DE1088546B - Electrostatic speaker - Google Patents

Description

The invention relates to electrostatic speakers. This means loudspeakers, whose excitation is based on the electrostatic attraction between a movable or flexible Membrane electrode and at least one rigid plate electrode attached parallel to it is based.

To reduce non-linear distortion, it is common to place the diaphragm between two such to attach solid plate electrodes, at least one of which is perforated or grid-like is designed to allow the passage of air.

To increase the sensitivity and to double the frequency caused by the attraction To avoid the audio frequency, it is also customary to use a constant DC voltage, a constant potential difference between the membrane electrode on the one hand and each of the two solid plate electrodes on the other hand, the plates opposite to the membrane are polarized.

The polarization voltage and the low frequency voltages can be fed in in two different ways, shown in FIGS. 1 and 2 in the equivalent circuit diagram shown are:

I. The positive terminal of the bias source 10 is connected to the fixed plate 11 and the negative terminal to the other fixed plate 12 (Fig. 1). The low frequency voltage of an AC power source 13 is applied between the speaker diaphragm 14 and point 15 of the center tap of the bias voltage source 10.

II. The low frequency voltage of the alternating current source 13 is by means of a transformer 16 the Fixed plates 11 and 12 fed, and the polarization voltage 10 is between the speaker diaphragm 14 and the center tap 17 of the secondary coil of the transformer 16 is applied (Fig. 2).

The higher the selected polarization voltage, the more sensitive the arrangement is; with the well-known Circuits described above, however, encounter difficulties in that between the Membrane electrode and the plate electrodes can pass over spark discharges.

With the circuit mentioned under II, the destructive effect of such a spark discharge can be be attenuated by placing a high resistor 19 (Fig. 2) in the line between the bias source 10 and the diaphragm 14; this resistance prevents the full polarizing voltage from being persistent, once arcing has occurred but does not fundamentally prevent sparks from occurring. All speakers - except the smallest - have an electrostatic speaker

Applicant:

Max Braun,
Frankfurt / M., Rüsselsheimer Str. 22

Claimed priority:
Great Britain 20 July 1954

David Theodore Nelson Williamson, Edinburgh,
and Peter James Walker, Huntingdon

(Great Britain),
have been named as inventors

however, this solution is not satisfactory because the charge stored in the loudspeaker is considerable and is sufficient to cause damage if a spark were to occur. In addition, is the collapse of the polarization voltage is very disturbing.

In the circuit mentioned under I is the use of a resistor that is large enough to Preventing a spark transition is not possible because all conductive connections also include audio frequency currents lead and increase the time constant of the circuit too much due to such a resistance would.

There are therefore semiconductor membranes for the oscillating electrode has been recommended to prevent the collapse of the polarization voltage in the event of arcing to avoid. The phenomenon is exploited that the electrostatic charge on a Semiconductor membrane is not as easy to move as on a highly conductive membrane. If between a spark should arise from a small area of the membrane electrode and one of the plate electrodes, so causes the in the neighboring of the discharge point Surface parts of the discharge current flowing to the discharge point such a reduction the voltage potential so that it is below the value required to maintain the discharge falls. Since the tension is already at a very small

009 590/271

The discharge of charge is sufficiently reduced, the spark becomes harmless in this way made. Known as membrane materials are animal skin - so-called gold bat skin - and certain semiconductors.

In spite of the improvement of the loudspeaker properties achieved with such membranes with regard to the background noise and despite the advantages which the known membranes with regard to the simple contact values in the order of magnitude of a few thousand megohms, however, can also be used. If the resistance of the membrane is driven too high, the second condition of uniform occupancy with charge carriers - even with peripheral feed - is no longer met. The upper limit is determined by the conductivity of dry air, which is between 10 ¹⁵ and 10 ¹⁶ ohm / cm. The clearances between mem-

limited to pronounced tweeters in small dimensions; its efficiency was low, the distortion factor was still too high for high outputs and / or frequency ranges.

The invention relates to an improved electrostatic loudspeaker which - without the above mentioned disadvantages - the use of high polarization voltages allows and its re-

with a very small distortion factor. A "full-range" loudspeaker according to the electrostatic principle seemed quite impossible in the past.

The membrane can also consist of a coated plastic film, as is known per se. It is advantageously only held by the peripheral electrode feeding in the polarization charge.

