Dual Section Electret Microphone

1. Field of the Invention

This invention relates generally to a capacitor microphone, and is directed more particularly to a bi-directional capacitor microphone having a pair of back-to-back transducer sections.

2. Description of the Prior Art

In general, a conventional bi-directional capacitor microphone is formed of a pair of conductive back plates and a conductive diaphragm disposed between the back plates with predetermined air gaps separating it from the back plates. In particular a first transducer section is formed of a first perforated back plate and a diaphragm spaced a predetermined amount from the first back plate, and a second transducer section is formed of a second perforated back plate and spaced another predetermined amount from the diaphragm. In such a microphone, the first and second back plates are positioned either to the left and right or above and below the diaphragm, and are electrically connected to each other. An electret layer is coated on the surfaces of both back plates that face the diaphragm. Thus, an electric charge is applied to the back plates so that vibrations of the diaphragm will induce an electric signal between the back plates and the diaphragm.

The conventional microphone described above can be provided with a bi-directional characteristic by selecting the electret of the first transducer section and the electret of the second transducer section to have opposite polarities.

Although the above microphone has a bi-directional characteristic, its frequency characteristic for low frequencies is rather poor. The reason for this is that the diaphragm is common to both the transducer sections, and the lower frequency sound waves of arrive virtually simultaneously at both surfaces of the diagram with almost no phase difference imparted to the two surfaces of the diaphragm, as a result, the diaphragm is not vibrated.

Another drawback of the above microphone is that unless the distance from the diaphragm to each back plate, that is, the width of each of the air gaps, is selected equal to the other, the characteristics of the respective transducer sections are not matched. This matching of characteristics is very difficult work in the microphones according to the prior art.

Accordingly, an object of the present invention is to provide a novel capacitor microphone which uses two transducer sections and avoids the drawbacks inherent in the prior art.

Another object of the invention is to provide a capacitor microphone in which a first transducer section having a diaphragm made of an electret and a second transducer section having a back plate with an electret are combined to provide the microphone with a superior bi-directional characteristic.

A further object of the invention is to provide a capacitor microphone which is simple in construction and superior in directional characteristic.

A further object of the invention is to provide a capacitor microphone in which left and right directional characteristics are both equal.

A yet further object of the invention is to provide a capacitor microphone in which the assembling of a diaphragm and a back plate can be performed simply.

A still further object of the invention is to provide a capacitor microphone whose directional characteristic is not changed even if it is used for a long time.

According to an aspect of the invention, a capacitor microphone comprises a first diaphragm including a first electret; a first conductive back plate having a surface facing the first diaphragm and spaced apart therefrom; a second conductive back plate electrically coupled to the first conductive back plate; a second electret on one surface of the second conductive back plate; a second diaphragm at least partially of conductive material facing the one surface of the second conductive back plate and spaced apart therefrom; and first and second output electrodes. The diaphragms are coupled to each other and to the first output electrode, while the back plates are coupled to each other and to the second output electrode, so that an output signal appears between the first and second output electrodes. Preferably, the back plates are disposed in a parallel, back-to-back configuration, and the electrets each have a negative polarity.

The above and other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings in which like reference numerals identify the same elements and parts.

FIG. 1 is a schematic cross-sectional view showing a capacitor microphone according to an embodiment the present invention;

FIGS. 2A and 2B are graphs showing directional characteristics of first and second transducer sections of, respectively, the microphone shown in FIG. 1;

FIG. 3 is a graph showing the directional characteristic of the microphone shown in FIG. 1; and

FIG. 4 is a schematic cross-sectional view showing a capacitor microphone according to another embodiment of the invention.

