DK1916872T3 - Differentially directional microphone system and hearing aid with such differential directional microphone system - Google Patents

Description

The invention relates to a differential directional microphone system for a hearing aid device, such as e.g. a hearing aid or an active noise protection apparatus, in which a lateral directional characteristic is produced by coupled differential directional microphones. Furthermore, the invention relates to a method for producing this lateral directional characteristic.

Modern hearing aids have audio processors that are tuned for computing power and energy efficiency. These offset hearing loss by means of a level and frequency dependent gain. Modern appliances also have powerful algorithms for reducing feedback and ambient noise. A particularly advantageous agent against localizable disturbance noise is adaptive directional microphone algorithms. Particularly powerful devices with overall classification systems can automatically detect important hearing situations and automatically select the best program for this. Thus, the carriers are offered an optimal hearing with at the same time high operating comfort.

However, directional microphones in hearing aids count towards the established methods for background noise reduction. Using differential directional microphones, language intelligibility can be proven to improve in hearing situations where the utility signal and the noise signal fall in from different spatial directions. The directional effect is produced by a differential processing of the output signals from two nearby microphones with circular characteristics. The signal processing of a first order differential directional microphone system consists essentially of the subtraction of the rear microphone signal, which is delayed a certain time, from the front microphone signal. This creates a direction-dependent sensitivity, the characteristics of which can be set by the delay time.

The strength of the directional effect is classified by a Directivity Index, which, in the case of a diffuse noise field and acoustic noise from the front direction, indicates the improvements in signal-to-noise ratio SNR (signal-to-noise ratio) relative to a radiant characteristic.

Particularly because of their resource-saving implementability in hearing aids, digital differential directional microphones using two single radiant microphones are very popular. They have the property that sound from a direction can be hidden. In this way, the preferred receiving direction is typically realized in the front (in the carrier's visual direction) so that signals from behind are attenuated. In many circumstances, it is desirable to set the preference direction aside. For example, when driving a car, it is meaningful to maximize the directional effect to the side, as the driver, after a conversation with the passenger, must look ahead, at the same time that directional microphone is desirable due to the background noise.

With existing hearing aids, directional microphones have so far been implemented without exception with a forward directional effect. This is because differential directional microphones only allow a so-called endfire device, that is, a maximum directional effect forward or backward. To achieve a lateral effect, a so-called beamformer is used, which as 'delay and sum' beamformers has a low directional effect with few microphones or also a so-called 'generalized sidelobe canceller' beamformer, which with a large filter length carries a high cost. Both aspects make a beamformer for the hearing aid unattractive.

In addition, second-order directional microphone systems are already known. Hereby the differential directional microphone principle is transferred to three microphones. This increases the directivity of the microphone system. The receiver direction of these known second order differential microphones is analogous to the receiver direction of a first order differential system also directed forward (in the carrier's line of sight). Such a second order directional microphone system is e.g. described in DE 103 10 779 B4, EP 1 465 453 A2 and DE 103 31 956 B3. In some digital hearing aids, adaptive directional microphone systems have also been implemented which, for the purpose of maximizing SNR gain, can adapt their directional characteristics in situations with directed noise cancellation continuously to the current noise field. Hereby, depending on the direction of the noise sound, the directionality of the microphone system is continuously changed from a dipole via a hyper-cardioid to a cardioid.

The object of the invention is to provide a hearing aid with a differential microphone system, thereby maximizing the directional effect. The object of the invention is further to provide a method by which the directional effect of a differential microphone system can be maximized. This is achieved by means of a differential directional microphone system according to claim 1. In addition, the object is achieved by a hearing aid according to claim 16. Further advantageous embodiments of the invention appear from the dependent claims.

According to the invention, a differential directional microphone system is provided for a hearing aid apparatus having a first directional microphone comprising a first differential directional microphone and with a second directional microphone comprising a further differential directional microphone, whereby the second directional microphone is engaged after the first directional microphone. Thereby, the directional effect of the first directional microphone is substantially opposite to the directional effect of the second directional microphone. The differential directional microphone system hereby has a directional characteristic, the directional effect of which is substantially perpendicular to an axis determined by the directional effects of the first and second directional steps. By means of the opposite oriented effect of the successively differentiated directional microphones, a directional characteristic of the side can be obtained in a particularly simple manner, each with a zero point in the front and rear directions. In an advantageous embodiment of the invention, the first directional microphone stage may comprise a second differential directional microphone, the directional effect of which corresponds substantially to the directional effect of the first differential directional microphone, whereby the first and second differential directional microphone output signals serve as input signals to the further differential directional microphone. With this arrangement, the signal proportions from the anterior and posterior directions are particularly effectively attenuated. By means of a targeted coupling of three and four circular microphones, respectively, by means of three differential directional microphone couplings, a directional effect can be obtained in Broadfire arrangement.

