EP1233647B1 - Electroacoustic capsule - Google Patents

Abstract

An electroacoustic capsule or electroacoustic transducer for an electroacoustic device has electrostrictive or magnetostrictive elements connected to a controllable power supply. Dimensional changes of the electrostrictive or magnetostrictive elements cause changes of the inner geometry of the electroacoustic capsule or electroacoustic transducer. This allows the adjustment of the capsule or transducer to the electroacoustic device in which it is mounted so that individual and dynamic adjustments are possible.

Description

The invention relates to an electro-acoustic device with a microphone capsule having an electrode and a membrane, and a microphone amplifier, wherein the electrode and the membrane are connected via an electrical contact with the microphone amplifier.

The transducer can operate either by the electromagnetic, electrodynamic, electrostatic or piezoelectric principle.

Such devices consist essentially of the actual electro-acoustic transducer, which is used in a so-called capsule, which in turn is used in a device housing in which all the necessary electronic components are located.

The non-generic JP 57-111200 A discloses a digital speaker in which an equal number of piezo valves, such as the number of bits transmitted, is used to generate the acoustic signal. The piezo valves are connected to the respective bit control and modulate the air flow with the digital signal. The valves are controlled so that they are either completely open or fully closed. An intermediate position is not provided. The controlled piezo valves are used directly to generate the acoustic signal. The piezo valves are exclusively "fed" by the incoming audio data stream, ie the signal to be converted itself.

The DE 4 342 169 A1 discloses an electromechanical transducer having a rigid electrode and an elastically compliant membrane. The electrode has an electrically conductive tip facing a conductor surface of the opposite membrane. With a control voltage, the electrostatic attraction between the electrode and membrane is adjusted so that in the region of the conductor tip, a tunnel current flows between the conductor surfaces of the electrode and the membrane. This tunneling current is amplified by means of an electronic amplifier and utilized as an output signal.

Electroacoustic devices include at least one so-called electroacoustic capsule which, in turn, can be implemented either as a sound generator or a sound receiver. For the purposes of linguistic simplification, in the present description and the claims in electroacoustic devices, the at least one designed as a sound receiver Capsule include, spoken by a microphone. Representing electroacoustic devices with at least one electroacoustic capsule, which is designed as a sound generator, is spoken here by a headphone.

However, there is a commonality between the two device groups: The acoustic properties of the devices are determined by the device manufacturer in the course of the production process and are therefore invariable to the end user. Put simply, one can speak of an unchangeable "sound character" of the device.

For example, the acoustic properties of a microphone with an electrostatic capsule depend essentially on the distance between the membrane and the electrode and on the design of the acoustic tuning elements of the capsule. When the geometrical parameters between the movable sound-field-exposed electrode, the membrane and the fixed electrode are fixed, and even though the acoustic tuning elements inside the capsule (narrow channels, closed volumes and only partially air-permeable regions) are calculated and mechanically realized, then the directional characteristic, the sensitivity and the frequency response are also fixed and unchangeable.

Therefore, the capsule is always designed in view of the intended use, and it is generally not possible to use an existing capsule in another case or device without a great loss of quality. This applies to both sound-absorbing and sound-emitting capsules.

This feature makes a number of capsule developments necessary, not to speak of warehousing and the creation of different tools for the production, which can be very quickly expensive especially in today's rapid replacement model.

The acoustic tuning of electroacoustic capsules, regardless of whether they are to be produced as a sound or Schallnehmer, now does not have to be determined at random by testing, but can be calculated in a wide range. This calculation is based on the agreement of the mathematical models for acoustics and electricity and is based on the electroacoustic analogy principle. It is carried out with the help of so-called equivalence circuits. Here, narrow and long channels in the acoustic range of a coil in the electrical range, closed volumes in the acoustic range correspond to a capacitor in the electrical area and covered with porous and only partially air-permeable material holes in the acoustic range ohmic resistance in the electrical field. Thus, the acoustic side can be transferred into a circuit diagram, this is dimensioned and tuned with the general rules of electrical engineering in the desired sense and the result is transferred back into the acoustics.

