Hearing Aid With Feedback Compensation

The present invention relates to a hearing aid with feedback compensation to prevent "howling" or similar oscillatory phenomena, said hearing aid being of the kind set forth in the preamble of claim 1.

A hearing aid of this kind is described in DK patent application No. 1479/88, filed on Mar. 18, 1988, and in the corresponding U.S. application Ser. No. 322,387, filed on Mar. 13, 1989.

The hearing aid described as an exemplary embodiment in the applications referred to above is a so-called in-the-ear hearing aid, that comprises a vent canal communicating the part of the ear's external meatus situated internally of the hearing aid capsule with the ambient air. In such a hearing aid, the predominant sound-transmission path from the transducer to the primary microphone receiving the ambient sound to be amplified will normally comprise the vent canal, all other paths, such as more or less temporary gaps between the hearing-aid capsule and the wall of the meatus having a considerably greater attenuation than said path comprising the vent canal and hence also having a correspondingly smaller ability to produce unwanted feedback.

In hearing aids without such a vent canal, the predominance of one single sound-transmission path may be less marked; in fact, there may exist a number of possible paths, in which the predominance, i.e. the minimum attenuation, shifts from one to the other depending on the actual physical relations in the external meatus, being influenced by the user's jaw movements, such as in yawning, chewing or speaking and other conditions, such as the orientation in space of the user's head.

From the above it will be understood that with hearing aids thus having several possible predominant sound-transmission paths between the transducer and the primary microphone, the use of a single secondary signal path effecting feedback compensation will be insufficient to prevent feedback from occurring, when either of said several possible paths is made active, such as by the user yawning.

It is the object of the present invention to provide a hearing aid of the kind initially referred to, in which the risk of unwanted feedback causing "howling" or the like is eliminated or at least considerably reduced, and this object is achieved in a hearing aid additionally exhibiting the feature set forth in the characterizing clause of claim 1. This makes it possible to provide feedback compensation for a number of possible more or less temporary sound-transmission paths capable of producing unwanted feedback, i.e. feedback causing "howling" or the like.

Advantageous embodiments of the hearing aid according to the present invention, the effects of which are explained in the following detailed portion of the present specification, are set forth in claims 2-11.

The present invention will now be explained in a more detailed manner with reference to the partly highly diagrammatic and simplified drawings, in which

FIG. 1 shows an example of a hearing aid employing feedback-compensating principles described in the above patent applications,

FIG. 2 is a block diagram of the electrical circuit in the hearing aid of FIG. 1,

FIG. 3 shows an exemplary embodiment of a hearing aid employing the principles of the present invention.

FIG. 4 shows another exemplary embodiment of a hearing aid employing the principles of the present invention, and

FIG. 5 shows an example of how the feedback compensation can be achieved by acoustical means.

FIGS. 1 and 2 illustrate the basic concept of feedback compensation as utilized in the hearing aid described in the above patent applications.

FIG. 1 shows a hearing aid 1 of the type adapted to be worn in the user's external auditory meatus, indicated by the surrounding tissue. The principles of the present invention may, however, be applied with equal effect to other types of hearing aids, in which there is a risk of acoustic feedback causing "howling" from the transducer supplying sound vibrations to the user's auditory sense organs to the microphone receiving the ambient sound to be amplified. For this reason, it will be obvious that the scope of the present invention, as defined in the claims, is not limited to the type of hearing aid described here in order to explain and exemplify the present invention, this hearing aid commonly being called an "in-the-ear" hearing aid or for short an ITE hearing aid. Thus, to mention but one example, the present invention may also be applied to "behind-the-ear" hearing aids, for short BTE hearing aids.

The hearing aid 1 comprises a housing or capsule consisting mainly of an outer panel 2 and a shaped portion 9. This housing or capsule contains

a primary microphone 3 adapted to receive ambient sound, symbolized by the black arrows, through a primary duct 3a, said primary microphone's output signal being delivered as a primary input signal to

an electronic signal processor 5, the output signal of which is delivered to

an electro-acoustic transducer or receiver 6, connected to the part 7 of the user's external auditory meatus proximal of the hearing aid through an output duct 8, and

a secondary or feedback-suppressing microphone 4, adapted to receive sound through a secondary duct 4a, said sound mainly originating from said proximal part 7 and being transmitted through the surrounding tissue as indicated by the white arrows or through passages (not shown) between the shaped portion 9 and the wall of the auditory meatus, the output signal of said secondary microphone being fed to a secondary input on said electronic signal processor 5.

