Pressure gradient microphone capsule

The invention relates to an electrostatic pressure gradient microphone capsule for essentially flush installation in or behind, in particular, flat installation surfaces with a capsule housing with two, optionally subdivided, sound inlet openings, a membrane held tensioned on a membrane ring, an electrode and, optionally, acoustic friction.

Electrostatic transducers for microphones, regardless of their physical mode of operation, have a membrane that is exposed to the sound field and is excited by it to vibrate. The subject of the invention is thus an electrostatic microphone.

The electrodes of the electrostatic transducer are an elastic, tensioned membrane and a rigid electrode, which is usually just called an electrode. Both form a capacitor, the electrical capacity of which changes due to pressure fluctuations in the sound field. Since an electric field is built up between the electrodes of the electrostatic transducer, it is possible to convert the changes in capacitance of the transducer into changes in electrical voltage with the aid of a downstream amplifier.

The electroacoustic properties of electrostatic microphone capsules are mainly dependent on the type of acoustic excitation of the membrane. In the sand of technology on the one hand the so-called pressure and on the other hand the so-called pressure gradient receivers are known. The membrane of the former is exposed to the sound field in only one direction (the so-called front direction), so they only react to changes in air pressure due to the sound waves. Since the air pressure is a scalar, microphones constructed in this way have a spherical directivity. In the case of a pressure gradient receiver, the membrane is exposed to the sound field from both directions; it is therefore stimulated to move by the currently prevailing pressure gradient between the front and the rear side of the membrane. Microphones constructed in this way have a directional characteristic due to the difference in transit time of the sound waves to the front and rear, which, through correct acoustic tuning of the microphone capsule, can assume almost any shape between figure-eight and kidney-shaped directional characteristics.

Up until now, pressure gradient microphone capsules have been built so that the front and rear sides of the diaphragm are aligned with the front and rear sound openings. This means that such microphone capsules have an axially symmetrical directional characteristic. 1 shows an electrostatic microphone capsule according to the prior art. It has a front sound inlet opening 1 and a rear sound inlet opening 2.

With regard to the assembly of microphone capsules, there is an increasing demand for them to be installed as flush as possible in a flat and relatively large surface. This can be a headliner of a car if a hands-free device for a mobile phone or another communication device is to be provided in the vehicle. However, microphone capsules according to the prior art are not able to meet these conditions and can only be built into a flat surface with great difficulty, at great expense and with considerable technical compromises. Fig. 2 shows the usual mounting situation ener, the prior art, working according to the electrostatic principle microphone capsule. In order to make both sound entry openings necessary for the function of the capsule accessible to the sound, it must be built into the mounting surface instead of as intended.

It is known from US Pat. No. 5,226,076 A to install a capsule like the one shown in FIG. 2 in a housing which has two sound openings on one of its longitudinal walls, one on each side of the capsule. This makes it possible to arrange the housing in or behind a flat installation surface. The disadvantage of this solution is the great installation height or depth, since the capsule with its axis of symmetry lies parallel to the installation plane and is accordingly high. The assembly of the microphone including its capsule with its housing, which actually represents an additional part completely separate from the finished microphone, is complex and expensive.

Microphone capsules operating purely on the principle of pressure receivers with only one sound entry can be integrated into the installation area without any problems due to the nature of a pressure receiver. But since they have a spherical directional characteristic, one then has to forego the great advantages of a directional characteristic. Due to their spherical directional characteristic, the pressure receivers are equally sensitive to both useful and interfering sound. As a result, they are rarely used in a noisy environment.

AT 410 741 B

The invention has set itself the task of creating a microphone capsule that works according to the electrostatic principle and is constructed as an electroacoustic transducer according to the pressure gradient principle, which can also be integrated into a flat surface without optical interference or acoustic compromises and at the same time has a low overall height has and is simple and inexpensive to manufacture.

According to the invention, these objectives are achieved in that the two sound inlet openings are arranged on one side of the capsule housing, the front side, and that one sound inlet opening is connected in an acoustically conductive manner to the front side of the membrane and the other sound inlet opening is connected in an acoustically conductive manner with the rear side of the membrane and that the Membrane is arranged essentially parallel to the front.

By rotating the plane in which the diaphragm lies and the design of the capsule with the acoustic connections between the respective sound inlet opening and the associated side of the diaphragm, two significant advantages are achieved over the aforementioned US-A: It is possible to access the housing to do without and the overall height is significantly reduced. In one embodiment, acoustic friction can even be dispensed with, which again saves costs and volume.

The invention is explained in more detail below with reference to the drawing. As already mentioned, FIG. 1 shows a pressure gradient capsule according to the prior art, FIG. 2 shows the installation situation of the capsule according to FIG achievable directional characteristic.

