microphone

Abstract

To solve the numerous sound quality challenges encountered when using MEMS microphones utilizing microelectromechanical systems (hereinafter referred to as MEMS) based on semiconductor manufacturing technology widely used in smartphones and other devices, which are compact and possess the advantages of being durable and highly reliable due to the use of semiconductor manufacturing technology, as high-fidelity recording microphones, due to their size and structure.SOLUTION: By using a plurality of MEMS microphones in parallel, suppressing the steep peaks in the high-frequency range inherent to MEMS microphones, minimizing the effects of phase differences caused by a plurality of units, driving the frequency response compensation circuit with low power, and preventing degradation in sound quality due to diffraction effects, a microphone for high-quality audio capture is realized.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a microphone suitable for picking up high-quality sound used in high-resolution audio and the like. [Background technology]

[0002] The extremely small microphones used in smartphones and other devices are not assembled as a standalone microphone unit like conventional microphones, but are integrated with associated circuits using a technology known as MEMS (microelectromechanical systems).

[0003] The general structure of a MEMS microphone is an extremely small microphone that combines a MEMS chip, which is an acoustic sensor, with an IC chip that processes signals. The MEMS chip forms a capacitor-type microphone unit, with a tiny diaphragm and back plate placed face-to-face between the sound hole for sound pickup and the back chamber. The change in capacitance caused by the diaphragm's fluctuation due to sound pressure is extracted as an electrical signal, and the circuit part is integrated into a package structure that outputs the signal through an integrated bias supply circuit and amplifier circuit.

[0004] FIG. 2 is a cross-sectional view illustrating the main body structure of a typical MEMS microphone 1. In the figure, sound hole 3 in substrate 2 is connected to the outside and serves as the entrance for sound. Diaphragm 4, a vibrating electrode plate, and backplate 5, a fixed electrode plate, are located midway through sound hole 3, separated by a small gap. A cavity that serves as back chamber 6 is formed behind diaphragm 4. Diaphragm 4 and backplate 5 act like parallel-plate capacitors; when sound pressure causes diaphragm 4 to vibrate, the gap length between diaphragm 4 and backplate 5 changes, resulting in a change in capacitance. This change is then output as an electrical signal. On the other hand, mounted on the board is a semiconductor chip 7 commonly known as an ASIC, which is for specific applications and integrates a boost circuit that supplies bias voltage to the back plate 5 and a preamplifier that amplifies the signal representing changes in sound pressure, and these are sealed together in a cap 8 to form an integrated structure. Reference numeral 9 denotes a solder pattern for attaching the MEMS microphone 1 to the board.

[0005] MEMS microphones are made into chips by applying semiconductor manufacturing technology, forming microstructures on silicon wafers using photolithography, etching, etc. They can be made small, measuring just a few millimeters, so they are often used in devices that require compactness, such as smartphones, earphone microphones for hands-free calls, and headphones with noise-canceling functions.

[0006] In this way, MEMS microphones can be made much smaller than conventional microphones. Moreover, because the manufacturing process uses semiconductor manufacturing technology, there are no parts that require fine adjustment skills, and they have the advantage of being durable and less prone to malfunction.

[0007] MEMS microphones have many advantages and are suitable for use in devices that require miniaturization. However, when attempting to use them as high-quality microphones for recording music, such as high-resolution audio, they have several problems, as described below.

[0008] High-resolution audio is a standard that achieves high-resolution sound quality that exceeds the sampling parameters of conventional CDs and DATs. It is usually a standard of 44.1kHz to 48kHz, or a quantization bit rate equivalent to 16 bits or more. In the case of recording microphones, a high-frequency characteristic of 40kHz or higher is required.

[0009] MEMS microphones have a small sound pickup area and a very small diaphragm diameter of about 1 mm, which makes it difficult to achieve a good signal-to-noise ratio.

[0010] With normal microphones, using multiple microphone units in parallel causes problems with phase differences between the units, so they are rarely used for high-quality sound applications.In contrast, with normal diaphragm microphones, the S / N ratio can be improved by increasing the diameter, so this is usually the method used. Although multiple units are used in microphones that use the MS method to record stereo with a single microphone, this is an assembly for recording two stereo channels, and does not involve using multiple microphone units with a single microphone to improve the signal-to-noise ratio.

[0011] Next, due to their structure, MEMS microphones have the problem that they can resonate strongly in the high frequency range, causing a peak in the frequency response, making it difficult to achieve a flat response.

[0012] With a normal diaphragm microphone, the peak that occurs in the high frequencies can be damped by narrowing the gap between the electrodes, taking advantage of the viscous resistance of the air, and this allows for adjustment to flatten the frequency response. However, with a MEMS microphone, it is difficult to adjust the gap in this way, so the peak ends up being sharp.

[0013] Furthermore, when using circuit technology to improve sound quality, using sophisticated and complex compensation circuits increases power consumption. Phantom power, which uses an audio cable to supply power to a microphone, is common, but this is limited to a maximum of 10 mA. Therefore, some ingenuity is required to ensure operation within this range.