According to a further advantageous embodiment of the invention, the first directional microphone stage may comprise three microphones. The first differential directional microphone thereby has a first coupling block whose inputs are connected to the first and second microphones, while the second differential directional microphone comprises a second coupling block whose inputs are connected to the second and third microphones. The directional effect of the two differential directional microphones thus essentially corresponds to the directional effect of the first differential directional microphone. In this arrangement, the second microphone is used jointly by the first and second differential microphones. Since only three microphones are used, the corresponding differential directional microphone can be implemented in a particularly simple way. In a further advantageous embodiment of the invention, the second microphone may be located at the same distance from the first and third microphones. The location of the microphones at the same distance allows a particularly effective directional effect of the differential directional microphone from the side. In a further advantageous embodiment of the invention, the second microphone is substantially disposed on the axis determined by the position of the first and third microphones. Also, the placement of the microphones along the particular axis allows a particularly effective directional effect of the differential directional microphone from the side.

According to a further advantageous embodiment of the invention, the first coupling block may be arranged to derive the second microphone's microphone signal at a predetermined time, to subtract the second microphone's delayed microphone signal and the first microphone's microphone signal from one another, and to output the resulting signal as the output signal to the first differential directional microphone signal output. Also, a second coupling block is arranged to retard the third microphone's microphone signal with the predetermined time, subtract the third microphone's delayed microphone signal and the second microphone's microphone signal from each other, and output the resulting signal as an output signal to a signal output in the second differential directional microphone. . In addition, the additional differential directional microphone has a further coupling block with a first signal input to the first differential directional microphone output signal and a second signal input to the second differential directional microphone output signal. The additional coupling block is hereby arranged to retard the first differential directional microphone's output signal with the predetermined time and subtract the first differential directional microphone's delayed output signal as well as the second differential directional microphone's output signal from each other. Due to the corresponding delay times, this special structure allows to determine the directional effect of the differential directional microphone.

According to a further advantageous embodiment of the invention, the first and second and third coupling blocks can each comprise a delay element, wherein the delay elements are arranged to delay the corresponding signals with a time corresponding to the duration that an audio signal uses for a delay signal. distance corresponding to the distance between the first and second microphones and between the second and third microphones, respectively. The advantage of this is that, due to the specially determined delay time, the directional effects of the two directional microphone are exactly opposite. Since the resets of the two differential microphones are also oriented exactly the opposite, a particularly high directional effect can be obtained from the side.

In addition, according to a particularly advantageous embodiment of the invention, the first directional microphone stage may comprise four microphones, the first differential directional microphone comprising the first and second microphones as well as a first coupling block, and the second differential directional microphone comprising the third and fourth microphones and a second coupling block. . The directional effect of the second differential directional microphone thus substantially corresponds to the directional effect of the first differential directional microphone. The four-m microphone arrangement is an alternative embodiment of the three-microphone arrangement. It allows for greater variation in the geometry of the microphones. In a further advantageous embodiment, the four microphones may be disposed substantially along an axis whereby the distance between the first and second microphones substantially corresponds to the distance between the third and fourth microphones. The positioning of the microphones along an axis allows for better directional effect from the side.

According to a further advantageous embodiment of the invention, the first coupling block may be formed such that the second microphone's microphone signal is delayed by a first predetermined time, and the second microphone's delayed microphone signal and the first microphone's microphone signal are subtracted from one another. In addition, the second coupling block is arranged to retrieve the fourth microphone's microphone signal with the first predetermined time and to subtract the fourth microphone's delayed microphone signal and the third microphone's microphone signal from each other. Finally, the additional differential directional microphone has an additional coupling block for retarding the first differential directional microphone output with a second predetermined time and subtracting the first differential directional microphone output signal and the second differential directional microphone output signal from each other. This embodiment exhibits a very simple construction which can be realized particularly simply in an advantageous way. In a further advantageous embodiment of the invention, the first, second and third coupling blocks can each comprise a delay element, wherein the first and second delay elements are arranged to delay the corresponding signals with a time corresponding to the duration as an audio signal. uses for a distance corresponding to the distance between the first and second microphones and between the third and fourth microphones, respectively. Thereby, the second delay time corresponds to a run time, which an audio signal uses for a distance corresponding to a combination of the distance between the first and second microphones, respectively between the third and fourth microphones and the distance between the second and third microphones. . Advantageously, the delay time thus determined allows an optimal directional effect from the side of the differential directional microphone system.