By combining all three electroacoustic elements, it is possible to make the desired tuning of the respective electroacoustic transducer. It has been found that, in particular, narrow channels play an essential role for a suitable tonal matching of electroacoustic transducers. This is due to the fact that a narrow channel not only has an inductive impedance component, but also has a significant proportion of the ohmic resistance. The emergence of the latter is due to the flow losses in narrow channels.

Based on this knowledge, the production of a so-called friction pill, which has both an ohmic and an inductive component in their impedance and in the AT 400 910 B is described. This document proposes to connect two made of hard material and provided with small openings on the edge of the plate by means of a screw in the middle of the platelets. By selectively rotating the plates against each other, it is possible to influence the impedance of this structure in the axial direction.

Another known way to change the impedance is not to twist the plates against each other, but to change the distance between the plates by means of the central screw. The impedance change of the resulting so-called friction pill mainly affects the sound of the microphone or the headphones. This means that not only the frequency response but also the directional characteristic of the microphone or the headphone is changed at the same time. In any case, and regardless of whether the tuning elements of the capsule are variable during production or not, the acoustic tuning is currently made only once, before the assembly of the capsule, and remains unchanged throughout the life of the acoustic device. This is the circumstance which is disliked by the users of the microphones or headphones.

Not only the sound character of the electroacoustic device is crucial for its proper use. Also its properties in terms of transmission quality are important. They are mainly determined by the sensitivity of the electroacoustic transducer.

Further correlations are the following: In addition to the already described influence of an acoustic impedance pill (friction pill), the distance between the electrode and the membrane influences the capsule capacity and thereby the sensitivity of the capsule. The capsule described above, by its incorporation into a microphone housing, electrically connected to the input of an amplifier located in the microphone housing. As a result, electroacoustic transmission characteristics of the microphone are substantially determined by both components. That is, both lowest and highest sound pressures, which can be transmitted without significant deterioration of the transmission quality, are dependent on the transmission characteristics of the microphone capsule and the microphone amplifier.

The lowest sound intensities that can still be transmitted are limited to the so-called inherent noise of the microphone down. It is about thermal noise, which occurs in all electronic devices. The strongest sound intensities to be transmitted are due to the limited power supply of the microphone amplifier, since the output voltage of an amplifier can not be higher than its supply voltage.

Development engineers in the field of electroacoustics are always anxious to build electro-acoustic devices so that they transmit both very quiet and very loud sound events without significant loss of quality. To build a microphone capsule for even smaller sound pressure, you have to build it so that it is as sensitive as possible to sound pressure fluctuations. That means their transmission factor should be as large as possible. This is achieved with electrostatic sounders in that the distance between the electrodes is kept as low as possible. On the other hand, but at very high sound pressure, the electrical voltage at the input of the amplifier so high that the output voltage of the amplifier, even at a lower sound pressure than before, reaches the level of the supply voltage of the amplifier as a natural gain limit. This means that a compromise has to be made with regard to the minimum and maximum sound pressure still to be transmitted, the so-called dynamics.

But if you know that in a recording situation is expected only with quiet sound events, for example, the piano passage of a concert, or only very loud Sound events, such as drum recording, then you can by skillful installation of the microphone partially overcome the disadvantages described. This means that with quiet sound sources the microphone is positioned closer to the sound source and conversely with loud instruments the microphone is further removed from the sound source. But it is obvious that this is difficult and possible in very rare cases.

Some microphone manufacturers help themselves to this dilemma by installing a so-called attenuator: between the capsule and the amplifier, a voltage divider is switched on manually as needed, so that in loud sound events, the amplifier does not receive too much capsule signal. The attenuation of the microphone capsule signal occurs in electrostatic microphone transducers in the high-impedance range, resulting in a number circuit difficulties. Above all, suitable switches must be used for high-resistance circuits. This means that only special and therefore expensive switches are suitable for the application. Since this example is a working on electrostatic principle microphone capsule, which is shown as a capacitor in electrical circuit of the microphone, one must work with so-called capacitive voltage dividers. They are realized with the help of electrical capacitors and allow the desired signal attenuation in a wide range. Unfortunately, the harmonic distortion (distortion of the output signal) is audibly increased when a capacitive attenuator is used in such capsules. Therefore, such microphones are avoided for high quality applications.

