EP1309225B1 - Method for determining the feedback threshold in a hearing aid - Google Patents

Description

The present invention relates to a method for determining a feedback threshold in a hearing aid.

Hearing aids are electronic devices in which noises are picked up by a microphone, processed or amplified in a signal processing unit and delivered via a loudspeaker, also called a handset, into the ear canal of the hearing device wearer. The amplified or processed sounds, which originate from the listener, can in turn be picked up by the microphone, with which the process repeats itself. In other words, this is a closed circuit consisting of the hearing aid, its output signal and the input signal. The path of the sound energy is not limited to acoustic energy, but optionally also includes a mechanical transmission from the output to the input, so for example on the housing of the hearing aid (so-called structure-borne noise). Furthermore, it has been found that signal feedback can occur through a vent channel that actually provides pressure equalization between the inner ear of the hearing aid wearer and the environment, or via electrical paths in the hearing aid. It has been shown that of all these possible Components the acoustic signal feedback has the largest share.

The mentioned effects can lead to a whistling in hearing aids, which is very unpleasant for the hearing aid wearer and finally renders the hearing aid unusable during the occurrence of whistling. Although it is possible to keep the gain in the hearing aid so small that no rocking and thus no whistling, which is just generated by the signal feedback arise. But this limits the usability of the hearing aid, especially in those applications in which a large hearing loss is to be compensated, as it occurs in the hard of hearing, since it is precisely in such patients, a relatively large gain in the hearing must be set to a relevant Compensate for the hearing loss.

Therefore, in order for all gain settings, in particular the maximum allowable gain setting, to be fully utilized in a hearing aid, it is necessary to determine the feedback threshold, i. to know the maximum gain setting in a hearing aid, in which just no signal feedback occurs.

Methods are already known for determining the feedback threshold in a hearing aid. So is in US 6,134,329 A method is described by means of which the transfer function of the hearing aid is derived on the basis of measurements made with the hearing aid inserted into the auditory canal of a hearing aid wearer. The total transfer function is calculated with different gain values without opening the closed circuit. Here, so-called optimal Wiener filter models are used. The transfer function in the forward path and that in the reverse path are then calculated together. From the transfer function in the forward path, the possible unstable frequencies and the maximum gain settings in the hearing aid can then be determined. Furthermore, it is also indicated how the transfer function in the forward path and that in the reverse path can be calculated from measurements of the overall transfer function. For the measurements while an additional microphone is inserted into the ear canal of the hearing aid wearer, the introduction into the ear canal preferably takes place through the vent channel.

Finally, the article entitled "Evaluation of feedback-reduction algorithms for hearing aids" [ Greenberg Julie et al., The Journal of the Acoustic Society of America, American Institute of Physics for the Acoustical Society of America, NY, NY, US, LNKD-DOI: 10.1121 / 1.1316095, Vol. 108, No. 5, 1 November 2000 (2000-11-01), pages 2366-2376, XP012002015 ISSN: 0001-4966 ]. Therein, adaptive algorithms are disclosed with which a feedback in a hearing aid can be further reduced.

It is obvious that this known method requires the provision of great computing power so that the desired information can be calculated. In addition, in the variant based on in-situ measurements, an additional microphone is required, by which the acoustic as well as mechanical properties of the overall system are adversely affected, so that as a result of which errors inevitably arise in the further calculations for determining the feedback thresholds.

Furthermore, it will open US-6,128,392 referring to the use of a hearing aid with a compensation filter in the reverse path in the form of an FIR (Finite Impulse Response) filter. To be compensated acoustic and mechanical signal feedback, wherein for determining the filter coefficients of the compensation filter, a pulse is delivered to the output of the hearing aid. At the input of the hearing aid, the impulse response is measured and from this the values for the coefficients for the compensation filter are determined. This is an integrated signal feedback attenuation, which changes the total transfer function of the hearing aid in some undesirable ways by simultaneously attenuating signal components of the useful signal.

For the sake of completeness, reference is made to a method commonly used in practice for determining the feedback threshold. It consists in gradually increasing the gain in the hearing aid until signal feedback occurs. The respective value for the gain at which just no signal feedback occurs, thus corresponds to the feedback threshold. This simple method has the great disadvantage that the hearing device wearer is exposed to a high level of noise, namely every time signal feedback occurs. In addition, during the determination of the Feedback threshold, the hearing aid perform a high performance.

The present invention is therefore based on the object to provide a method which does not have the disadvantages mentioned above.

This object is achieved by the measures specified in claim 1. Advantageous embodiments of the invention are specified in further claims.

The present invention takes advantage of the fact that the gain in the forward path of a compressive system, such as a hearing loss hearing loss hearing aid, is in the closed circuit, i. in closed-loop operation, which is damped by the attenuation in the reverse path. By simply measuring the gain in the forward path of the hearing aid, the feedback threshold can thus be determined.