Finally, in a further advantageous embodiment of the invention, the directional characteristics of the differential directional microphone system can be adapted adaptively. This advantageously allows for adaptation of the directional characteristic to different hearing situations. The invention is described below with reference to the drawing, in which: Figure 1 shows a second-order differential directional microphone system with three interconnected differential directional microphones; Figure 2 shows the internal structure of a switch block in a differential directional microphone; Figure 3 shows a second-order differential directional microphone system. Figure 4A and 4B show two variants of a second-order differential directional microphone system of the invention with four round-beam microphones; and Figure 5 shows a polar diagram for displaying the directional characteristics of a second-order differential microphone system of the invention.

Figure 1 shows a second-order differential directional microphone system, such as e.g. is already used today for noise suppression. The differential directional microphone system is two-stage and has three microphones M1, M2, M3, which are typically positioned along a straight line (microphone axis A). The first microphone stage I is thereby formed by two differential directional microphones 10, 20. Each of the two differential directional microphones 10, 20 each consists of two of the three input microphones M1, M2, M3 and a switch block I, II. In such a switch block, the signals of the two input microphones M1, M2, M3 are typically combined with each other and applied to the input of the respective differential directional microphone 10, 20. The output signals of the two differential directional microphones 10, 20 in the first microphone stage I form the two input signals to the differential directional microphone 30 of the second microphone stage II. After a processing in the differential directional microphone's switch block 30 of the second microphone stage II, in which the two input signals are combined in a typical manner, an output signal is provided for further processing at the output of the second microphone II. Such differential directional microphone systems are used to amplify the forward direction, i.e. in the hearing aid carrier's view and to hide the background noise from the side. With the help of the two directional microcontrollers, the directional effect of the first directional microcontroller is enhanced, so that ambient noise from the side is attenuated more strongly. The output of the second micro-stage II in the known second-order differential microphone system thus has no or only very small proportions from the lateral directions, ie from 90 ° - 270 ° direction respectively.

Figure 2 shows schematically the typical structure of a coupling block with such a differential directional microphone. Thereby, a first input signal located at the input of the respective switch block is first delayed at a predetermined time T by a delay element. The delayed signal is then subtracted by an adder from the second input signal. The combined signal is eventually output at the switch block signal output. Thus, in principle, the signal of the first microphone M1 can also be subtracted from the signal of the second microphone M2 retarded. The set delay time T hereby determines the direction from which the respective differential directional microphone preferably receives audio signals.

In order to obtain a side directional effect (Broadfire arrangement), the couplings of the differential microphone systems are formed in such a way that the directional effect of the two directional microphone I, II is oriented opposite. Thus, the first stage I hides sound from the rear direction, while the second stage II hides sound from the front direction. This results in a directional effect in Broadfire usage. The corresponding structure of such a second order directional microphone system is e.g. The three microphones M1, M2, M3 attached to the first directional microphone stage are preferably positioned exactly along the microphone axis A. The second microphone M2 is furthermore spaced apart from the first and third microphones M1, M3. This is illustrated in Figure 3 by corresponding mapping of the microphone distances d. The three microphones M1, M2, M3 output signals rm (t), rri2 (t), rri3 (t) serve as input signals to the two differential directional microphones 10, 20 in the first directional microphone stage I , whereby the second microphone M2 is associated with the first and second differential microphones 10, 20, respectively. In order to obtain a directional effect with a zero point at the rear, the delay time Ti in the first delay element 12 is chosen a time Two corresponding to the duration of a sound wave for the distance indicated by the microphone distance d. The first to the second microphone M1, M2 signals are then subsequently combined with an adder 13 with each other. Thereby the retained microphone signal rri2 (t-To) of the second microphone M2 is subtracted from the first microphone M1 microphone signal m-i (t). Also at the second differential directional microphone 20, as delay time T2 in the corresponding delay element 22, time is selected to obtain a directional effect with a posterior zero. Subsequently, the third microphone's delayed microphone signal rri3 (t-To) is subtracted by an adder 23 from the second microphone's microphone signal rri2 (t). Since the two differential directional microphones 10, 20 in the first microphone stage I have a zero point in the posterior direction and a forward direction effect, their cardioid cone overlaps.