The upper limit for the value of the polarization

The construction of the device has the electrostatic io bran and external electrodes on which the Polari loudspeakers still not be able to enforce. He is sationssspannungs, namely form a sideline on pronounced tweeters in the final path for the propagation paths on the and

through the membrane. Dielectrics such as shellac and rosin with a specific resistance 15 equal to that of air and greater are therefore unsuitable, because the charge density decreases towards the center of the membrane.

The subject matter of the invention is therefore a peripherally fed semiconductor membrane which consists of a plastic gift There is almost 20 foil over an enlarged frequency range, the electrical resistance of which is so great frequency linear and almost free from distortion. is that each on the surface elements of the mem-

The invention is based on the knowledge that the electrical charge present in the branch remains constant the resistance of the loudspeaker membrane within in spite of the vibrating with the sound frequency Certain critical limits must lie in order to constantly change the capacitance of a membrane The sound of the arrangement extends over a wide frequency band, but is small enough to accommodate a High-performance speakers with a very low distortion factor, even distribution of the from a peripheral to accomplish. From the point of view of function, the polarization charge fed in can be via the electrode Express this in such a way as to allow for a flawless sound membrane.

playback even with large amplitudes, i. H. for loudspeakers built in series according to this rule

The linearity of the loudspeaker even at medium and 30 frequencies work over a frequency range from 45 Hz to low frequencies, as even as possible and 18,000 Hz almost linearly in frequency (drop-6db) and constant occupancy of the membrane with charge carriers
Requirement is.

With regard to the electrical resistance of the membrane, two considerations must be made 35 will:

The resistance has to be great enough to
any electrical charge attached to any
Body is present on each side of the membrane, im
to keep essentially constant when the membrane is 40 potentials partially due to the onset of the audio frequency oscillates. As is well known, with every ionization in the spaces between the membrane loudspeaker, the membrane does not act like a rigid electrode and the plate electrodes determine what piston, because it requires holding organs and because the acoustic resistances at approximately 70VoIt each 0.025mm of the distance are not always even between each of the plate electrodes and the meme are distributed over their entire surface. While the 45 bran electrode is the case.

Throttling of sparking a membrane resistance. Further details result from the ^{following description of some exemplary embodiments in the order of magnitude of 10 5} Ohm / cm, such as a gold bat skin or thin soot and the drawing. In this show layers, because then the charge carriers Fig. 1 and 2 each have a schematic representation of the

are already difficult to move, must for the inven- 50 construction of known electrostatic loudspeakers, in accordance with the purpose of the membrane resistance. Fig. 3 an electrostatic loudspeaker stage.

Lich can be chosen higher in order to achieve that the measure of the invention, partly as a circuit diagram and at any point, d. H. on a surface element partially as a longitudinal section through a loudspeaker, The charge sitting on the membrane remains constant, even with the thickness of the electrodes - for the purpose of better if the capacity relationships or the field - 55 clarity - is exaggerated,

forces against the vibrating diaphragm constantly Fig. 4, 5 and 6 each have a partial section through a

change. Well then, when the charge is on the mem- um-converted embodiment of a loudspeaker the The burn surface can no longer fluctuate, Fig. 3,

Attractive forces proportional to the alternating voltage, Fig. 7 shows a longitudinal section through a

otherwise, they follow a quadratic law that divides electrostatic sounds into 60 frequency ranges with larger amplitudes in audible distortion speakers.

gen makes noticeable. _ An electrostatic loudspeaker according to the

On the other hand, the upper limit for finding the value consists, for example, of a number of of resistance is determined by the need for loudspeaker units, each of which is a flexible one that the polarizing charge is uniformly used over 65 membrane 21 (Fig. 3) made of a film material entire membrane electrode must be able to spread. sits, such as B. from a plastic film of suitable resistance values on the order of some tem resistance - e.g. nylon - which - to achieve a hundred megohms per cross-sectional unit are sufficient - a suitable resistor - also coated so was found - can be normal for all speakers. The speaker diaphragm 21 is smaller Size requirements; Against the distance between two nearly parallel

Plate electrodes 22 and 23, for example rectangular in shape and about 15-75 cm in size clamped. The distance between the speaker cone and the plate electrode !! is so big that the membrane can oscillate freely at the greatest oscillation amplitude that occurs. The plates 22 and 23 can be made of metal or some other electrically conductive material. Your size will due to structural conditions and / or due to the only in the outer electrodes; a membrane electrode resistor, which is large enough to satisfy the inventive rule would be much too high than that the membrane electrode according to the arrangement I with the low frequency currents without considerable Attenuation and distortion could be fed. The use of the power supply strips 25 recommended because of the high resistance of the membrane electrode. When the polarization potential

acoustic requirements. Namely, suitable values io at a single point or at some of the electrical resistance of the loudspeaker meme points at the edge of the membrane electrode

"should and if the

bran are given below.