The importance of knowing the feedback threshold has already been mentioned in detail in the introduction. This is especially true if a hearing aid has no suitable feedback suppression. But even in the case of a suitable feedback suppression, knowing the feedback threshold is useful. So is through the present invention provides the ability to check the quality of the hearing aid, especially in in-ear devices, and / or the quality of the earpiece.

• der Vorwärtspfad muss zur Bestimmung der Rückkopplungsschwelle nicht aufgetrennt werden;
• am Mikrophoneingang des Hörgerätes wird kein Signal-zu-Rausch-Abstand benötigt; d.h. bei einem gegebenen maximalen Schalldruck P am Ohr und einem Umgebungsstörlärm S können Rückkopplungsschwellen V_KRIT bis zu einer Grösse von $${\mathrm{V}}_{\mathrm{KRIT}}=\mathrm{P}-\mathrm{S}$$
  
  bestimmt werden. Die bekannten Methoden benötigen am Mikrophon einen Signal-zu-Rausch-Abstand DS, so dass Rückkopplungsschwellen nur bis zu einer Grösse von $${\mathrm{V}}_{\mathrm{KRIT}}=\mathrm{P}-\left(\mathrm{S}+\mathrm{DS}\right)$$
  
  bestimmt werden können.
• bei einem gegebenen Umgebungslärm und bei gleichem Schalldruck am Ohr während der Bestimmung der Rückkopplungsschwelle kann somit eine höhere Verstärkung erreicht werden;
• das erfindungsgemässe Verfahren kann mit den bestehenden Signalverarbeitungsmöglichkeiten, welche bei modernen Hörgeräten eingesetzt werden, ohne oder mit geringem Mehraufwand realisiert werden.

The invention further has the following advantages:

• the forward path need not be separated to determine the feedback threshold;
• no signal-to-noise ratio is needed at the microphone input of the hearing aid; ie at a given maximum sound pressure P at the ear and an ambient _interference noise S, feedback _thresholds V _KRIT up to a magnitude of $${\mathrm{V}}_{\mathrm{KRIT}}=\mathrm{P}-\mathrm{S}$$
  
  be determined. The known methods require at the microphone a signal-to-noise ratio DS, so that feedback thresholds only up to a size of $${\mathrm{V}}_{\mathrm{KRIT}}=\mathrm{P}-\left(\mathrm{S}+\mathrm{DS}\right)$$
  
  can be determined.
• with a given ambient noise and at the same sound pressure at the ear during the determination of the feedback threshold, a higher gain can thus be achieved;
• The inventive method can be realized with the existing signal processing options that are used in modern hearing aids, with little or no additional effort.

In a further embodiment of the invention, it is provided to perform the gain measurement in the forward path in the steady state in different frequency bands, whereby the critical gain, i. the feedback threshold is determined in each of the frequency bands.

Fig. 1

ein Blockschaltdiagramm eines an sich bekannten Systems mit einem Vorwärts- und einem Rückwärtspfad,

Fig. 2

ein Blockschaltdiagramm eines Hörgerätes mit einem Rückwärtspfad, welcher stellvertretend für alle möglichen Signalrückkopplungen bei einem Hörgerät vorgesehen ist,

Fig. 3

einen Verstärkungsverlauf, bei dem die Verstärkung in Funktion eines Eingangspegels eines Hörgerätes in doppeltlogarithmischer Darstellung aufgetragen ist, und

Fig. 4

eine weitere Variante für einen Verstärkungsverlauf in der zu Fig. 3 analoger Darstellungen.

The invention will be explained in more detail with reference to drawings, for example. Showing:

Fig. 1

2 is a block diagram of a system known per se with a forward and a reverse path;

Fig. 2

1 is a block circuit diagram of a hearing aid with a reverse path, which is provided for all possible signal feedbacks in a hearing aid,

Fig. 3

a gain curve in which the gain is plotted as a function of an input level of a hearing aid in logarithmic representation, and

Fig. 4

another variant for a gain curve in the zu Fig. 3 analogous representations.

Fig. 1 shows a block diagram for a feedback system, as is well known. Denoted at 100 is a processing unit having a transfer function G and at 200 a feedback unit having a transfer function K. An input signal I is supplied to one of the two inputs of an adder unit 10 whose single output is fed to the processing unit 100. In the processing unit 100, an output signal O is generated, which, in addition to the fact that it is led to the outside, is fed via the feedback unit 200 to the second input of the adder unit 10.

