Hearing aid control circuit for suppressing background noise

Abstract

In a control circuit for a hearing aid, a threshold switch is connected to the low-frequency channel which transmits an input signal from a microphone to an earphone. This switch is arranged to block transmission of the signal below a set threshold level of the signal. The low-frequency signal in the channel controls the switch.

Description

The present invention relates to improvements in a control circuit connected between a microphone and an emitter of a hearing aid. The microphone receives an input signal and the circuit is arranged to transmit the input signal to an emitter, the circuit including a low-frequency channel comprising an initial amplifier connected to the microphone and a final amplifying transistor connected to the emitter.

Many hard-of-hearing persons, particularly those using hearing aids for the first time, complain about the annoying audibility of background noises. People of normal hearing capacity and with two healthy ears have the ability to "tune out" diffuse noises and to concentrate on the desired audible signal, i.e. speech or music. The hard-of-hearing person, however, who normally is provided with a hearing aid in one ear only, has lost this capacity to a large extent. As a result, many people using hearing aids subjectively receive background noise at a high and annoying level and, for this reason, persons who are hard of hearing and would benefit from the use of a hearing aid refuse it entirely.

It is the object of this invention to provide a hearing aid wherein undesired background noises are suppressed by normally blocking their transmission from the microphone to the emitter of the hearing aid. The signal transmission is opened only when the input signal has reached a predetermined threshold level significantly higher than the level of the usual background noises.

Threshold value switching means are well known in the radio art to provide noise attenuation, quiet tuning, squelch, etc. Obviously, however, all of these circuits use the high-frequency carrier of the received signal as controlling criterion for switching the low-frequency transmission path in and out. However, since there is no high-frequency carrier in a hearing aid circuit, the control signal must come from the low-frequency signal itself. Up to now, circuits of this type could not be used in hearing aids because of the required circuit components and the low voltages used in hearing aids.

In accordance with the invention, this difficulty has been overcome in the type of circuit described hereinabove by connecting a threshold switching means to the low-frequency channel and arranging it to block transmission of the input signal from the microphone to the emitter below a predetermined threshold level of the signal, the low-frequency signal in the low-frequency channel controlling the switching means.

The above and other objects, advantages and feature of the present invention will become more apparent from the following detailed description of a now preferred embodiment thereof, taken in conjunction with the accompanying drawing wherein

FIG. 1 is a graph illustrating the input-output characteristics of a desirable hearing aid, and

FIG. 2 is a circuit diagram illustrating the invention.

Referring now to FIG. 1, while there is a linear ratio between the input decible level p.sub.i and the output decible level p.sub.o in conventional hearing aids even when the input noise level is low (see broken line), the control circuit illustrated in FIG. 2 will suppress any output signal p'.sub.o if the input noise level is smaller than threshold level p'.sub.i. Once the threshold value has been reached, the sound amplification will increase linearly (see full line) until the output decible level has reached its saturation value at level p'.sub.o.

Referring now to the circuit diagram for a hearing aid illustrated in FIG. 2, there is shown microphone 1 connected to the base contact of output stage amplifying transistor T.sub.3, condenser C.sub.2, potentiometer 3 and amplifier 2 being interposed between transistor T.sub.3 and microphone 1. The collector of the transistor is connected to the positive terminal of a battery, which provides the voltage source for the hearing aid, earphone or emitter 4 being interposed between the transistor collector and the battery terminal, while the transistor emitter is directly connected to the negative terminal of the battery. Amplifier 2 and transistor T.sub.3 form a low-frequency channel.

The collector-emitter path of threshold switching transistor T.sub.2 is connected between the base contact of output stage transistor T.sub.3 and the positive terminal of the battery, resistance R.sub.6 being interposed between the base contact and the connector emitter path of transistor T.sub.2. Condenser C.sub.4 is connected in parallel to transistor T.sub.2.

The output of amplifier 2 is connected to the base contact of amplifying transistor T.sub.1, condenser C.sub.1 being interposed between the amplifier output and the transistor base contact. Potentiometer 5, with two slide contacts 7, 8, connects the collector of transistor T.sub.1 to the positive battery terminal while the emitter of transistor T.sub.1 is connected directly to the negative battery terminal.

The base contact of switching transistor T.sub.2 may be connected either to first slide contact 7 by a first pair of terminals of switch unit 6 or to second slide contact 8 by a second pair of terminals of the switch unit, condenser C.sub.3 being interposed between the base contact of transistor T.sub.2 and the switch unit. Furthermore, the base contact of transistor T.sub.2 may be connected to the negative battery terminal by a third pair of terminals of switch unit 6, resistance R.sub.2 being interposed between the switch unit and the negative battery terminal.

The base contact of amplifying transistor T.sub.1 is connected to the connecting point between resistances R.sub.3 and R.sub.4 which are connected in series between the collector of transistor T.sub.3 and the negative battery terminal, resistance R.sub.1 being interposed between the base contact of transistor T.sub.1 and the connecting point between resistances R.sub.3 and R.sub.4. Resistance R.sub.5 is connected between this connecting point and the base contact of switching transistor T.sub.2.

The above-described circuit operates in the following manner:

The low-frequency signal received by microphone 1 is amplified at 2, the loudness of the signal being individually adjustable by potentiometer 3, and the adjusted, amplified signal is transmitted through condenser C.sub.2 to the base contact of output stage transistor T.sub.3. At rest, when the input signal to the microphone is negligible, i.e. caused primarily or exclusively by ambient noises, the voltage transmitted to switching transistor T.sub.2 via voltage dividers R.sub.3, R.sub.4 and resistance r.sub.5 is such that the switching transistor is blocked. At the same time, the base contact of output stage transistor T.sub.3 also receives no appreciable voltage via resistance R.sub.6 and is also blocked. Therefore, earphone or emitter 4 produces no output signal. Point 9 of the circuit is approximately at full working voltage.