The output signals vi (t), V2 (t) from the two differential directional microphones in the first microphone stage generate two input signals for the differential directional microphone 30 in the second microphone stage II. In order to obtain a desired directional effect, here, analogous to the two differential directional microphones 10, 20 of one of the input signals, one of the input signals is obtained by a suitable delay element 32 with a predetermined delay time T3, and the signals are subsequently combined with each other by an adder 33. Hereby, the first differential directional microphone 10 output signal vi (t) is delayed with time To, and the second differential directional microphone 20's output signal V2 (t) is subtracted subsequently from the first differential directional microphone 10's delayed output signal vi (t-To). In this way, the differential directional microphone 30 of the second microphone stage II, which has a cardioid directional characteristic, achieves a zero in the posterior direction.

This also follows from an analysis of the network. For the output signal as the differential directional microphone system:

For rear signals:

For front signals, the same applies:

When To = d / c is selected as delay time (microphone distance d, sound speed c), for the proportions of the output signals of the differential microphone system from the front and rear direction:

Thus, since the two micro-stages I, II each have zeroes in the opposite direction, the output of the differential microphone system has no proportions from the front and the rear. Thus, by means of the combination of the two microfronts I, II, a directional effect is obtained.

However, in order to achieve the desired directional effect of the differential microphone system, it is not imperative that the second microphone M1 is positioned directly on the microphone axis A formed by the shortest connection between the first and third microphones M1, M2. On the other hand, what is crucial to the resultant directional effect of the differential directional microphone system is that the projections of the connecting axis between the first and second microphones M1, M2 and the distance between the second and third microphones M2, M3 are equal. Thus, e.g. also possible with a triangular position of the three microphones M1, M2, M3 to obtain a corresponding directional effect from the side, if they are individually at the predetermined axis A related distances d between the two microphone pairs M1, M2 and M2, M3 have the same value.

Figure 4A shows a further embodiment of a differential microphone system according to the invention. Hereby, the first microphone stage I comprises four radiant microphones M1, M2, M3, M4, which are preferably arranged along the microphone axis A. The first and second microphones M1, M2 as well as the third and fourth microphones M3, M4, each of which form a pair of microphones, each having a predetermined distance d from each other. Also, the distance d 'between the second and third microphones M2, M3 in Figure 4 corresponds to the regular microphone distance d. In any case, this distance d' can be varied as needed. In particular, in order to obtain the desired directional characteristic, the delay time T3 of the retaining element 32 in the additional differential directional microphone 30 must be adjusted.

This delay time is thereby set depending on the distance d 'between the second and third microphones M2, M3. The relationship between the distance d 'between the second and third microphones M2, M3 and the required delay time T3 of this delay element 32 can be determined as follows:

Since the distance d 'between the second and third microphones M2, M3 in the example shown in Figure 4 corresponds to the regular microphone distance d, the retention time T3 of the retaining element 32 for the further differentiated directional microphone 30 is chosen the double retention time Two to obtain an orthogonal oriented to the microphone axis A directional effect, each with a zero point in the anterior and posterior directions (Broadfire arrangement). If the distance d 'between the second and third microphones M2, M3 is reduced to zero, the position of the second microphone M2 along the microphone axis A covers the third position of the third microphone M3. In this case, instead of two separate microphones, a simple one can be used. Such an arrangement is then similar to the differential microphone system shown in Figure 3. Since the distance d 'between the second and third microphones M2, M3 is zero, the above equation for the second directional microphone II delay element 32 gives a delay time T3 of exactly Two.

However, the location of the two microphone pairs belonging to the first and second differential directional microphones 10, 20 may also differ. As shown in Figure 4B, the second differential microphone M2 of the first differential microphone 10 is then between the third and fourth microphone M3, M4 of the second differential microphone 20. Also in this case, the retention time T3 of the second directional microphone II can be determined by the context between the delay time and the microphone distance on which the above equation is based. In any case, it must be taken into account that the distance from the second to the third microphone M2, M3 now extends opposite the distance from the first to the second and from the third to the fourth microphone M3, M4, respectively. Second, the delay time T3 of directional microphone II in such an arrangement is as follows:

In the present case, since the distance d 'between the second and third microphones M2, M3 corresponds exactly to half of the regular microphone distance d, from the equation above, the value of the retention time T3 of the second directional microphone II is exactly 2/2. In other words, the first directional microphone's delay time T1, T2 is twice as long as the second directional microphone's II delay time.

