MICROPHONE ASSEMBLY WITH SUPPRESSED FREQUENCY RESPONSE

Abstract

The present invention relates to a microphone assembly comprising a microphone unit for converting incoming acoustical sound to an electrical signal, and a rear volume comprising acoustically connected rear volume compartments, said acoustically connected rear volume compartments setting an effective acoustical impedance of said rear volume in order to reduce the sensitivity of the microphone assembly with respect to a resonance peak. The present invention further relates to a hearing device comprising a microphone assembly.

Claims

1. A microphone assembly comprising a microphone unit for converting incoming acoustical sound to an electrical signal, and a rear volume comprising acoustically connected rear volume compartments, said acoustically connected rear volume compartments setting an effective acoustical impedance of said rear volume in order to reduce the sensitivity of the microphone assembly with respect to a resonance peak.

2. A microphone assembly according to claim 1, wherein the acoustically connected rear volume compartments form, in combination, a substantially closed rear volume.

3. A microphone assembly according to claim 1, wherein the effective acoustical impedance of the rear volume is adapted to reduce the sensitivity of the microphone assembly in a frequency range including the resonance peak.

4. A microphone assembly according to claim 3, wherein the rear volume comprises a first and a second rear volume compartment being acoustically connected via an acoustical filter.

5. A microphone assembly according to claim 4, wherein the acoustical filter comprises a band-stop filter.

6. A microphone assembly according to claim 4, wherein the acoustical filter comprises a notch filter.

7. A microphone assembly according to claim 4, further comprising one or more additional rear volume compartments, said one or more additional rear volume compartments being acoustically connected to the first and/or the second rear volume compartment via one or more acoustical filters.

8. A microphone assembly according to claim 7, wherein the acoustical filter comprises a band-stop filter.

9. A microphone assembly according to claim 7, wherein the acoustical filter comprises a notch filter.

10. A microphone assembly according to claim 4, wherein a number of the rear volume compartments are separated by a substantially rigid separation member having the acoustical filter arranged therein or attached thereto.

11. A microphone assembly according to claim 10, wherein the acoustical filter comprises a number of through-going openings, such as tube-shaped through-going openings, in the substantially rigid separation member.

12. A microphone assembly according to claim 10, wherein the acoustical filter comprises a discrete acoustical filter attached to the substantially rigid separation member.

13. A microphone assembly according to claim 10, wherein the discrete acoustical filter comprises a porous material.

14. A microphone assembly according to claim 10, wherein the discrete acoustical filter comprises a flexible membrane.

15. A microphone assembly according to claim 10, wherein the discrete acoustical filter comprises a passive MEMS structure.

16. A microphone assembly according to claim 1, wherein the microphone unit comprises a MEMS microphone.

17. A microphone assembly according to claim 1, wherein the microphone unit comprises an electret microphone.

18. A microphone assembly according to claim 1, further comprising an amplifier for amplifying the electrical signal from the microphone unit, and a front volume being acoustically connected to an acoustical sound inlet for receiving incoming acoustical sound.

19. A microphone assembly according to claim 1, wherein the microphone assembly comprises a plurality of microphone units, and wherein a substantially closed rear volume comprising acoustically connected rear volume compartments is associated with each microphone unit.

20. A hearing device comprising a microphone assembly according to claim 1, said hearing device comprising a hearing aid being selected from the group consisting of: behind-the-ear, in-the-ear, in-the-canal and completely-in-the-canal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will now be explained in further details with reference to the accompanying figures, wherein

[0028] FIG. 1 shows a prior art frequency response,

[0029] FIG. 2 shows a MEMS microphone assembly having a rear volume with two compartments and an associated lumped element model,

[0030] FIG. 3 shows frequency responses with different alpha's,

[0031] FIG. 4 shows the open noise levels associated with the frequency responses of FIG. 3 for different alpha's in case of a second order band-stop filter,

[0032] FIG. 5 shows measured and simulated response curves in case of a second order band-stop filter,

[0033] FIG. 6 shows various rear volume configurations,

[0034] FIG. 7 shows frequency and noise response curves for different band-stop filters, and

[0035] FIG. 8 shows various filter implementations.

[0036] While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] In its broadest aspect the present invention relates to a microphone assembly with the capability of suppressing the microphone assembly response at or around a resonance peak, while leaving the frequency response at frequencies outside a filter range essentially unaffected. The present invention is also applicable in relation to other applications, including the suppression of a microphone response at certain ultrasonic frequencies, or the suppression of unwanted resonances.