In the embodiment of the invention shown in FIG. 1, a capacitor microphone 1 is formed of first and second audio-electric transducing members (which will be hereinafter referred simply to as transducer sections) 1a and 1b. The first transducer section 1a comprises a diaphragm 2a and a back plate 3a which is made of a conductor and has a number of perforations or sound openings 4a. The back plate 3a faces the diaphragm 2a and is separated therefrom by a spacer 6a made of an insulator. The second transducer section 1b comprises a diaphragm 2b and a back plate 3b which is made of a conductor and has a number of performations or sound openings 4b and faces the diaphragm 2b and is separated therfrom by a spacer 6b made of an insulator. In this embodiment, the diaphragm 2a of the first transducer section 1a is made of an electret, and the diaphragm 2b of the second transducer section 1b is formed of a thin diaphragm material such as a thin conductive metal plate or thin synthetic resin layer with conductive material deposited thereon by vaporization. An electret 5 having apertures 4b' matching the sound openings 4b of the back plate 3b is coated on the surface of the back plate 3b facing the diaphragm 2b. The diaphragm 2a is provided with a conductive surface, which can be formed by vapor deposition. In other words, in the first transducer section 1a, the diaphragm 2a itself is formed as the electret, while in the second transducer section 1b, the back plate 3b includes the electret 5.

In the embodiment of FIG. 1, the electrets are arranged so that their surfaces facing the back plates 3a and 3b are of like polarity. Further, it may be desired that negative charge is used as the charge imparted by the electrets. The back plates 3a and 3b are electrically coupled by a conductive bar 10. Both of the first and second transducer sections 1a and 1b are covered by a housing 7 made of, for example, aluminium. The housing 7 comprises peripheral portions 7a and 7b which press against conductive rings 11a and 11b to bias the diaphragms 2a and 2b towards the back plates 3a and 3b, respectively. Insulating layers 12 are provided between the housing 7 and the back plates 3a and 3b. The diaphragms 2a and 2b are electrically connected to one another and to a first lead 8, and the conductive bar 10, which couples the back plates 3a and 3b, is connected to a second lead 9. Any conventional, well-known electret can be used as the diaphragm 2a and the electret 5.

With to the capacitor microphone of the present invention constructed as above, since the diaphragm 2a of the first transducer section 1a contains negative charge, the first transducer section 1a exhibits cardioid directional characteristic which has a maximum response at the left side as shown in FIG. 2A. Since the electret 5 on the back plate 3b in the second transducer section 1b is also negatively charged, the second transducer section 1b exhibits a cardioid directional characteristic which has a maximum response at the right side as shown in FIG. 2B. Since the two directional characteristics have maximum response directions that are different by 180.degree., the combined outputs therefrom, which appear at the leads 8 and 9, will exhibit the bi-directional characteristic shown in FIG. 3.

In the embodiment of the invention shown in FIG. 1, the diaphragms 2a and 2b are located outside the back plates 3a and 3b, respectively, so that a distance W between the diaphragms 2a and 2b may be suitably selected, for example, to be 12 mm, for providing a sufficient phase difference between both the diaphragms 2a and 2b for low frequency sound waves as compared with the prior art microphone, so that the low frequency characteristic according to the microphone of this invention is improved.

Further, by using the electrets of like charge as the diaphragm 2a and the electret 5 on the back plate 3b, the outputs of the same characteristic can be derived from both the transducer sections. Also, when the charge on the electret is selected to be negative, the charge holding time can be made long as compared with an electret of positive charge. As a result, the microphone of this invention can be used for an extended period of time.

If, in the second transducer section 1b, the diaphragm 2b is made of an electret diaphragm, and hence the electret 5 on the back plate 3b is removed, then, the electret diaphragm must be selected to have a different polarity from the electret in the first transducer section 1a. For example, when a negatively-charged electret is used as one of the diaphragms, for example, diaphragm 2a, and a positively-charged electret is used as the other diaphragm 2b, the charge in the electret that is charged positively becomes greatly attenuated over an extended period, as compared with the electret which is charged negatively. As a result, the outputs from both the transducer sections become different. In other words, such a microphone will not provide the desired bi-directional characteristic for any great length of time.

According to this invention, the above defects can be avoided by use of the preferred construction as described above.

FIG. 4 shows another embodiment of the capacitor microphone according to the present invention. In this embodiment, a single back plate or block 3 with openings 4 is made by form the back plates 3a and 3b of FIG. 1 as an integrated unit, while the remaining construction and operation of this embodiment are substantially the same as in the embodiment shown in FIG. 1. Therefore, in FIG. 3 the parts and elements corresponding to those shown in FIG. 1 are identified with the same reference numerals, and their detailed description is omitted.

It will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirit or scope of the present invention which is intended to be defined by the appended claims.