As can be seen from FIG. 3, a pressure gradient capsule according to the invention has the following schematic structure: A membrane 1 is pretensioned and glued onto a membrane ring 2. With the aid of a spacer ring 3, it is brought to a distance of about 10 to 60 μm from an electrode 4 and together with it forms a capacitor. The electrode 4 is provided with bores. A so-called acoustic friction 5 is provided on the side of the electrode facing away from the membrane 1. The acoustic friction 5 usually has a plastic ring, the opening of which is covered with a porous material (metal mesh, sintered material, plastic fabric or natural fiber). The acoustic friction 5 is used for acoustic tuning of the microphone capsule. This structure corresponds in principle to the state of the art.

According to the invention, the capsule is accommodated in a capsule housing 6. The capsule housing is closed on its upper side, the front side 11, with a cover. The front side 11 has at least two sound openings, one opening, the front opening 7, allowing the sound waves to enter the front side of the membrane 1 and the second opening, the rear opening 8, consisting of a sound channel 9 leading past the built-in parts of the capsule from sections or Areas 9a, 9b and 9c, the access of the sound waves to the rear side of the membrane 1 is allowed. The sound channel 9 can replace or supplement the acoustic friction 5, as indicated by the area 9c; for this purpose, a separate sound friction can be provided in the interior of the sound channel 9, for example in the area 9b. Such acoustic friction in the sound channel 9 can be accommodated either at its beginning, in the vicinity of the rear opening 8, or in partial sections or in the entire area of ​​the sound channel.

The objectives according to the invention are thus achieved in that the capsule has a larger cross-section than the membrane ring 2 in planes parallel to the membrane 1 and that the remaining area is used to provide a sound channel, namely the sound channel 9a, 9b, 9c within the capsule to lead the "around the membrane" to the rear of the "membrane - electrode" unit. Since the diameter of the membrane ring 2 is significantly larger than the height of the unit "membrane - electrode" including rings and friction, it is possible through the invention to produce pressure gradient microphones with a significantly flatter construction than before, and that with significantly reduced production costs, since the assembly in a train, is possible on the same machines as before, since only the capsule housing has a slightly different shape.

Due to its design, the microphone capsule according to the invention can be installed in such a way that the surface of the capsule housing 6 is flush with the installation surface 10 and is therefore optically inconspicuous. The capsule is installed in a directional manner so that the front opening is closer to the sound source to be recorded than the rear opening 8. The microphone capsule is

AT 410 741 B acoustically tuned so that its directional characteristic shows its maximum sensitivity in the presumed direction of the speaker. This means that the directional characteristic is positioned parallel to the installation plane 10, which makes it possible to mask out interfering signals from other directions.

In Fig. 3 the type of assembly is not shown in detail, the capsule can be detachably or permanently connected to the installation surface by means of a press fit, by gluing, by screws, bolts, etc. The electrical contacting is also not shown, with knowledge of the invention this does not present a problem for the person skilled in the art. Depending on whether a friction 5 is provided or not, the overall height can be further reduced.

The capsule housing can have an oval cross-section normal to the membrane plane or can be constructed from two semicircles with a rectangular central part and thus support the circular components of the actual transducer over a substantial part of the circumference. When choosing a different shape, it is also possible to connect these components individually and to store them together in the capsule housing, but this is only advisable in special cases because of the larger space requirement and the additional effort.

The storage or mounting of the actual components, namely the friction, the electrode and the membrane ring, is not shown in detail in FIG , take place, the parts in the area adjoining the sound channel 9 are designed to be self-supporting, knobs, webs, elevations or the like can also be used. protrude upwards from the bottom of the capsule housing 6 (indicated in the area 9b of the sound channel) and support the friction or the electrode.

Fig. 4 shows a directional characteristic that can be achieved with the invention, indicated by the dashed line 13. A transducer designed according to the invention is mounted with its capsule housing 6 in alignment (flush) in the installation surface 10; Drawing plane. The arrow P essentially indicates the direction in which the sensitivity of the built-in microphone is greatest. If this corresponds, for example, to the direction towards the driver's head when installed in a vehicle, it is easy to see that the microphone hardly transmits sound from the direction of the passenger, arrow N, from which background noise is most likely, although the capsule is flat and inconspicuous is behind the mounting surface 10.

It is of course possible to perforate the installation surface 10 only at the locations of the sound inlet openings 7, 8 or to break through it in some other way and to arrange the capsule completely behind the material of the installation surface 10. This must be taken into account when designing the dimensions of the capsule and the acoustic friction, but this does not present any difficulties for those skilled in the field of acoustics with knowledge of the invention.

All materials used in conventional electroacoustic transducers can be used as materials; processing and assembly is also easily possible by means of conventional techniques for the person skilled in the art with knowledge of the invention.