[0038] Generally, the suppression of the frequency response at or around the resonance peak is provided by introducing an acoustical filter in a substantially closed rear volume of the microphone assembly. The acoustical filter is specific to a single frequency (notch filter) or to a specific frequency band (band-stop filter). The order of the acoustical filter can be changed to alter frequency specificity. Increasing the order of the acoustical filter sharpens the filter transitions and hence increases the filter specificity.

[0039] The acoustical filter is implemented by placing a structure inside the rear volume of microphone assembly such that the effective acoustic impedance of the rear volume is changed to the required filter impedance. The effective acoustical impedance of the microphone assembly changes in a manner so that it acts as a rejection filter to the volume flow that passes through a sensing element (microphone) of the microphone assembly. The volume flow is only allowed to reach the rear volume via the sensing element (microphone). The reduced volume flow effectively reduces the sensitivity of the microphone.

[0040] Thus, according to the present invention the impedance of the substantially closed microphone rear volume is changed such that it acts as a rejection filter to the volume flow (q.sub.v) that passes through the sensing element 209, cf. FIG. 2. The filter is implemented by dividing the substantially closed microphone rear volume into two or more rear volume compartments, cf. rear volume compartments 204, 205 in FIG. 2a, and placing a filter structure with a specific acoustic impedance (.sub.za,filter) between the compartments. The main function of the structure is to add an acoustic mass and (optionally) an acoustic resistance (damping) in between the compartments, such that the ensemble of the filter structure and the acoustic compliances of the rear volumes function as a rejection filter to q.sub.v.

[0041] FIG. 2a illustrates the above-mentioned principle for a MEMS microphone 200. In FIG. 2a the filter 206 is implemented by a perforated plate, which divides the rear volume with compliance C.sub.a,rv, into two rear volume compartments with cavities 204, 205 with compliances C.sub.a,rv1 and C.sub.a,rv2. The filter structure 206 is created by a number of perforated holes, which create a flow path with between the two compliances. The MEMS microphone further comprises a substrate 201 having a sound inlet 207, a housing 202, a MEMS sensing element 208, 209 and an ASIC 210. As addressed above incoming sound is only allowed to reach the two rear volume compartments 204, 205 via the MEMS sensing element 208, 209.

[0042] FIG. 2b shows a lumped element model of the modified rear volume only, assuming no change in the rest of the microphone system. The model also shows that the initial volume flow (q.sub.v) is divided into two flows; one (q.sub.v,1) that flows directly to the first compartment 204 with compliance C.sub.a,rv1 and one (q.sub.v,2) that flows through the filter structure to the second compartment 205 with compliance C.sub.a,rv2.

[0043] The following relationships apply between the compliances of the total rear volume, Ca,rv, the first compartment 204, C.sub.a,rv1, and the second compartment 205, C.sub.a,rv2:

C.sub.a,rv1=C.sub.a,rv

C.sub.a,rv2=(1)C.sub.a,rv

C.sub.a,rv1+C.sub.a,rv2=C.sub.a,rv

01

[0044] An important design parameter for the acoustical filter is the ratio, a, between the volume of the first compartment 204 and the volume of the initial rear volume. This ratio can be between 0, i.e. the second compartment 205 (in this case only the second) have a sum of volumes equal to the initial rear volume and 1, i.e. the first compartment 204 has the same volume as the initial rear volume. In general, a smaller alpha allows for a larger flow q.sub.v2, which results in a stronger filter with higher rejection factor. However, as a smaller alpha also increases the (unwanted) self-noise of the microphone, there exist a (application specific) trade-off between filter efficiency and added noise. The effect of different alpha's on the peak damping and noise performance in case of a second order band-stop are shown in FIGS. 3 and 4.

[0045] The acoustic mass of the filter structure Z.sub.a,filter is chosen such that the filter resonance is at the required frequency. To do this the following relationship is used:

[00001]$f_0=\frac{1}{2.Math..Math.\sqrt{M_{a,\mathrm{filter}}.Math.C_{a,\mathrm{rv}}\left(1-\right).Math.}}$

[0046] As such, the acoustic mass M.sub.a,filter depends on the chosen value for alpha, the given compliance of the original microphone rear volume C.sub.a,rv and the selected frequency f.sub.0. When alpha and the acoustic mass are set, the sharpness and rejection factor of the filter (Q) is further controlled by selecting the appropriate value of the acoustic resistance R.sub.a,filter according to:

[00002]$Q=\frac{1}{R_{a,\mathrm{filter}}.Math.\sqrt{\left(1-\right)}}.Math.\sqrt{\frac{M_{a,\mathrm{filter}}}{C_{a,\mathrm{rv}}}}$

[0047] When R.sub.a,filter is chosen 0, the Q.sub.n goes to infinity, and the filter will act as a notch filter that only works at f.sub.0. Any other value for R.sub.a,filter will dampen the notch and will lower the sharpness Q.sub.n of the filter. Consequently, the filter then acts as a band-stop filter to a frequency range centered at f.sub.0.

