US2631196A

US2631196A - Electrostatic loud-speaker

Info

Publication number: US2631196A
Application number: US119639A
Authority: US
Inventors: Arthur A Janszen
Original assignee: Individual
Current assignee: Individual
Priority date: 1949-10-05
Filing date: 1949-10-05
Publication date: 1953-03-10
Anticipated expiration: 1970-03-10

Description

March 10, 1953 A. A. JANSZEN ELECTROSTATIC LQUD SPEAKER 3 Sheets-Sheet 1 Filed Oct. 5, 1949 Fig.l

March 10, 1953 A. A. JANSZEN 2,631,196

ELECTROSTATIC mun SPEAKE Filed 001:. 5, 1949 s Sheets-Sheet s SPEAKER ,yg

INPUT 6 TERMINALS (F =F T "1 Fig.7

T0 44 STATIONARY ELECTRODE SPEAKER INPUT TERMINALS 6 8 a T T 2 1 INPUT TERMINALS i l l l l.lu l.l ll l 50 lgVENTO/i Patented Mar. 10, 1953 UNITED STATES PATENT OFFICE 2,631,196 ELECTROSTATIC LOUD- SPEAKER Arthur A. J anszen, Cambridge, Mass.

Application October 5, 1949, Serial N 0. 119,639

The present invention relates to electroacoustical transducers and more particularly to loudspeakers.

Because of the inherent simplicity of a system in which a driving force is essentially constant over the area which is to vibrate (in contrast to the application of force at or near the apex of a conical structure used as a diaphragm in the conventional electrodynamic loudspeaker), many attempts have been made in the past to develop a loudspeaker'utilizing the electrostatic principle, in which the electrostatic forces existing between essentially parallel surfaces of a condenser are used to set one of the surfaces in motion. So far as I know, however, no electrostatic loudspeaker having adequate protection against electrical breakdown and having satisfactory frequency response, power output, and polar energy distribution characteristics, has been developed. Bass response has been poor because of the use of diaphragm materials which have low compliance, the use of small vibrating segments into which the total vibrating surface was broken up, and the presence of trapped air between the vibrating surface and the stationary surface. This trapped air constituted a capacitive reactance which added to the effective stiffness of the vibrating system. High frequency response (greater than 5,000 C. P. S.) has been poor because of the high mass per unit area of the moving surface, and the difiiculty of efiecting an efficient coupling to ordinary power amplifiers at high frequencies because of the fact that the impedance looking into the loudspeakers has been essentially the capacitive reactance of a condenser.

Furthermore, when diaphragm areas sufiicient for the production of high acoustic. energy levels at low frequencies were used, the polar distribution of energy at the high frequencies was unsatisfactory because of the highly directive properties of flat radiators at frequencies where their circumference is many times the wavelength; in other words, the speakers were too directive at high frequencies.

The principal object of the present invention is to obtain a high acoustic power output per unit area over the wide range of audio-frequencies necessary for effective loudspeaker operation.

With this object in view, one of the principal features of the present invention comprises an" acoustically transparent stationary-or back electrode which improves the high frequency response by allowing the diaphragm to vibrate at the driving frequency without hindrance due to 7 Claims. (Cl. 179-111) entrapment of air. In one form of the invention the back electrode comprises a mesh or zig-zag of Wire in which the successive lengths are sufficiently close together to provide a substantially uniform electrical field at the diaphragm. In view of the high polarizing and driving potentials necessarily used in an electrostatic speaker, the wires are preferably coated with a high dielectric material in order that the diaphragm may not have to assume any insulating function and may be as thin as possible.

Another feature of the invention resides in a construction whereby one or both of the electrodes constituting the condenser of the transducer are provided with insulation of such character that the air gap canbe operated at or near its ionization potential. While the use of solid dielectric materials in loudspeakers and microphones is not new, it has been used mainly to provide points of support for the diaphragm. So far as I am aware, it has not previously been proposed to operate the air gap at or near ionization potential. As will be pointed out in the specific description of the apparatus according to the present invention, the realization of this feature requres the use of a dielectric material having certain qualities, namely, high dielectric strength, high dielectric constant, and an appropriate leakage resistance. Various materials may be used for this purpose, but a polyvinyl chloride dielectric has been found satisfactory in practice.

A further feature consists in electrically dividing the diaphragm into relatively small areas which are separately excited ,throughnet'works of such character that a'desirable frequencyresponse characteristic, polar energy distribution characteristic, and impedance characteristic can be'obtained.