Amplifying transistor T.sub.1 as well as amplifier 2 are constantly in operation, independently of the level of the input signal to the microphone. The low-frequency signal at the base contact of transistor T.sub.1 is practically independent of the adjustment of potentiometer 3. The low-frequency signal coming from transistor T.sub.1 is transmitted through operating resistance 5 by the first pair of terminals of switch unit 6 and condenser C.sub.3 to the base contact of switching transistor T.sub.2. This signal is rectified in the base contact-emitter path of this transistor and opens the transistor as soon as the signal has reached a predetermined amplitude. A small input signal will not reach this amplitude.

When the input signal reaches a threshold level of loudness exceeding a predetermined decible value, a higher low-frequency voltage will be produced at the output of amplifier 2, this voltage being further amplified by transistor T.sub.1 and being sufficient to make switching transistor T.sub.2 partially conductive. This will cause a current to flow from transistor T.sub.2 through resistance R.sub.6 to the base contact of output stage transistor T.sub.3, which makes this transistor partially conductive, too. The resultant voltage drop at receiver 4 will make the voltage at circuit point 9 more negative, thus causing transistor T.sub.2 to be opened more widely and placing output stage transistor T.sub.3 into full operation. The connection of resistance R.sub.4 to circuit point 9 causes the transition from the blocked to the conductive condition of the transistors to arise like an avalanche, the circuit operating like a flip-flop circuit.

When the low-frequency signal falls below the threshold level, the recitified signal voltage at the base contact of switching transistor T.sub.2 will be reduced again. The direct current transmitted through resistance R.sub.5 cannot keep the transistor T.sub.2 open, the transistor becomes highly resistant again and the circuit returns or flips back into the blocked condition. Condenser C.sub.4 prevents the low-frequency signals to pass via transistors T.sub.1, T.sub.2 to transistor T.sub.3 and also somewhat delays the flip-flop action so that vey short error signals will not switch on the circuit.

The level of the input signal causing the flip-flop circuit to become conductive depends on the amplitude of the low-frequency signal at the base contact of switching transistor T.sub.2. Since switching unit 6 may be operated by the hard-of-hearing person, the operator can adjust the threshold of the circuit to the ambient acoustical conditions. In very noisy surroundings, the operator will set the threshold at a higher level since the operating noise (speech) will then be louder, too, the automatic reaction of a speaker being to adjust his voice to the surrounding noise so as to be intelligible.

When switching unit 6 is adjusted to the illustrated position to make the uppermost pair of terminals operative, operating resistance 5 will make only a small portion of the amplified signal voltage effective at switching transistor T.sub.2. In this position, the input signal must have a very high level to open transistor T.sub.2. On the other hand, the entire signal voltage will be transmitted by resistance 5 in the central position so that transistor T.sub.2 will become conductive even when the level of the input signal is relatively low. When the lowest pair of terminals of switching unit 6 is in operation, the voltage coming from resistance R.sub.2 will keep transistor T.sub.2 constantly open, i.e. the threshold switch is inoperative. In this position of the switching unit, the circuit operates with a conventional hearing aid with linear amplification.

The described circuit does not essentially differ in its requirement for components from other hearing aid circuits, for instance conventional hearing aids with automatic adjustments of noise level, and it may readily be built into hearing aids of conventional size.

While the threshold value switch has been illustrated in connection with the circuit of a conventional hearing aid, it could also be combined with other types of control circuit for hearing aids. A hearing aid with the control circuit of the present invention operates very economically since the output stage, which normally consumes about 60 to 75% of the entire current requirements, is switched on only when an input signal of a predetermined threshold value is present.

Claims

What is claimed is:

1. In a control circuit connected between a microphone and an emitter of a hearing aid, the microphone receiving an input signal and the circuit being arranged to transmit the input signal to the emitter, and the circuit including a low-frequency channel comprising a signal connected to the microphone and an output stage amplifying transistor connected to the emitter, the improvement of a threshold switching means connected to the low-frequency channel and arranged to block transmission of the input signal from the microphone to the emitter below a predetermined threshold level of the signal, the low-frequency signal in the low-frequency channel controlling the switching means.

2. In the hearing aid control circuit of claim 1, means for adjusting the threshold level of the signal.

3. In the hearing aid control circuit of claim 1, the low-frequency channel comprising at least one transistor and the threshold switching means being connected to the transistor.

4. In the hearing aid control circuit of claim 3, the transistor being the amplifying transistor connected to the emitter of the hearing aid.

5. In the hearing aid control circuit of claim 1, the threshold switching means comprising a switching transistor having a collector-emitter path, and the low-frequency channel comprising at least one transistor having a base contact, the collector-emitter path being connected to the base contact.

6. In the hearing aid control circuit of claim 5, the transistor in the low-frequency channel being the amplifying transistor.

7. In the hearing aid control circuit of claim 5, the switching transistor and the low-frequency channel transistor connected thereto constituting a bistable flip-flop stage.

8. In the hearing aid control circuit of claim 5, the amplifier having an output, a further amplifying transistor having an input and an output, the output of the amplifier transmitting alternating circuit to the input of the further amplifying transistor, and the output of the further amplifying transistor being connected to the switching transistor.

9. In the hearing aid control circuit of claim 8, a decible adjusting means connected between the amplifier and the further amplifying transistor, the high point of the decible adjusting means transmitting alternating current to the input of the further amplifying transistor.

10. In the hearing aid control circuit of claim 8, a resistance connected between the further amplifying transistor and the switching transistor for transmitting a control voltage to the switching transistor, and means for adjusting the control voltage whereby the threshold value may be adjusted.