[0048] In general, microphones for hearing aids applications can have a peak resonance between 5 kHz and 40 kHz, where electrets microphones generally have their resonance below 10 kHz and MEMS microphones generally have a peak between 10 kHz and 40 kHz. This difference between ECM and MEMS is mostly explained by the difference in acoustic compliance of the sensor diaphragm and the overall acoustic mass, i.e. the diaphragm acoustic compliance of current MEMS designs is 3 to 5 times lower than the diaphragm compliance of ECMs.

[0049] Because the rear volume compliances of existing ECM and MEMS microphones are more or less in the same order of magnitude, the value for the filter acoustic mass will typically be smaller for MEMS microphones than for ECMs. For example, when setting alpha at 0.8, a MEMS microphone with f.sub.0 at 20 kHz and rear volume of 6 mm.sup.3 needs M.sub.a,filter to be about 9.510.sup.3 kg/m.sup.4. For a specific ECM with f.sub.0 at 6 kHz and also 6 mm.sup.3 rear volume, M.sub.a,filter now needs to be 104.710.sup.3 kg/m.sup.4. This is an order of magnitude larger than for the MEMS.

[0050] FIG. 5 shows the measured and simulated results of actual built demonstrators. It concerns two MEMS microphones with identical buildup. However, one of the MEMS microphones does not have a filter structure (reference microphone), while the other MEMS microphone has a perforated plate with thickness of 80 um dividing the rear volume into a first compartment of 4 mm.sup.3 and a second compartment of 1 mm.sup.3. As such, the design parameter alpha was set at 0.8. The filter structure has 5 holes of radius 45 um. From these dimensions and using known theory, the acoustic mass and resistance of the filter holes were calculated as 1.210.sup.4 kg/m.sup.4 and 5.410.sup.8 Pa.Math.s/m.sup.4, respectively. Simulations were found to match the measurements results when the values for the acoustic mass and resistance were set to 1.310.sup.4 kg/m.sup.4 and 5.610.sup.8 Pa.Math.s/m.sup.4, respectively. This result indicates a good theoretical understanding of the principle underlying the present invention and demonstrates the practical feasibility of the present invention.

[0051] As indicated in FIG. 6 the rear volume compartments may be arranged in different ways. In FIG. 6a the filters 604, 605 are coupled in parallel when separating rear volume compartments 601, 602 and 603. The parallel coupled filters 604, 605 are denoted 607, 606, respectively, in the associated lumped element model. The capacitors indicate the respective compliances of the rear volume compartments 601, 602 and 603. In FIG. 6b the filters 611, 612 are coupled in series when separating rear volume compartments 608, 609 and 610. The series coupled filters 611, 612 are denoted 613, 614, respectively, in the associated lumped element model. Again, the capacitors indicate the respective compliances of the rear volume compartments 608, 609 and 610. In FIG. 6c the filters 618, 619, 620 are coupled in both series and parallel when separating rear volume compartments 615, 616 and 617. The series and parallel coupled filters 618, 619, 620 are denoted 622, 623, 621, respectively, in the associated lumped element model, and the capacitors indicate the respective compliances of the rear volume compartments 615, 616 and 617. It should be noted that the three rear volume compartments may be arranged differently compared to the illustrations given in FIG. 6. For example, the two small rear volume compartments do not need to be adjacent to each other. In FIGS. 6a-c only a single microphone unit is depicted. It should be noted however that a plurality of microphone units could be applied. In case of a plurality of microphone units a rear volume comprising acoustically connected rear volume compartments is associated with each microphone unit. It should be noted that the rear volumes of FIGS. 6a, 6b and 6c are substantially closed rear volumes although they are all divided into rear volume compartments.

[0052] FIG. 7 shows simulation results of the effect of using a higher order filter on the frequency response and noise performance in case of parallel coupled rear volume compartments. Clearly, using the 4th order filter (dashed lines) gives a flatter frequency response (FIG. 7a) and less added noise (FIG. 7b), compared to conventional damping (dotted lines) and a 2.sup.nd order filter (solid lines).