[0014] Furthermore, microphones for high-quality sound pickup must be designed with a shape that avoids diffraction effects. The diffraction effect occurs due to reflection, and sound reflection is caused by discontinuities in the shape, such as corners or straight bends. In other words, the principle is that diffraction is less likely to occur if the microphone housing is made of a continuous curved surface without corners. Therefore, the shape needs to follow this principle: the sound picked up by the microphone in the sound pickup section travels to the surrounding area, and if there are corners, it will return, but if there are no corners, it will not return.

[0015] The present invention realizes a MEMS microphone as a high-performance music recording microphone by utilizing these various improvements alone or in combination with each other. Summary of the Invention [Problem to be solved by the invention]

[0016] This invention makes use of the advantages of MEMS microphones, such as their small size and low failure rate, while resolving some of the problems involved in using them as high-quality microphones, thereby obtaining a microphone that can be used to record high-quality sound. [Means for solving the problem]

[0017] The first invention is to use multiple MEMS microphone units in parallel to solve the problem of a small sound collection area.

[0018] A second aspect of the present invention is to arrange two or more MEMS microphone units closely together so that the distance between the sound receiving openings of each MEMS microphone unit is minimized.

[0019] As mentioned above, using multiple microphone units in parallel, especially in microphones for high-quality sound, causes problems with phase differences between the units, so this type of configuration is not used.However, because MEMS microphone units are extremely small, if multiple units are placed closely together, the sound pickup ports can be installed close to each other.

[0020] Next, the third invention is the characteristic that the sharp peaks that occur in the high frequencies of MEMS microphones are difficult to control on the microphone unit side. With diaphragm microphones, the peaks that occur in the high frequencies can be damped by narrowing the gap between the electrodes, taking advantage of the viscous resistance of the air, and this is where the flattening of the frequency characteristics can be adjusted. However, with MEMS microphones, it is difficult to adjust this gap, so the peaks end up being sharp.

[0021] In this invention, this problem is corrected using an electrical circuit. Microphones have a phenomenon known as the proximity effect, which produces a lot of low-frequency sound. To prevent this, it is common to adjust the frequency characteristics using an electrical circuit. However, MEMS microphones have a sharp peak of about 20 dB in the high frequency range of around 25 kHz, making it difficult to repair using a normal electrical circuit and making them unusable. Furthermore, even if such a need exists, peaks are usually suppressed using an LC resonant circuit, but in this invention, in order to avoid the impact on sound quality caused by magnetic distortion of magnetic materials, we adopted a twin-T circuit with bootstrap applied as the optimal circuit for suppressing sharp resonances while maintaining high sound quality. This circuit can simultaneously optimize the f0 and Q0 characteristics to compensate for the shortcomings of MEMS microphones, a method that is rarely seen in microphone circuits that aim for flat characteristics. As an alternative, a staggered twin-T circuit, in which the constants of the twin-T circuit are intentionally slightly shifted, can also simultaneously optimize the f0 and Q0 characteristics. A similar circuit, the bridged-T circuit, also exists, but experiments showed that it is not suitable for compensating for a high Q0.

[0022] Furthermore, the fourth invention uses a compensation circuit based on an electrical circuit such as the one described above, which increases power consumption. Phantom power supply is a convenient method of supplying power through a signal cable, but it has a current limit of 10 mA at 48 V. When the current is low, a simple method of supplying current through a resistor can be used, but when the current increases, a constant current circuit is used. This invention uses a constant current circuit, and this circuit is also characterized by using a FET circuit, which is desirable for sound quality. However, when viewed from the power supply side, an op-amp tends to appear to have constant current characteristics. Therefore, if a constant current is supplied, the voltage will be difficult to stabilize. This problem is solved by giving the output resistive characteristics for DC and constant current characteristics for AC audio signals. An example of this design is shown outside the dotted line in Figure 1.

[0023] Furthermore, as a fifth invention, the surface of the microphone housing near the sound pickup part is constructed as a continuous curved surface without corners, as a shape improvement to avoid diffraction effects caused by the microphone housing.

[0024] Microphone sound pickup is affected by the diffraction effect. Diffraction occurs when sound reflects. Sound reflections are caused by discontinuities in shape. In other words, diffraction is less likely to occur if the microphone housing has a continuous, curved surface with no sharp edges, eliminating any discontinuities. Therefore, considering the phenomenon of a microphone picking up sound at its pickup port and then propagating to the surrounding area, returning if there are sharp edges but not if there are no sharp edges, the ideal operation would be to place the microphone at the bottom of an onion-shaped or baseball bat-like grip. The remaining sound would then travel around the periphery, passing through the sphere and then smoothly travel back through the stem of the onion or the shaft of the baseball bat, eliminating the diffraction effect and reducing the factors that impair sound quality. [Effects of the Invention]

[0025] According to this invention, MEMS microphones, which have been difficult to use in the past, can now be used for applications that allow high-quality sound recording, such as high-resolution audio.