Thus, the position of the 10.20 microphone pairs of the two differential directional microphones relative to one another can be varied arbitrarily along the microphone axis A. Using the shown relationships between the distances d, d 'and the delay times Τι, T2, T3 in the two microphone stages I, II, the differential directional microphone system coupling is individually adapted to provide the desired directional characteristic.

In the examples shown in Figures 3, 4A and 4B, the combination of the signals in the adder of the corresponding couplings can in principle also be reversed, so that e.g. by the coupling shown in Figure 3, the retarded output signal M2 (t-To) of the second microphone M2 is not subtracted from the first microphone M1 output signal m (t), but vice versa. In this case, the subtraction of the corresponding microphone signal rri3 (t-To), rri2 (t) in the second differential directional microphone 20 and the signals vi (t, V2 (t-To) in the further differential directional microphone 30 must also take place accordingly.

Figure 5 shows the directional characteristics of the differential microphone system according to the invention with an arrangement of three radiating microphones from Figure 3 as a polar diagram. The directional characteristic describes the sensitivity of the differential microphone system as the output level, depending on the angle of sound. Hereby, the leading direction of the axis A described by the microphone arrangement, that is, the hearing aid carrier's visual direction, is at 0 °. Accordingly, the rear direction is at 180 °. 90 ° and 270 ° respectively correspond to the left and right sides of the hearing aid. As can be seen from the polar chart plotted in a horizontal plane, the differential with the crophone system s zero points at 0 ° and at 180 °. By contrast, the maxima are in the 90 ° and 270 ° directions orthogonal to the forward axis. This is similar to the so-called Boardfire event.

All embodiments of the invention can be realized both by analog and digital. With a differential microphone system that works digitally, any analog microphone signals that may be present must first be digitized before they are further processed. The delay and subtraction of the signals can thereby be carried out by means of hardware and software. In principle, the distances d and d 'in this specification refer, respectively, to a stretch along the microphone axis A. If the microphones M1, M2, M3, M4, especially the second microphone M2, in the 3 microphone arrangement or the second or third microphone M2 respectively. , M3 in the 4 microphone arrangement, does not exactly lie on the microphone axis, it is meant by the microphone distance d or d 'preferably the projection of the connection distance between the respective microphones on the microphone axis A.

Claims (15)