[0053] FIG. 8 shows ways of implementing the acoustical filter within a microphone assembly. In FIG. 8 a MEMS package has been used for illustration purposes. However, the principles are applicable to ECM packages as well.

[0054] In FIG. 8a the acoustical filter 803 is formed by a perforated plate. Thus, the filter impedance is implemented by introducing a plate of a certain thickness that splits the rear volume into two compartments 801, 802. The two compartments 801, 802 have a total combined volume preferably being equal to the initial rear volume. The inserted plate has a number of through-going openings. The openings may be perforated, lasered, cut, molded, pressed or otherwise machined. The shape, size and number of the openings are chosen such that, in combination with the plate thickness and the acoustic impedances of the compartments, the ensemble of acoustic impedances of the openings creates the required acoustic impedance of the filter. The applied plate should be rigid and it may be of any suitable material.

[0055] In FIG. 8b tube-shaped openings are applied. The filter impedance is thus implemented by having two compartments 804, 805 that are acoustically connected via one or more tubes 806. As seen in FIG. 8b the tubes define conduits, having a certain wall thickness, that protrude into the compartments beyond the thickness of the separation structure. The number, length, cross-sectional area and cross-sectional shape of the tubes are chosen such that, in combination with the acoustic impedances of the cavities, the ensemble of acoustic impedances of the tubes creates the required acoustic impedance of the filter. The tube wall should be rigid and can be of any material.

[0056] In FIG. 8c the filter is implemented as one or more flexible membranes 809. Again, tilter impedance is implemented by having two compartments 807, 808 that are acoustically connected via one or more flexible membranes. The size, thickness and material of the membrane(s) are chosen such that, in combination with the acoustic impedances of the cavities, the acoustic impedance of the membrane creates the required acoustic impedance of the filter. The membrane(s) implementation should include a vent opening 825 that allows for barometric pressure relief between the sub-compartments and the pressure outside the microphone. The vent opening should not change the filter function.

[0057] In FIG. 8d a semiconductor process-based device 812 is applied. The impedance that connects the sub-compartments 810, 811 can be realized by means of a semiconductor device, such as a MEMS. The semiconductor device 812 can be a passive device with a fixed and technology inherent highly accurate impedance.

[0058] In FIG. 8e a path 815 of a through-housing connection is applied. The filter impedance is implemented by having two rear volume compartments 813, 814 that are acoustically connected via path or paths 815 on the exterior of the microphone system. The acoustic path or paths 815 are connected to the sub-compartments 813, 814 via a through-housing connection, which can again be a perforation or a tube. The number, length, cross-sectional area and cross-sectional shape of the path or paths 815 and through-housing connections are chosen such that, in combination with the acoustic impedances of the cavities, the ensemble of acoustic impedances creates the required acoustic impedance of the filter.

[0059] In FIG. 8f a filter structure 818 on a support is provided. The filter impedance is implemented by either concepts a), b) or c), but the filter structure is realized in a separate assembly. The separate assembly is then placed on a support inside the rear volume of the microphone whereby compartments 816, 817 are provided. This closely relates to concept d), but the assembly build extends also to non-semiconductor based processes.

[0060] In FIG. 8g a porous material 821 is applied between the two compartments 819, 820. This concept is similar to concept f) with the difference that the filter structure now consists of a porous material that has a certain acoustic impedance.

[0061] In FIG. 8h an application aided configuration is provided. The filter impedance is implemented by either concepts a) to g), but now at least one of the sub-compartments 822, 823 is created inside the system of the application but external to the microphone assembly itself.

[0062] In FIGS. 8a-h only a single microphone unit is depicted. It should be noted however that a plurality of microphone units could be applied. In case of a plurality of microphone units a substantially closed rear volume comprising acoustically connected rear volume compartments is associated with each microphone unit.

[0063] It should be noted that the rear volumes of FIGS. 8a-8g are all substantially closed rear volumes although they are divided into various arrangements of rear volume compartments. In FIG. 8h the rear volume compartment 822 is provided external to the microphone assembly itself. However, despite this design variant incoming sound is only allowed to reach the rear volume compartments 822, 823 via the MEMS sensing element. The rear volume compartments 822, 823 thus constitute, in combination, a substantially closed rear volume.

[0064] As a general consideration the location of the filter structure on the structure that separates the rear volume compartments is arbitrary. The locations of filter sub-structures on the structure that separates the rear volume compartments is also arbitrary. The total size of the filter structure depends on the required filter function and is in the limit constrained by the system dimensions. Implementation principles can be combined to achieve the required filter impedance.