[0026] Conventionally, microphones suitable for high-quality sound recording, such as those used in high-resolution audio, have complex structures and require skilled engineers to individually adjust each one to achieve optimal characteristics, resulting in poor productivity. They are also vulnerable to shocks and require careful use. The present invention uses MEMS microphones, enabling mass production technology to be established, resulting in high reliability and stable quality. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a circuit diagram showing the configuration of a high-performance MEMS microphone for music or the like according to the present invention. [Figure 2] FIG. 1 is a cross-sectional view showing the structure of a typical MEMS microphone. [Figure 3] 1 is a cross-sectional view showing the structure of a sound pickup section of a housing of a MEMS microphone according to the present invention. [Figure 4] FIG. 10 is an explanatory diagram illustrating a situation in which a plurality of MEMS microphone units are closely arranged. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

[0029] Figure 1 shows an example circuit suitable for this invention. In the figure, four MEMS microphones 1 are connected in parallel. The background noise of the MEMS microphone 1 used in this example is 24.5 dB, which is a little high for a music microphone, but connecting four microphones in parallel improves it by about 6 dB. In addition, the supply voltage VCC is normally 2.75 V, but increasing the voltage by 1.25 times to 3.4 V also contributes to improving the noise level.

[0030] The MEMS microphone 1 has a sharp peak in the high frequency range, but this is suppressed by passing the signal through a twin-T circuit 11 and a bootstrap circuit 12. This circuit does not generate magnetic distortion in magnetic materials, as occurs with an LC resonant circuit.

[0031] Furthermore, when a compensation circuit such as the one described above is installed, the phantom power supply system used for professional applications imposes current limitations, so some ingenuity is required to configure the circuit within this range. In this invention, an operational amplifier tends to appear to have a constant current characteristic when viewed from the power supply side. Therefore, if a constant current is supplied, the voltage will be difficult to stabilize. Therefore, the circuit is given a resistive output characteristic for DC and a constant current characteristic for AC audio signals. This circuit also uses a field-effect transistor (FET) 13, which is considered desirable for sound quality. The output is output from an XLR connector 14, which is commonly used for professional applications.

[0032] 3 shows the state in which the MEMS microphone 1 is mounted on the sound pickup section of a microphone housing. In the figure, microphone housing 15 is made by cutting or pressing a rod-shaped metal material into a cylindrical shape, with sound pickup section 16 at one end and a connector section (not shown) at the other end, and the microphone unit and circuit components built into the cylindrical interior.

[0033] A sound pickup port 16 is formed with a step at the tip of microphone housing 15. Multiple MEMS microphones 1 are mounted on circuit board 17, and sound holes (not shown) of MEMS microphones 1 are capable of recording external sounds through holes in board 17. Sound pickup port 16 is closed with sound pickup port cap 17, and sound pickup port 18 has an opening 19 at a position that coincides with the sound hole of MEMS microphone 1, allowing external sounds to be picked up.

[0034] The microphone's sound pickup area is entirely formed of curved surface 20, which is shaped to prevent deterioration of sound quality due to diffraction.

[0035] Figure 4 shows an arrangement of multiple units of the MEMS microphone 1. In the figure, (a) shows an example where four MEMS microphone 1 units are arranged, (b) shows an example where six MEMS microphone 1 units are arranged, and (c), (d), and (e) show an example where eight MEMS microphone 1 units are arranged.

[0036] The unit of the MEMS microphone 1 is generally a chip of several mm square, and the sound pickup port 16 is unevenly located, so it is sufficient to place the MEMS microphone 1 itself in close contact and also place the sound pickup port 16 as close as possible.

[0037] The high-frequency characteristics required for a high-resolution microphone are 40 kHz to 100 kHz, but the wavelength in this band, assuming the speed of sound is 340 m / s, is between 8.5 mm and 3.4 mm. Since the MEMS microphone 1 is extremely small and is attached closely, phase difference is not an issue.

[0038] As described above, the present invention provides a microphone suitable for picking up high quality sound used for high resolution audio and the like. [Explanation of symbols]

[0039] 1 MEMS microphone 2. Multilayer ceramic substrate 3. Sound hole 4 diaphragm 5 Backplate 6 Back Chamber 7. Semiconductor chips 8 Metal Cap 9 Solder Pattern 11 Twin-T Circuit 12 Bootstrap Circuit 13 Field-effect transistor (FET) 14 XLR connectors 15 Microphone housing 16. Sound pickup port 17 Circuit Board 18. Sound pickup cap 19 Transparent 20 curved surface

Claims

1. A microphone characterized by using at least two or more microelectromechanical system (MEMS) microphone units connected in parallel.

2. 2. The microphone according to claim 1, wherein the two or more MEMS microphone units are closely arranged so that the distance between the sound receiving openings of the respective MEMS microphone units is minimized.

3. The microphone of claim 1, characterized in that the high-frequency peaks of the MEMS microphone unit are suppressed by applying a bootstrap to the twin-T circuit, or by controlling f0 and Q0 by shifting the constants of the twin-T circuit.

4. 4. The microphone according to claim 1, wherein a field effect transistor (FET) is used in a constant current circuit for a power supply for a band compensation circuit of the MEMS microphone unit.

5. 5. The microphone according to claim 1, wherein the surface of the microphone housing that houses the MEMS microphone unit in the vicinity of the sound pickup portion is formed as a continuous curved surface without corners.

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