Other features of the invention consist of oertain novel features of construction and combinations and arrangements of partshereinafter described and .particularly defined in the claims.v

In the accompanying drawings, Fig. 1 is a front elevation of one form of loudspeaker according to the present invention; Fig. 2 is a sectional detail view of the electrodes; Fig. 3 is an enlarged detail view showing the spacers;'

Fig. 415 a plot of the impedance characteristics; and Figs. 5 and 6 are diagrams of equivalent networks of the speaker; Fig. '7 is an elevation of a modified form of the invention; Fig. 8 is av diagram of another equivalent network; and Fig. 9 shows another modified form of the invention. r

The loudspeaker according to the preferred a 3 form of the present invention as shown in the drawings comprises a back electrode indicated generally at 6 and a front electrode or diaphragm 8. The back electrode is in the form of parallel stretches of wire threaded back and forth between pins 9. The individual lengths of wire are insulated with a dielectric material indicated at H] in Fig. 2. While the individual lengths or passes of wire are close together, there is sufficient space between them to prevent entrapment of air and the back electrode may therefore be considered as acoustically transparent. The lengths; of wire are so mounted that the entire back elece. trode has only a small inductance and the entire electrode assumes practically a uniioifm llql ential, throughout its entire area at all frequencies within the audio range.

The front electrode comprises a diaphragm; preferably of rubber chloride (Pliofilm) in a very she t, wh ch. m y b b t; .0 04 h k: 1.65 The electrode is mounted in a suitable. frame and Stretched taut at a small distance in, front of the back electrode 6. Thediaphragm sv sp c d. from. th back e e r de byme ns of we ten s-sh wn as pie s of di l c c a erial. Th spacers a e-about -04; inch in e ght. and a e ir g lar y distrib ted v r the sur ce of. the electrodes whereby the diaphragm is me-. chamcel y iyidedintoir s ar a s d s g e n. a. generalway by thenumerals 14, t etc. Fig. 3,. Theseareas. are of non-uniform size and configu a ons.- a d, non-symm trical wh reby; h ir n tur i equencies d ner sli h ly rom-cam v anothe dienh eem. is Pro ide h a csndu ns. coating, preferably in the nature of a dag coating n. w i hi the gele izi g and r in p e t al-s. are BR iQ- As. w ll c de-s bed. ater. heeqet ne s. rzreIe h y pplied. n. a. manner o; divi e th iaphragm. ele t ically n o. areas hayi g. ect e re pons s; in dif erent requ nc ban pa t. iron; this; featu e howev r. the. transducer; s shown in n a eedsecti n nEie- 72.5

n F g: sev re, re ches f the rire 6; or. t e: ccnductina baekn a are shown... ea hw reb ns.

' ith.asheath-v Q: of.

hitheff r, re s ns t9. h rexnla edlateLis pref:-

m lxe t elw .%9.IL ma be, m de-h pre erred dimensipna. namely: wire diameter 0.015"; diameter-ofsheath. -04.07 air n 4.1%! 21l li! ,ess:ofdiaphragm 0 ,0o04

e; neratien Qt.- the peaker he principal:

e1 ises item the. fact, that. the dielec: .i ,te l, l fish s an: appreciable. but not. infie, ta, leaka e resistance- This; is. indicated by. the equivalent circuit of the loudspeaker which.

sh wn; n; 51s.. 5; which:

Cd is the capacity between the two surfaces of. he d hr G is. h ci y. betw en he nner s rf e of the, diaphra m a d... he ut r su face f he pqlyvi yl h eri e sheath. 1- t e ai sea capacit 5. s, he an i st between e stationar el ctmde conductor and the outer surface ofv the polyviny hloride. e, andvRea e the r spectiv z e a eres sts; ances cr sshe h e an citan s,

es 1 2mm Ep is the polarizing voltage, t. the cu rent. th l9ulsnee ran ectric material;

1' hloride. While exact dimene. inven ion.

he seri s. esistance is lating he-p larizin Iac is the A. C. current in the loudspeaker due to the signal voltage appearing across the secondary winding of the output transformer of the power amplifier.