There is thus a great need for transducers or capsules whose electroacoustic properties can still be changed in a targeted and simple manner after their production, preferably on the occasion of the installation of the capsule in a housing. Of course, the users of electro-acoustic devices are interested in adapting the acoustic properties to the particular application.

According to the invention, these objects are achieved with an electroacoustic device of the type mentioned above in that the device comprises a controllable voltage source and the microphone capsule comprises electrostrictive or magnetostrictive elements, preferably piezoelectric components, which via a second contact with the controllable voltage source for applying an electrical voltage are related to the electrostrictive or magnetostrictive elements and that the dimensional changes the electrostrictive or magnetostrictive elements cause changes in the internal geometry of the microphone capsule.

By "change in internal geometry" in the specification and claims is meant both the change in the distance between the electrode and diaphragm of an electrostatic transducer and the change in the distance of components of the capsule to one another, as for example in one of the above-mentioned friction pills the opening or closing or changing the size of an opening od.dergl. Understood

In the description and the claims, "electrostrictive or magnetostrictive elements" are understood as meaning all components which, upon application of an electrical voltage, reversibly change a characteristic body dimension in a measure dependent on the applied voltage. In addition to the aforementioned piezoelectric components, which reversibly change their geometric dimensions by applying a voltage, examples are also magnetostrictive elements which reversibly change their geometric dimensions by the action of a magnetic field.

• die Fig. 1 einen elektrostatischen Wandler gemäß dem Stand der Technik,
• die Fig. 2 eine Gegenüberstellung der elektroakustischen Analogie,
• die Fig. 3 eine vorbekannte Reibungspille in schematischer Seitenansicht,
• die Fig. 4 eine erfindungsgemäße elektroakustischen Reibungspille,
• die Fig. 5 einen erfindungsgemäß ausgebildeten Wandler und
• die Fig. 6, 7 und 8 Details.

The invention will be explained in more detail below with reference to the drawing. It shows

• the Fig. 1 an electrostatic transducer according to the prior art,
• the Fig. 2 a comparison of the electroacoustic analogy,
• the Fig. 3 a previously known friction pill in a schematic side view,
• the Fig. 4 an electroacoustic friction pill according to the invention,
• the Fig. 5 an inventively designed transducer and
• the FIGS. 6, 7 and 8 Details.

The Fig. 1 shows as an example working on the electrostatic principle sound-absorbing capsule for installation in a microphone. The acoustic properties of the microphone depend essentially on the distance between the diaphragm 1 and the electrode 2 and the design of the acoustic tuning elements 3 (size of the rear volume, friction in the rear sound inlet, size and number of openings in the electrode 2) Capsule off. When the geometric parameters between the movable and sound field exposed electrodes, the diaphragm and the fixed electrode 2 are fixed, and also the acoustic tuning parameters 3 inside the capsule (narrow channels, closed volumes and only partially air permeable areas) are calculated and performed mechanically are, then the directional characteristic, the sensitivity, the frequency response is also fixed and unchangeable. The "boundary conditions" for the capsule shown are determined by the (not shown) microphone housing, when they change the corresponding tuning parameters 3 inside the capsule are no longer able to ensure the desired transmission behavior.

The Fig. 2 shows the corresponding elements of the electroacoustic analogue, on the left side the acoustic elements, on the right the corresponding electrical: Narrow and long channels 31 in the acoustic range correspond to a coil 32 in the electrical range, closed volumes 33 in the acoustic range correspond to a capacitor 34th In the electrical field and with porous and only partially air-permeable material covered holes 35 in the acoustic range correspond to an ohmic resistance 36 in the electrical field.

The Fig. 3 shows a friction pill according to the above-cited AT-B: Two made of hard material and provided at the edge with small openings 39, 40 plates 36, 37 are connected by means of a screw 38 in its center. By selectively rotating the plates 36, 37 against each other, it is possible to influence the acoustic impedance of this structure in the axial direction, since the length of the paths is changed by the twisting.