As may be seen from FIG. 2, the output signal from the primary microphone 3 is supplied to the negative input of a difference amplifier 12, the output of which is connected to the receiver 6. The output signal from the secondary microphone 4 is fed to an attenuator 10 and a delay unit 11 before arriving in an attenuated and delayed condition at the positive input on said difference amplifier 12. Both the attenuator 10 and the delay unit 11 are adjustable, and by suitably adjusting these units, that part of the output signal from the primary microphone 3 due to this microphone receiving sound transmitted along the paths indicated from the output duct 8 may be counteracted by a signal of the same magnitude. This adjustment mainly entails adjusting the attenuation in the attenuator 10 and the delay in the delay unit 11 to correspond to the attenuation and delay suffered by the sound from the output duct 8 when passing through the distance c from the secondary duct 4a to the primary duct 3a, if necessary taking account of the length of the ducts.

If the secondary sound transmission path symbolized by the white arrows and extending past the secondary duct 4a always is the only one or the predominant secondary sound path, through which sound may reach the primary microphone 1 from the output duct 8, then the arrangement shown in FIGS. 1 and 2 will function satisfactorily to suppress acoustic feedback of a magnitude capable of producing "howling". It may well happen, however, that the user, particularly by moving his/her jaw in such activities as chewing, yawning or merely speaking, may change the shape of the external auditory meatus to such an extent, that other secondary paths arise in various positions around the housing or capsule 2, 9, even in the form of more or less open channels or ducts, and these other secondary paths may well have lower values of attenuation than the path shown in FIG. 1.

It will be obvious that under such circumstances, unwanted feedback may easily arise through the other secondary paths, and it will hence be necessary to make arrangements for suppressing also secondary signals transmitted by such paths. An example of such an arrangement according to the present invention will now be described with reference to FIG. 3.

For the sake of good order it should be mentioned that FIGS. 3 and 4 show some of the components outside of the hearing-aid housing, whereas they in actual practice will be found inside the housing, together with the necessary batteries, switches etc.

The hearing aid 21 can be imagined as physically resembling the hearing aid 1 of FIG. 1. It also comprises a primary microphone 23 feeding into the negative input of a difference amplifier 32, but instead of having one single secondary microphone as in the embodiment of FIG. 1, it has three secondary microphones 24a, 24b and 24c, each feeding into a delay unit 31a, 31b and 31c, respectively, and an attenuator 30a, 30b and 30c, respectively, the outputs of the attenuators being individually connected to three separate inputs of an operational control unit 33, the output of which is connected to the positive input of the difference amplifier 32. The latter feeds the receiver 26 with its output duct 28 in the same manner as described above with reference to FIGS. 1 and 2.

The operational control unit 33 contains electronic circuitry and/or so-called logical elements, arranged in such a manner that the output of that one of the attenuators 30a-30c at any moment producing the strongest signal is connected to the positive input of the difference amplifier 32. This means, of course, that that one of the three possible feedback paths served by the three secondary microphones 24a-24c constituting the predominant feedback path is used to produce the counter-signal at the positive input of the difference amplifier 32. In this manner, whichever of the three possible--or at least contemplated--feedback paths carrying the greater risk of producing unwanted feedback capable of making the hearing aid "howl" will be brought into operation by the operational control unit 33.

A person skilled in the art of electronics and logical circuitry will know how to construct an operational control unit having the functions of the unit 33 described above, for which reason further detailed description is deemed unnecessary. It may, however, be mentioned that such a control unit should at least comprise means or functions capable of

measuring the output signals from all attenuators 30a-30c separately,

comparing such measurements, and

connecting the output of that one of said attenuators carrying the strongest signal to the positive input of the difference amplifier 32.