The plate electrodes 22 and 23 are so thick that they are practically rigid, and both are - as at 24 shown - provided with holes through which the acoustic waves exit and a damping of the Vibrations is prevented. The speaker membrane is held taut that they along their entire edge between "would have to be electrically conductive" and if those in between If parts of the edge were only separated from the plate electrodes by the insulating strips 26, then the Leakage current from the membrane electrode through the insulating strips 26 a uniform charge prevent the membrane electrode. The interposition of the electrically conductive feed belts 25 holds between membrane electrode 21 and insulation

the elements pinched, which is the shape of ao these leakage paths from the one high cons

Ribbons or foils 25 have. These conductive elements are in turn between the edges of the plate electrodes 22 and 23 pinched, with strips 26 made of plastic, e.g. B. Polythene or another suitable insulating material, are arranged between the feed belts 25 and the plate edges such that the loudspeaker diaphragm 21 is isolated from the plate electrodes 21 and 23.

The electrically conductive strips 25 should - as in
3 and 4 - protrude inwardly over the iso 30 of another material, as in FIG.

Plates- ~ -......-.-

stand away from the membrane electrode and conducts the leakage current directly to the voltage source, where it cannot impair the proper functioning of the loudspeaker.

If the membrane area is small compared to the emitted wavelength, the frequency response be equalized by the fact that individual parts of the membrane are weighted, be it through thickening or by attaching button-like parts

currents between the membrane 21 and the electrodes 22 and 23 to prevent what about a free part of the membrane would cause a voltage drop; because such a voltage drop would prevent the full polarization voltage from taking effect on the membrane, which reduces the performance of the loudspeaker unit would make it insufficient. For electrical connection with the plates 22 and 23 serve the connection terminals 22 'and 23', while the connection terminal 21 'with the power supply strips 25 and is therefore connected to the membrane 21. The loudspeaker units can be placed side by side - with the outer surfaces of the Plates provided with holes lying in one plane - can be assembled.

The two plate connection terminals 22 'and 23' of each loudspeaker unit are connected to the ends of the Secondary winding 28 of a parts shared by all units relatively "stationary, and the Vibrations take place mainly in the fields of lesser mass in between, of which an effective radiation takes place. The membrane electrode vibrates at the low frequencies as a whole, with the larger mass lowering the bass resonance frequency.

In the exemplary embodiment according to FIG. 3, some fields which do not contribute to the sound emission have the membrane has a harmful capacitive resistance. These fields are practical on the one hand formed by the plate electrodes 22 and on the other hand by the feed strips 25, wherein the insulating strips 26 act as a dielectric. To prevent such a loss, the in Fig. 5 shown modified embodiment can be applied.

In this embodiment, the plate elec-

seeds of the low-frequency output transformer 29 take the form of metal layers 34 attached to the

bound, whose primary winding 30 through the low frequency amplifier output stage 31 is excited. Each aggregate is polarized by a source 32 of high voltage, for example a few hundred Volts per millimeter of distance between the membrane connection terminal 21 'and a center tap 33 of the Secondary winding 28 of transformer 29 is connected; as a precaution, it will be easier accessible of the two plate electrodes, e.g. B. the plate 23, grounded.

If the charge on the speaker diaphragm 21 is during a half period of the lowest occurring Frequency can practically not change, you can use a simpler circuit, and the polarizing high voltage source can be replaced at the center tap at each end of the secondary winding " 28 -be connected.

The circuit diagram is that described at the beginning of the arrangement II (Fig. 2), i.e. the low frequency interior Areas of rigid insulating disks 35 are attached, between the edges of the power supply strips 25 and the membrane 21 are clamped in, as has already been described in connection with FIG. 3. the Carrier plates 35 and 'the metal layers 34 are - as shown at 36 - with holes for the clay outlet Mistake. The metal layers 34 have a certain distance 37 from the edge of the membrane 21, so that no parts of the plate electrodes formed by these layers with the power supply strips 25 come into contact. Therefore, no part of the low frequency signal goes through capacitive loss lost.

The distance 37 between the edge of each metal layer 34 and the inner edge of the power supply strip 25 on the same side of the membrane electrode 21 should be large enough to cause an ionizing discharge to prevent.