The Fig. 4 shows an inventive embodiment of an electroacoustic friction pill. It consists of two at the edge with small openings 8 provided platelets 6, 7 of piezoelectric material. The electrical contacting of the platelets 6 and 7 via any of the previously known types of contacting 4. The platelets are on the metallized upper and lower side and electrically connected in series. By connecting to a DC voltage source they expand so that the height of the gap 5 between the plates 6, 7 is reduced.

The change in the voltage connected to the platelets caused by the change in the distance 5 between the platelets 6, 7, a change in the acoustic impedance in the axial direction. Because of that it is possible; to influence the sound of the microphone or the headphones in which this friction pill is built in from the outside, without having to disassemble or even remove the microphone or headphone capsule or the microphone or headphones.

It is also possible to replace one of the two plates 6 or 7 by a plate made of conventional material, for example of plastic or metal. As a result, only one plate contributes to the reduction of platelet spacing. The platelets do not have to be circular, and all other geometric designs from rectangular to oval are conceivable. But you must have at least one opening 8 at the edge or inside for the air or sound passage. The initial distance of the platelets 6, 7 is determined in the embodiment shown by a small step 9 at the edge of the plate 7. Instead of level 9, a spacer ring can be used. By reversing the polarization voltage, it is possible to reduce the distance between the plates both (at the radial distance from the step 9), as well as to increase.

The Fig. 5 shows the inventive application of an electrode made of piezoelectric material, which can be used in electrostatic microphone capsules. The difference to Fig. 1 It has now been given a second role and is not only connected via the electrical contact as one of the two capacitor electrodes of the electroacoustic transducer to the microphone amplifier, but is also connected via a second contact 14 connected to a second electrical circuit. This makes it possible to change the electrode 12 by applying a control voltage to the contact 14 in its thickness and thus also the distance between the electrode 12 and the membrane 1. It is of course also possible to arrange the piezoelectric elements in the region of the retaining ring 15 for the membrane and thus the distance between membrane and Elcktrode directly and not by way of the change in the thickness of the electrode 12 to change.

It is particularly advantageous to influence the sensitivity of the microphone in this way. One can then dispense with the external attenuation capacitors discussed above and instead change the distance between the membrane and the electrode directly. In this case, a reduction in the distance between the electrodes 11, 14 of the transducer caused by the application of a control voltage to the electrode corresponds to an increase in the capsule sensitivity. As the capacitance of the capsule is increased by reducing the distance between the membrane and the electrode, one obtains the advantage that the sensitive capsule automatically has a large capacity. Since the noise of a C-microphone is smaller, the larger its capsule capacity, the invention makes it possible to build high-sensitivity and low-noise microphones, but still have a wide dynamic range, because so for the recording of loud sound events, the capsule on insensitive (large Distance between the electrode and the membrane) can be switched.

In order to provide better reproducible results, the capsule capacitance can be used as a measured variable for a control loop in the microphone. But this also allows manufacturing tolerances and temperature influences, which have a negative effect on the distance between electrodes, to be compensated in a simple and reliable manner. The provision of appropriate electronics is not a problem for those skilled in the field of tuning microphones in the knowledge of the invention.

Since the piezoelectric plates are electrically high impedance in both application examples, no noticeable current flows through them, which has a positive effect on the total power consumption of the electroacoustic device. From an electrical point of view, the platelets described are to be regarded as the plates of a capacitor, which in turn means that there is only a short charge current in the electrical control circuit, and only until the capacitor has charged to the connected voltage (a few milliseconds). For the reason described above (no current flow), the voltage to which the plates are connected can be referred to as the polarization voltage.

The size of the polarization voltage can be changed either continuously or in predetermined stages. The voltage source itself is a DC voltage source and their voltage can be up to several 100V as needed. Since the voltage source does not have to provide any appreciable current intensity, it is also possible to dispense with all current protection measures (current limitation). The voltage can either be obtained from the power supply of the device (phantom power for condenser microphones) or from a control voltage connected to the device.