With the terms of the transfer functions in the forward and backward paths G and K, respectively, the following total transfer function for the system is obtained Fig. 1 : $$\frac{Q}{I}=\frac{G}{1-K\cdot G}$$

Fig. 2 shows a block diagram of a hearing aid 1, consisting of the processing unit 100 with the transfer function G on the basis of the representation according to Fig. 1 , The processing unit 100 upstream or downstream of a microphone 20 and a speaker 30, which is also referred to as a handset in the hearing aid industry. The output signal of the hearing aid 1 or the handset 30 is in turn fed via a feedback unit 200, and in addition to the input signal I, the microphone 20. Accordingly, the microphone 20 is preceded by an adder 10, which has both the input signal I and the output signal of the feedback unit 200 as input signals.

It is expressly pointed out that the processing unit 100 represents the simplest structure of a hearing aid 1. In fact, any other functional units - such as analog / digital converters, monitoring units for the monitoring of supply voltage, digital / analog converter, etc. - may be provided without departing from the concept of the present invention.

The feedback unit 200 with the transfer function K is the actual equivalent circuit diagram for the effects mentioned at the outset, which can lead to signal feedback. In this regard, reference is made to what has already been explained and the general remarks in US 6,134,329 directed.

Apart from additional influences on the overall transfer function due to specific Transmission characteristics of the microphone 20 and the handset 30, corresponds to the overall transfer function of the block diagram according to Fig. 2 according to that Fig. 1 ,

Fig. 3 shows, in a schematic representation, a course for the amplification of a compressive system, as it is used in a hearing aid to compensate for a hearing loss. While the abscissa represents the level of the input signal I using a logarithmic scale and the unit decibel (dB), the ordinate plots the gain V, also using a logarithmic representation. The course of the gain as a function of the input signal level has a negative slope, which is just one of the characteristics of a compressive system.

Is the forward path a compressive system, as in Fig. 3 is for the gain curve in function of the input signal level is visible, and the gain V _A for an input signal I _{A is} greater than a supposed, ie still unknown feedback threshold, so the gain in the forward path to the attenuation in the reverse path sets. Thus, according to the present invention, by measuring the gain in the forward path, the feedback _threshold V _KRIT can be determined, since at this measured gain just no feedback will occur.

In a further embodiment of the invention, it is provided to set the slope of the gain _curve V in a first phase to -1, in order to ensure a quick settling on the feedback _threshold V _CRIT . In a chronologically second phase, a flatter slope - ie a slope which is less than -1 - is then selected for the gain _curve , whereby a higher accuracy for the feedback _threshold V _{KRIT is} obtained.

In a further embodiment of the invention, it is provided to subdivide the audible range of human hearing into frequency bands, in each of which a feedback _threshold V _{KRIT is determined} according to one of the methods described above. It is conceivable both the determination of feedback _thresholds V _KRIT in one or individual and in all frequency bands. In a preferred embodiment of the invention, the frequency bands used are the so-called characteristic frequency bands, which are predetermined by the structure of the human ear.

Based on Fig. 4 another embodiment of the invention will be explained. Shown is a gain curve V in the forward path of a hearing aid 1, wherein the same scale as in Fig. 3 has been used. The gain _curve V corresponds to that which is set after the determination of the feedback _threshold V _KRIT , where four ranges I, II, III and IV can be identified. _According to the invention, the gain _curve V in the hearing aid 1 is limited to the maximum gain V _KRIT by means of a limiting unit present in the hearing aid, with which signal feedback can be avoided.

Claims (7)

1. Method for determining a feedback threshold (V_KRIT) in a hearing device with an amplification course of a compressive system, the method being characterized in

   - that an input signal (I_A) is fed to the hearing device inserted to a hearing device user, which input signal (I_A) results in a higher amplification (V_A) than a supposed feedback threshold, and

   - that an amplification (V_KS) is measured in the forward path of the hearing device after termination of a transient process,

   the measured amplification (V_KS) being set equal to the feedback threshold (V_KRIT) .
2. Method according to claim 1, characterized in that an amplification course (V) is adjusted as a function of the level of the input signal (I) in the hearing device as follows:

   - a slope of -1 is chosen in a first phase for the amplification course (V) represented double-logarithmically, and

   - a slope less than -1 is chosen in a second phase for the amplification course (V) represented double-logarithmically.
3. Method according to claim 1 or 2, characterized in that the human hearing range is split into frequency bands, preferably in so-called characteristic frequency bands, a feedback threshold (V_KRIT) being determined in at least one of the frequency bands.
4. Method according to claim 3, characterized in that a feedback threshold (V_KRIT) is determined in each frequency band.
5. Method according to one of the proceeding claims, characterized in that the amplification (V) is limited in the hearing device on the basis of the feedback threshold (V_KRIT) or the feedback thresholds, respectively.
6. Method according to one of the claims 1 to 5, characterized in that a maximum amplification of the hearing device is adjusted in dependence on the determined feedback threshold (V_KRIT).
7. Method according to claim 6, characterized in that the maximum amplification will be set equal to the feedback threshold (V_KRIT).

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