A second order differential directional microphone system for a hearing aid comprising: - a first directional microphone stage (I) with a first and second differential directional microphone (10, 20), the directional effect of the second differential directional microphone (20) substantially corresponding to the first differential directional effect of directional microphone (10), and - a second directional microphone stage (II) coupled after the first directional microphone stage (I), whereby the first and second differential directional microphone (10, 20) output signals (vi (t), V2 (t ) serves as input signals to the second directional microcontroller (II), whereby the first microcontroller (I) has a directional characteristic with a zero point in a first direction, characterized in that the second microphone stage (II) has a directional characteristic with a zero point in a second one. direction which is oriented opposite to the first direction. Differential directional microphone system according to claim 1, characterized in that the first directional microphone stage (I) comprises three microphones (M1, M2, M3), wherein the first differential directional microphone (10) comprises a first coupling block (11) whose inputs are connected to the first and second microphones (M1, M2), whereby the second differential directional microphone (20) has a second coupling block (21) whose inputs are connected to the second and third microphones (M2, M3), thereby providing the second differential directional effect of directional microphone (20) substantially corresponds to the directional effect of first differential directional microphone (10). Differential directional microphone system according to claim 2, characterized in that the second microphone (M2) is arranged at the same distance from the first and third microphones (M1, M3). Differential directional microphone system according to one of claims 1 to 3, characterized in that the second microphone (M2) is arranged substantially on the axis (A) determined by the first and third microphones (M1, M3). ) position. Differential directional microphone system according to one of claims 1 to 4, characterized in that the first coupling block (11) is arranged to delay the second microphone (M2) microphone signal (rri2 (t)) with a predetermined time (Two), subtracting the second microphone (M2) delayed microphone signal (rri2 (t-To)) as well as the first microphone (M1) microphone signal (mi (t)) from each other and delivering the resulting signal as the output signal (vi (t)) at a signal output belonging to the first differential directional microphone (10), the second coupling block (21) being arranged to retrieve the third microphone (M3) microphone signal (rri3 (t)) with the predetermined time (Two), to subtract the third microphone (M3) distorted microphone signal (rri3 (t-To)) as well as the other microphone (M2) microphone signal (rri2 (t)) from each other and to output the resulting signal as output signal (V2 (t)) to a signal output of the second differential directional microphone (20), and that the additional differential e directional microphone (30) comprises a further coupling block (31) having a first signal input to the output signal (vi (t)) of the first differential directional microphone (10) and a second signal input to the second differential microphone (20) output signal (V2 (t)) , wherein the additional coupling block (33) is arranged to retard the output signal (vi (t)) of the first differential directional microphone (10) with the predetermined time (To) and subtract the delayed output signal (vi (t-To)) and the second differential directional microphone (20) output signal (V2 (t)) apart. Differential directional microphone system according to claim 5, characterized in that the first, second and third coupling blocks (11, 21, 31) each comprise a delay element (12, 22, 32), the delay elements (12, 22, 32). ) is arranged to retard the corresponding signals (rri2 (t), rri3 (t), vi (t) with a time (Two) corresponding to the duration that an audio signal uses for a distance corresponding to the distance ( d) between the first and second microphones (M1, M2) and between the second and third microphones (M2, M3) respectively. Differential directional microphone system with a differential microphone system according to claim 1, characterized in that the first directional microphone system (I) comprises four microphones (M1, M2, M3, M4), the first differential directional microphone (10) comprising the first and second microphones. (M1, M2) as well as a first coupling block (11), wherein the second differential directional microphone (20) comprises the third and fourth microphones (M3, M4) and a second coupling block (21), whereby the second differential directional microphone (20) directional effect essentially corresponds to the directional effect of the first differential directional microphone. Differential directional microphone system according to claim 7, characterized in that the four microphones (M1, M2, M3, M4) are arranged substantially along an axis (A), whereby the distance (d) between the first and second microphones (M1). , M2) substantially corresponds to the distance (d) between the third and fourth microphones (M3, M4). Differential directional microphone system according to claim 7 or 8, characterized in that the first coupling block (11) is adapted to delay the second microphone (M2) microphone signal (rri2 (t)) with a first predetermined time (To), and subtracting the second microphone's delayed microphone signal (rri2 (t-To)) and the first microphone (M1) microphone signal (rrn (t)) from each other, the second coupling block (21) being arranged to delay the fourth microphone (M4) microphone signal (rri4 (t)) with the first predetermined time (Two), and subtracting the fourth microphone (M4) delayed microphone signal (rri4 (t-To)) and the third microphone (M3) microphone signal (rri3 (t)) ), and that the additional differential directional microphone (30) has an additional coupling block (31) for retarding the output signal (vi (t)) of the first differential directional microphone (10) with a second predetermined time (T3), and that subtract the delayed output signal of the first differential directional microphone (10) (vi (t-T3)) as well as the output signal (V2 (t)) of the second differential directional microphone (20). Differential directional microphone system according to claim 9, characterized in that the first, second and third coupling blocks (11, 21, 31) each comprise a delay element (12, 22, 32), wherein the first and second delay elements (12, 22) is arranged to retard the corresponding signal (rri2 (t), rri3 (t)) with a time (Two) corresponding to the duration used by an audio signal for a distance corresponding to the distance between the first and the second microphone (M1, M2) between the third and fourth microphones (M3, M4), respectively, whereby the second delay time (T3) corresponds to the duration used by an audio signal for a distance corresponding to a combination of the distance ( d) between the first and second microphones (M1, M2), respectively, the third and fourth microphones (M3, M4) and the distance (d ') between the second and third microphones (M2, M3). Differential directional microphone system according to claim 10, characterized in that the second microphone (M2) is arranged between the first and third microphones (M1, M3) and the third microphone (M3) is arranged between the second and fourth microphones ( M2, M4), whereby the relationship between the first and second delay times (Two, T3) is determined by the following equation: Differential directional microphone system according to claim 10, characterized in that the second microphone (M2) is arranged between the third and fourth microphones (M3, M4) and the third microphone (M3) between the first and second microphones (M1, M2), whereby the relationship between the first and the second delay time (Two, T3) is determined by the equation: Differential directional microphone system according to one of the preceding claims, characterized in that the directional characteristics of the differential directional microphone system can be adapted adaptively. Differential directional microphone system according to one of claims 2 to 13, characterized in that the microphones (M1, M2, M3, M4) are formed substantially radially. A hearing aid with a second-order differential directional microphone system according to any one of the preceding claims.