Of course, the use of piezoelectric elements having a particularly large coefficient of expansion is preferred. This makes it possible to influence individual electroacoustic elements individually. Thus, in the region of the capsule or the friction pill by the excitation with control voltage channels 16 in a component 19 can be opened or closed individually by a piezoelectrically reacting plate 21, as in the Fig. 6 is shown. It is thus also possible to increase the size of an acoustically significant volume 17 by connecting it in parallel with another volume 18, like the latter Fig. 7 shows. Entire friction pills, which are arranged, for example, in sound passage openings 35, can also be mechanically displaced or "covered", as can be seen from FIG Fig. 8 seen. In this case, each 21 is a made of piezoelectric material, and in the manner described above operated with a control voltage plate is characterized. A so excited with control voltage plate 21 opens or closes the provided for the acoustic vote of the capsule not shown in detail.

A dynamic adaptation of an electroacoustic transducer or capsule, which is based on the electrostatic principle and operates as a microphone, is characterized in that between the main sound source and the microphone a sound level determining sound receiver is arranged, the measured value for controlling the voltage for the electrostrictive or Magnetostrictive element is used. Due to the fast data processing and the rapid adaptation of piezoelectric components, the sensitivity of the microphone can be adjusted as a function of the current sound level during recording.

Claims (9)

1. An electroacoustic appliance with a microphone capsule which has an electrode (12) and a diaphragm (11), and with a microphone amplifier, wherein the electrode (12) and the diaphragm (11) are connected via an electrical contact with the microphone amplifier, characterised in that the appliance comprises an adjustable power supply and the microphone capsule has electrostrictive or magnetostrictive elements (6,7;12;21), preferably piezoelectric components, which via a second contact (4,14) are connected to the adjustable power supply for applying an electrical voltage to the electrostrictive or magnetostrictive elements (6,7;12;21), and in that the variations in the dimensions of the electrostrictive or magnetostrictive elements (6, 7; 12; 21) bring about variations in the internal geometry of the microphone capsule.
2. An electroacoustic appliance according to Claim 1, characterised in that the electrode (12) is the electrostrictive or magnetostrictive element.
3. An electroacoustic appliance according to Claim 1, characterised in that the diaphragm (11) is maintained at a distance from the electrode (12) by means of an annular spacer (15), wherein the spacer is the electrostrictive or magnetostrictive element.
4. An electroacoustic appliance according to any one of Claims 1 to 3, characterised in that the capsule capacitance is used as a measured value for a control loop for determining the voltage for the electrostrictive or magnetostrictive element (6,7;12;21) so as to compensate for production tolerances and temperature influences which have an adverse effect on the spacing between the electrode (12) and the diaphragm (11).
5. An electroacoustic appliance according to any one of Claims 1 to 3, characterised in that a sound receiver is disposed between the main sound source and the microphone, which sound receiver determines the sound level and the measured value of which is used to control the voltage for the electrostrictive or magnetostrictive element (6,7;12;21).
6. An electroacoustic appliance according to Claim 1 with at least one sound inlet, characterised in that an electro-acoustic friction pellet is disposed in the vicinity of a sound inlet and comprises two plates (6,7) of electrostrictive or magneto-strictive material, preferably of piezoelectric material, which plates are preferably provided with small openings at the edge, in that the plates (6,7) are metallised on their upper and lower sides and are provided with an electrical contact (4), and in that they are electrically connected in series.
7. An electroacoustic appliance according to Claim 1, characterised in that the electrostrictive or magnetostrictive elements (21) open or cover a sound transmission opening (35) as a function of their respective geometry.
8. An electroacoustic appliance according to Claim 1, characterised in that the electrostrictive or magnetostrictive elements connect or separate a first hollow space (17) to or from a second hollow space (18) as a function of their respective geometry.
9. An electroacoustic appliance according to Claim 1, characterised in that the electrostrictive or magnetostrictive elements (21) open or cover a channel (16) of a component (19) as a function of their respective geometry.

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