
Acoustic feedback of the second type is conventionally reduced by
mounting the receiver within a compartment which is effectively
sealed off from the microphone when the hearing aid is in its
operative position.
Mechanical vibrations may be reduced by acoustically isolating each
transducer from the hearing aid case. A transducer suspension
system which may be used to isolate the transducer from the case is
described in my U.S. Pat. No. 3,048,668, issued Aug. 7, 1962. A
bone conduction receiver which utilizes torsional suspension to
reduce mechanical case vibration is described in my U.S. Pat. No.
3,019,304, issued Jan. 30, 1962. While these arrangements are
capable of substantially reducing acoustical and mechanical
coupling between the transducers, feedback remains as a principal
limitation on the amount of gain which may be employed in a hearing
aid.
It is accordingly a principal object of the present invention to
reduce the coupling between electroacoustical transducers close
proximity within in a hearing aid unit or the like.
It is a further and related object of the present invention to
permit the gain of a sound amplifying device to be increased
without causing self-oscillation.
It is a still further object of the present invention to provide a
highly stable hearing aid of simple and inexpensive
construction.
SUMMARY OF THE INVENTION
The present invention is based upon the recognition that a
transducer may be coupled to the case in which it is mounted by
virtue of acoustical pressure waves within the compartment which
retains the transducer. In accordance with the invention, this
acoustical coupling is reduced through the provision of openings in
the compartment for venting acoustical pressure waves and thereby
reducing their intensity.
The principles of the present invention are particularly useful
when applied to reduce feedback in a bone conduction hearing aid
since, when such an aid is in operation, the vibration of the
relatively large surface areas within the bone conduction receiver
creates substantial pressure waves within the receiver compartment.
These pressure waves tend to cause vibrations of the hearing aid
case and consequent coupling to the microphone. A profound
improvement may be obtained by providing openings in the receiver
compartment for venting these pressure variations. This novel
feature of venting the acoustical pressure waves in the hearing aid
receiver compartment has substantially reduced these pressure waves
and feedback to the microphone even when positioned relatively
close to the receiver and has permitted the use of much higher gain
amplifiers.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present
invention may be more clearly understood through a consideration of
the following detailed description. In the course of this
description, reference will frequently be made to the attached
drawings in which:
FIG. 1 is a perspective view of an eyeglass hearing aid embodying
the principles of the invention;
FIG. 2 is an enlarged, side elevational view showing the bone
conduction receiver compartment as constructed in accordance with
the invention;
FIG. 3 is an enlarged, end view of the bone conduction receiver
compartment shown in FIGS. 1 and 2;
FIG. 4 is a schematic diagram illustrating the manner in which the
feedback characteristics of a typical hearing aid may be measured;
and
FIG. 5 is a Nyquist plot of the characteristics of a hearing aid
both with and without the venting openings contemplated by the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a perspective view of a binaural, bone conduction,
eyeglass hearing aid. Each of the elongated temple members
indicated generally at 11 and 12 contains a complete bone
conduction hearing aid unit. In the temple member 12, for example,
the volume control is seen at 14 and the battery housing at 15. The
pickup microphone an the electronic amplifier are included within a
compartment behind the cover plate 17. The output transducer, or
"receiver," is mounted within a compartment at 20 such that the
bone conduction button 21 engages with the mastoid bone behind the
wearer's ear. As seen more clearly in the enlarged view of FIGS. 2
and 3, the end of stem 12 is provided with a comfort cushion 25
which surrounds the button 21. The cushion 25 is affixed to a cover
plate 27 which is in turn affixed by screws 29 and 30 to the rigid
temple member 12.
As seen most clearly in FIG. 3, the cover plate 27 is held in
spaced relation from the body of the compartment 20 by means of
spacing discs 33 and 34. A tapered slot 35 is thus formed in the
compartment 20 which holds a receiver indicated at 40 in FIG. 3.
The slot 35 extends around a major portion of the periphery of the
compartment 20. Receiver 40 imparts vibratile motion to the button
21 and preferably takes the form disclosed in my U.S. Pat. No.
3,019,304, issued Jan. 30, 1962. As seen in FIG. 2, the button 21
extends through an opening 41 which is substantially larger than
would be necessary to permit the passage of button 21 therethrough.
The opening 41 accordingly allows pressure waves within compartment
20 to pass into the comfort cushion 25 where they are at least
partially absorbed. Opening 41 and the elongated slot 35 vent the
internal pressure variations within compartment 20 to reduce the
acoustic coupling between transducer 40 and the temple member
12.
In prior art bone conduction and air conduction hearing aids,
extreme efforts have been made to seal off the receiver from the
surrounding air when the hearing aid is in its operative position.
In prior bone conduction transducers, a sealing ring which
surrounds the vibratile portion making contact with the wearer's
mastoid bone has been employed to prevent the passage of sound from
the receiver to the microphone. It has been found, however, that
the acoustical pressure vibrations within the compartment cause
significant coupling between the mechanically suspended receiver
and the hearing aid case. The resulting mechanical case vibrations
couple the receiver to the microphone. According to the invention,
improved results are obtained by providing venting openings through
the sidewalls of the receiver compartment.
The improvement obtainable through the application of the
principles of the invention is illustrated by FIGS. 4 and 5 of the
drawings. As shown in FIG. 4, a variable frequency oscillator 50 is
connected to the input of an amplifying state 51 in the hearing aid
circuit. The output of an earlier chain of stages 52 is temporarily
disconnected from stage 51 and connected instead to a signal
measuring device 54. The signal from oscillator 50 is amplified by
stage 51 and applied to energize a receiver 53. Because of
acoustical and mechanical feedback, schematically illustrated by
the feedback path 55, microphone 56 detects at least some
acoustical energy from receiver 53. The amplified signal from
microphone 56 is measured by the device 54. The ratio of this
measured value to the amplitude of the signal injected by
oscillator 50 is a complex number whose value changes with
frequency.
As shown in FIG. 5, these values may be plotted both with and
without the venting openings. As shown by the heavy line plotted in
FIG. 5, when the receiver compartment is unvented the measured
ratio crosses the 0.degree.radial at a value of approximately 1.5.
This means that, at a particular frequency, the signal which is fed
back is in phase with the injected signal and is approximately 1
1/2 times larger than the injected signal. The result would be
self-oscillation when the aid is put into operation. To prevent
such oscillation, the gain of the amplifier must be reduced until
the in-phase feedback signal is smaller than the injected
signal.
As illustrated by the lighter line plotted in FIG. 5, the inclusion
of venting slots in the receiver compartment significantly reduces
the magnitude of feedback energy reaching the microphone from the
receiver. In consequence, the stability of the hearing aid is
greatly increased and, if desired, the gain of the amplifiers
employed may be increased without causing oscillation.
Prior to the invention disclosed herein, it was not known or
appreciated that the hearing aid receiver was coupled to the
receiver compartment by the acoustical pressure waves within the
compartment, and that this coupling caused vibrations of the
hearing aid case which were coupled to the microphone so as to
cause feedback unless limited gain amplifiers were employed. This
problem has been overcome by the present invention through the
provision of openings in the receiver compartment for venting the
acoustical pressure waves - without necessarily venting the
receiver itself -- thereby reducing the intensity of such pressure
waves, thereby permitting the use of higher gain amplifiers.
It is to be understood that the embodiment of the invention which
has been described is merely illustrative of one application of the
principles of the invention. Numerous modifications may be made by
those skilled in the art without departing from the true spirit and
scope of the invention.
* * * * *