MEMS microphone

1. A MEMS microphone comprising:

a) a case with a front side open to the exterior, the case comprising an interior volume, and a closed side;

b) a MEMS membrane mounted on one face of a base, the base being mounted inside the case on said closed side; and

c) a mesh covering the open front side, said front side covered by said mesh being substantially transparent acoustically to at least some of a range of operating frequencies at which the microphone is sensitive, the mesh being conductive and grounded;

wherein:

said range comprises ultrasound frequencies and defines a highest operating wavelength; and

said mesh includes perforations with a largest opening being less than half of said highest operating wavelength across.

2. A microphone according to claim 1 , also including an amplifier mounted in the case.

3. A microphone according to claim 1 , wherein there is a first frequency greater than at least one member of the group comprising 25 kHz, 40 KHz, 70 KHz, and 100 KHz, at which the microphone has a sensitivity of more than −60 dB, with 0 dB defined as 1 volt per pascal.

4. A microphone according to claim 3 , wherein the sensitivity at the first frequency is one member of the group comprising: greater than −50 dB, greater than −40 dB, greater than −30 dB, no lower than 40 dB below the sensitivity of the microphone at 20 kHz, no lower than 30 dB below the sensitivity of the microphone at 20 kHz, no lower than 30 dB below the sensitivity of the microphone at 20 kHz, is no lower than 20 dB below the sensitivity of the microphone at 20 kHz, no lower than 40 dB below an average of the sensitivity in decibels of the frequencies between 20 kHz and the first frequency, and no lower than 40 dB below a maximum of the sensitivity of the frequencies between 20 kHz and the first frequency.

5. A microphone according to claim 3 , wherein the sensitivity varies by one member of the group comprising: no more than 6 dB over at least 10 kHz, somewhere between 20 kHz and the first frequency, no more than 6 dB over at least 20 kHz, somewhere between 20 kHz and the first frequency, and no more than 6 dB over at least 40 kHz, somewhere between 20 kHz and the first frequency.

6. A microphone according to claim 1 , wherein the directional sensitivity of the microphone varies by one member of the group comprising: no more than 30 dB, for directions in front of the mesh, for the range of operating frequencies for which the sensitivity is within 40 dB of the most sensitive frequency, above 20 kHz, and no more than 20 dB, for directions in front of the mesh, for said range of operating frequencies.

7. A microphone according to claim 1 , wherein the substantially solid side is a back side, opposite the front side, and the base is mounted with the MEMS membrane facing the front side.

8. A microphone according to claim 1 , wherein the mesh comprises at least one member of the group consisting of: a fill factor of less than 70%, substantially all the perforations therein having a diameter less than 0.6 mm, and the centers of the perforations therein being arranged substantially in a rhombic grid.

9. A microphone according to claim 1 , wherein, for at least one frequency in the range of operating frequencies, defined as the frequencies above 20 kHz which have sensitivity within 40 dB of the most sensitive frequency above 20 kHz, and one direction in front of the mesh, the sensitivity is reduced by less than 10 dB relative to the microphone if the mesh were removed, or wherein for all frequencies in the range of operating frequencies, for at least one direction in front of the mesh, the sensitivity is reduced by less than 10 dB relative to the microphone if the mesh were removed, or wherein, for at least one frequency in the range of operating frequencies, for all directions in front of the mesh, the sensitivity is reduced by less than 10 dB relative to the microphone if the mesh were removed, or wherein for all frequencies in the range of operating frequencies, for all directions in front of the mesh, the sensitivity is reduced by less than 10 dB relative to the microphone if the mesh were removed.

10. A microphone according to claim 1 , wherein the front side includes a solid area, for picking and placing, the solid area being at least one member of the group consisting of at least 1 mm square and having an area of filling of the case directly thereunder.

11. A microphone according to claim 1 , wherein the membrane is less than 1 mm behind the mesh.

12. A microphone according to claim 1 , wherein the case has a plane of substantial symmetry, and the center of the MEMS membrane is within a distance equal to 10% of the largest diameter of the case, of the plane of substantial symmetry, or wherein the case comprises a flat side that the base is mounted on, with the center of the MEMS membrane aligned with the center of the flat side to within 10% of the largest diameter of the case.

13. A microphone according to claim 1 , with largest dimension less than 7 mm.

14. The MEMS microphone of claim 1 wherein the case does not have an interior empty volume that includes a cube 2 mm on a side.

15. The MEMS microphone of claim 14 wherein the case does not have an interior empty volume that includes a cube 1.5 mm on a side.

16. The MEMS microphone of claim 14 wherein the case does not have an interior empty volume that includes a cube 1 mm on a side.