Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Microphone capsule only
Utility Patent
1997-04-15
2001-01-02
Le, Huyen (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Microphone capsule only
C381S113000, C381S191000, C367S170000
Utility Patent
active
06169810
ABSTRACT:
This invention relates to an electroacoustic transducer having a substantially closed case with an internal microphone, wherein the microphone has a diaphragm, clamped on a carrier and provided with a conducting layer, and a backplate arranged parallel to the diaphragm. Such a transducer is known, for example, from published European patent application 0,533,284.
In general, the operation of such a transducer is based on the phenomenon that the capacitance of a capacitor is dependent on the mutual distance between its capacitor plates. If as a result of, for instance, acoustic vibrations, one of those plates is caused to vibrate so that the effective distance between the plates varies, the capacitance which varies as a result of this can be detected as an electrical signal. A widely applied embodiment of an electroacoustic transducer is of the so-called electret type where one of the capacitor plates is provided with a predetermined amount of charge. The 0 533 284 European patent application describes an example of such an electroacoustic transducer of the electret type, and in the following the present invention will be explained specifically for such an electroacoustic transducer of the electret type, but it is expressly noted that the invention is not limited thereto.
In general, such a transducer comprises a substantially closed case which is provided with an opening through which the interior of the case can communicate with the surroundings. Arranged in the case is a microphone capsule which, in the above-mentioned case of an electret type, is designated as electret system, which comprises a so-called backplate and a diaphragm arranged adjacent the backplate, which diaphragm is at least partly provided with a conducting layer. The electret system further comprises an electret layer which can be applied to the backplate or to the diaphragm; the diaphragm can even be manufactured from electret material. In the embodiment known from the publication mentioned, the electret layer is applied to the backplate.
Upon entry of sound waves into the case, the diaphragm is caused to vibrate, so that through the combination of the diaphragm and the backplate an electrical signal is generated which is representative of the sound waves, and which can be presented to an amplifier for further processing.
The object of the present invention is to improve the known transducer.
A first aspect of the present invention concerns the assembly of the backplate and the diaphragm in the case. Usually, the diaphragm is clamped on a frame-shaped carrier. The frame-shaped carrier has a substantially rectangular shape with rounded corners, the outer contour of the carrier corresponding to the inner contour of the case. The carrier with the diaphragm is arranged adjacent the bottom of the case, and above it the backplate is arranged. With a view to reducing parasitic capacitance between the backplate and the carrier, the carrier is located at the underside of the diaphragm, that is, on the side of the diaphragm remote from the backplate (see, for instance, as shown in FIG. 8 of EP-0,533,284). The distance between the diaphragm and the backplate is determined by distance elements or projections formed on the backplate; the constructional rigidity of the electret system with respect to the case is ensured by supporting means which support the diaphragm at the location of the distance elements mentioned. In the known device those supporting means are formed as supporting posts extending upright from the bottom of the case. For that matter, the diaphragm and the backplate are individually secured to the case.
In the conventional transducer, the height dimension of the carrier (also referred to as thickness) is less than the height dimension of the supporting posts, because otherwise it would be impossible for the diaphragm to be supported by the supporting posts. In the assembly of such a construction, a risk exists that when the carrier (with the diaphragm) is being placed in the case, the carrier is not moved down far enough, in which case the supporting posts do not support the diaphragm, or that the carrier is moved down too far, in which case the supporting posts deform the diaphragm and adversely affect the state of tension in the diaphragm. The subsequent placement of the backplate is also a precision job: if the backplate is mounted too high, the distance between backplate and diaphragm is too great and hence the sensitivity of the transducer too low. In this connection, it is a drawback that the positioning of the backplate relative to the diaphragm only occurs when the backplate is being fitted in the case.
The object of the invention is to overcome the disadvantages mentioned.
According to an important aspect of the present invention, the supporting means for the diaphragm are formed on the diaphragm carrier. Thus the carrier no longer functions solely for clamping and retaining the diaphragm along the edge thereof, but also for supporting the diaphragm at the location of at least some of the distance elements of the backplate. An important advantage of this is that the transducer, to work properly, is no longer dependent on supporting posts formed on the case. The case can even be constructed without supporting posts formed on the case, which is constructionally simpler and cheaper.
Another important advantage of the present invention is that the electret system, that is, the combination of carrier, diaphragm and backplate, prior to being placed in the case, can be formed as an electret unit, with the backplate being attached to the carrier, for instance by means of a suitable glue. Positioning the backplate relative to the diaphragm with great accuracy has then become relatively simple. Another advantage in this connection concerns the fact that the shape of the backplate can now be simpler, and that the backplate does not need to be secured to the inner wall of the case, which saves a processing step. In the conventional transducer as known from European patent application 0,533,284, the backplate has the shape of a rectangle whose dimensions are less than the corresponding internal dimensions of the case to allow air displacement, which rectangle, adjacent its corner points, is provided with projections for connecting the backplate mechanically and electrically with the case. In the transducer according to the present invention, the backplate can simply have the shape of a rectangle, and no separate operation is necessary to secure the backplate mechanically to the case.
Forming an electret unit (subassembly) consisting of the combination of carrier, diaphragm and backplate is known per se from U.S. Pat. No. 4,730,283. There, however, the carrier and the backplate are located on the same side of the diaphragm, with the backplate being placed within the carrier. The backplate is glued through its corner points to the carrier, with either the outside corners of the backplate or the inside corners of the carrier being provided with projections. The carrier thus does not serve for supporting the diaphragm at the location of the distance elements of the backplate. This entails as a disadvantage that the positioning of the backplate in the carrier is particularly cumbersome and should be done with particular accuracy: if the backplate is not pressed far enough into the carrier, the distance between backplate and diaphragm is too large and hence the sensitivity of the transducer is too low, whereas if the backplate is pressed too far into the carrier, the diaphragm can be deformed. Thus the problem of the cumbersome positioning of the carrier with the diaphragm in the case has shifted to the problem of the cumbersome positioning of the backplate in the carrier.
A second aspect of the present invention concerns the processing of the electrical signal. As has been described extensively in EP-0,533,284, the transducer is provided with an electric circuit for amplifying the electric signal generated by the electret system. Conventionally, one of the components of the electret system is connected with t
de Blok M.
Geschiere O.
van Doorn J. M.
Van Halteren A. Z.
Wilmink E.
Le Huyen
Michaelson Peter L.
Michaelson & Wallace
Microtronic Nederland B.V.
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