For electrostatic speakers where the

Currents that excite each loudspeaker unit flow 70 audio frequency field applied between the fixed plates

is, the working conditions are occasionally such that the voltages applied are so high that a Spark transfer between the solid plates can occur through the membrane. This damages the membrane and is acoustically highly undesirable. Although it is possible to do so by embedding To prevent the solid plates in insulating material, difficulties arise in practice Manufacture of perforated, shielded electrodes when the shield is to be closed.

By using a perforated dielectric plate 41 (see Fig. 6) of high permeability, which is placed between each fixed electrode 42 and the membrane 43, the field conditions be influenced so that the field strength is formed for the most part in the air gap, so that thereby essentially the working conditions of the speaker are not changed.

However, this increases the distance between the fixed electrodes and the field strength in the air between the conductive surfaces through the perforations is significantly reduced. Now in order to maintain the polarizing field in the gap and around the dielectric plate 41 in front To protect against undesired charging, the material of this plate must be sufficiently conductive and are in electrical contact with the subsequent fixed electrode.

Electrostatic loudspeaker units according to the present invention are particularly suitable for so-called loudspeaker columns. An example is shown in FIG. 7, which shows a cross section through a shows circular loudspeaker, again the thickness of the electrodes is exaggerated. The circular one Membrane 71 is made conductive at two separate parts; it has an annular part 72 for the reproduction of the lower frequencies and a circular part 73 concentric with it for playing the higher frequencies. So in reality there are two separate membrane electrodes

72 and 73 available.

The corresponding plate electrodes are similar to those already described and shown in FIG built up. The plate electrodes cooperating with the membrane electrode 72 are in the form of a annular metal layer 74 which is attached to the inner surface of the rigid insulating material plates 75. Annular Power supply strips 76 serve - as described above - to the high potential to the To apply the periphery of the membrane. The openings 77 are used for the exit of sound.

The plate electrodes cooperating with the diaphragm 73 are circular in shape Metal layer 78, which is also supported by the insulating plates 75. Ring-shaped power supply strips 79 are on the periphery of the membrane

73 attached.

The plates 75 are shaped so that the metal layers 78 of the associated membrane 73 are closer, than the metal layers 74 of the associated membrane 72 are removed, since the latter is a larger one Requires vibration amplitude. Of course, these different distances also make the creation different Polarization potentials required. There are no difficulties in practice because the lead strips 76 and 79 are electrically isolated from one another.

Claims (7)

PATENT VISIONS: 1. Electrostatic loudspeaker with a membrane made of a semiconductor, which between two perforated plate electrodes arranged to swing freely and connected to a polarization voltage source is connected, characterized in that the membrane (21) consists of a plastic film, the electrical resistance of which is so great that any electrical charge on the surface elements of the membrane (21) remains constant despite the constant caused by the membrane (21) vibrating with the audio frequency Changes in the capacity of the arrangement, but small enough to be evenly distributed the polarization charge fed in from a peripheral electrode (25) across the membrane (21) to enable. 2. Electrostatic loudspeaker according to claim 1, characterized in that the plastic film is covered with a layer, the electrical resistance of which is within the prescribed range Area lies. 3. Electrostatic loudspeaker according to claim 1 or 2, characterized in that the Membrane (21) only from which the polarization ¹ charge-feeding peripheral electrode (25) is held. 4. Electrostatic loudspeaker according to claims 1 to 3, characterized in that the rigid plate electrodes (22) provided with holes (24), breakthroughs or the like with their Edges the peripheral electrode with the interposition of a frame-shaped insulation (26) Clamp (25). 5. Electrostatic loudspeaker according to claims 1 to 4, characterized in that the peripheral electrode (25) inwards over which it separates from the rigid plate electrodes (22,23) Insulation (26) survives, so that leakage currents between membrane (21) and insulation (26) or plate electrodes (22, 23) can be avoided with certainty. 6. Electrostatic loudspeaker according to claim 1, characterized in that between a perforated dielectric screen (41) is arranged between the membrane (43) and each plate electrode (42) and is conductively connected to the electrodes, so that the build-up of an undesirable Charge on it is prevented. 7. Electrostatic loudspeaker according to claims 1 to 6 with in two different frequency ranges divided membrane, characterized in that its membrane (71) has a central, circular part (73) for high frequencies with a small spacing of the circular disk-shaped Has electrodes (78), which is of an annular part (72) for low frequencies is surrounded, which on both sides by circular electrodes (74) at a greater distance is flanked. Considered publications:
German patent specifications No. 573 873, 531 299, 339 British Patents Nos. 370 248, 348 573, 744, 222 507. 1 sheet of drawings © 009 590/271 8.60