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.

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.

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.

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.