The exemplary embodiment shown in FIG. 3 comprises three secondary microphones 24a-24c in the form of "normal" acoustic-electrical transducers, possibly comprising a small housing containing a diaphraghm, a sensor, such as a moving-coil arrangement, an electret element or a piezo-electric crystal. It will be obvious that the use of a great number of such secondary microphones in a small hearing aid especially one of the "in-the-ear" type, may cause problems due to lack of space or difficulties in making the microphones sufficiently small. FIG. 4 shows an example of an approach to solving this problem.

The hearing aid 41 shown in FIG. 4 comprises a primary microphone 43 feeding into the negative input of a difference amplifier 52, as well as a plurality of secondary microphones 44a, 44b, 44c . . . 44x, each feeding into a delay unit 51a, 51b, 51c ..... 51x and an attenuator 50a, 50b, 50c . . . 50x, respectively, the outputs of the attenuators being individually connected to a corresponding plurality of separate inputs of an operational control unit 53, the output of which is connected to the positive input of the difference amplifier 52. The latter feeds the receiver 46 with its output duct 48 in the same manner as in the embodiment of FIG. 3.

The operational control unit 53 functions in the same manner as the operational control unit 33 in the embodiment of FIG. 3, for which reason further description or explanation should be unnecessary. The important difference between the embodiment of FIG. 4 and that of FIG. 3 is that in the latter, the secondary microphones 24a-24c are "discrete components", i.e. separate components fitted into the hearing aid after being manufactured singly, whereas in the embodiment of FIG. 4, the secondary microphones 44a, 44b . . . 44x are elements integral with, more or less embedded in or adhered to the hearing-aid housing. Any type of element capable of producing an output signal (voltage) as a result of being influenced by sound waves in the surrounding medium may be used. Such elements could constitute piezo-electric elements or so-called electrets, i.e. permanently electrically stressed dielectric elements, or any other type of element suitable for the purpose. It will be obvious that the use of substantially "two-dimensional" elements of the type referred to as the secondary microphones 44a, 44b . . . 44x will result in a considerable saving of space, thus making it possible to equip the hearing aid with secondary microphones in a number sufficiently great to take into account practically all possible acoustic feedback paths, which may arise during the user's various activities as described above.

FIG. 5 shows a hearing aid 61, in which the feedback compensation means are purely acoustical in nature. This hearing aid comprises a differential microphone 62, the front chamber of which is adapted to receive ambient sound (symbolized by the black arrows) through a primary duct 63, the rear chamber being adapted to receive sound from the receiver 66 and output duct 68 through a secondary duct 64. The output of the differential microphone (or sound-gradient microphone) 62 is connected to the input of an amplifier 65 feeding the receiver 66 producing sound waves in the output duct 68.

In order that the sound transmitted from the output duct 68 (as symbolized by the white arrows) to the primary duct 63 may be cancelled, the effective length of the secondary duct 64 should equal the effective length of the sound-transmission path from the entrance of the secondary duct 64 past the hearing-aid housing and through the primary duct 63 into the front chamber of the differential microphone 62. For this purpose it may be necessary to introduce bends or convolutions in the secondary duct 64 to make it sufficiently long. Also, the strength of the secondary signal entering the rear chamber of the differential microphone 62 should be adjusted, such as by an acoustical attenuator of any suitable type. An example of how this may be achieved is described in the above-mentioned patent applications.

For simplicity and ease of understanding, the embodiment of FIG. 5 is shown as having only one set of feedback-compensating means, consisting of the secondary duct 64 and its associated attenuator (not shown). In order to carry out the principles of the present invention, there should be at least two such sets of feedback-compensating means, each having a secondary duct having its entrance placed in a position different from that of the others, and having its length and attenuation adjusted for optimum compensation in the case that the feedback path it serves is made active, such a by the user chewing or yawning.

It also lies within the scope of the present invention to combine features from the various embodiments shown, e.g. by using one or two sets of acoustic feedback-compensating means as described with reference to FIG. 5, combined with a number of those partly electrical sets of feedback-compensating means shown in FIG. 3 or 4.