Typical values of Cd, 02., Cs, Re, Rs, and Ip which have been obtained experimentally are noted in Fig. 5. The values of the Us are such that practically all .2%). of the A, C si nal voltage appears; acros the air gap when the dimensions are those given in Fig. 5. The distribution of the D. C. polarizing voltage, however, is not determined by the relative values of the Us, but by the-relative values of the Rs, once a steady state condition exists, And the relative values of these Rs change since Re. is not constant, but is infihfifld @iHIiQtiQHQi-j the potential existing across it, decreasing as the potential increases. If the polyvinyl chloride were a perfect insulator, i. e., if its leakage resistance were infinite, no D. C. polarizing potentialcould existacross theair gap, since the surface of the poly-vinyl chloride would: attain the same potential as the surface of the diaphragm facing it, and therefore theacoustic output of the diaphragm would consist onlyof the second harmonic of the applied A. C. electrical signal; On the other hand, if the leakage resistancewere very low, the D. C. currents-which couldflow betweendiaphragm andthe conductor ofthe stationary electrode would be of amagni tude suflicient to causeinjurytocomponents of the loudspeaker. Therefore, the sheathing isone that has (1) highdielectric-constant, (2) moderately high leakage resistance, and 3 highdielectric strength.

Forthe conditions above described the polariz ing potential divides in such a manner that a substantially Q nstantpolarizing voltageappears across theaingap. In other words; an increaseintotal polarizing" potential betweenterminals increases the-ionization current and decreases the resistance of the air gap to such an extentthata the actual potential across the air gap is not materially changed,- whilethe potential across the dielectric sheaths isincreased.- It has-beenfound possible. in aspeakerof the dimensions; indicated above tomaintain a substantially 0011- stant air. gap polarizing voltage with a total polarizing-- potential anywhere inthe range ofabout 3500 to: 6800 volts. Insuch a case the actual voltage-across-the gap itseh is about 2100- volts. Thiscorrespendstoa voltage gradientof about 50,000 voltsper inch which is substantially in-t-he ionization range. The large electrostatic forces-which resultfrom the application ofsuch'a' high electric field together with the large separation between electrodes rnakeittpossible tool),- tain high acoustic, power output. per unit, area. over the entire range of audio-frequencies. By large en at on'l'" er to. a seuarati nsnmicient to permit the largediaphragm excursionsrreguired, for 0 r q ener nm n tmn, .atthe high e er y s. w h a ob ai i b1.ev by main aining. the; air gap gradient in or nearthe ionization range.

It has b en. stat d. above: that. hevv deg atin snr ferably not appl ed. uniformly erhe u facehut in areas whereby. th dianhr inl s dirvided into. areas moreon ess ele tively, l ssen:- siv'e to certain frequency bands. The areas are preferably of. g neral an ular shape.- l usaas shown n F g, 1) the e is asmell C ntral coated area; W n he form-of.asqnarer.asec ndarea.2&1. urtounding he cent al a ea, and anoutcr area; lemimdthe entiremers ni. The-areas. lit-land; 2.9.; ress getetesii by; anmn atedzpertion; mend;

the

areas

20 and 22 are separated by an uncoated area 26. By this means the effective radiating area of the diaphragm may be made a function of frequency, since the various areas may be formed as portions of an electrical network. Thus, a direct lead 28 is preferably run to the inner area [8, a second lead is run to the intermediate section 20 through an impedance 3%, and a third lead is connected to the outer lead through .an impedance 32. The

impedances

30 and 32 may be pure resistances, and may be constituted, in part at least, by the resistance of the dag coating.

Since the dag coating does not have a high current-carrying capacity, a stripe of conducting silver paste is preferably coated on each dagcoated area as indicated at 34, 36, and 38, in order to conduct the current with a better distribution to each of the rings.

A network showing diagrammatically the effect of the division of area is presented in Fig. 6, wherein C1 represents the capacitance of the central portion l8 with respect to the back electrode and C2 and 03 represent the capacitances of the rings 2!] and 22 with respect to the back electrode. The resistances 3B and 32 are in series with C2 and C3 respectively. This network diagram'is not to be confused with Fig. 5 since Fig. 5 represents the equivalent network in section, that is, across any portion of the speaker, whereas Fig. 6 represents the distribution of the resistance and capacitance over the surface. As illustrated by Fig. 6 the higher frequencies are largely limited to the central portion I8, while the intermediate frequencies are restricted to the central portion l8 and ring 20, and the low frequencies are radiated by the entire area. This has a two-fold result: first, the impedance looking into the speaker is more nearly constant with frequency, and second, the smaller radiating area for the high frequencies results in a lower directivity. In respect to the latter, it is apparent that if the high frequencies were allowed to radiate from the entire surface, the speaker would be highly directional. The smaller radiating area afforded by the central portion for the high frequencies spreads the angular pattern into a satisfactory angle.

The electrical division of the diaphragm into a central zone and annuli is not essential, since the division may be effected in the form of bands, or as square, rectangles, circles, etc., but preferably in regular geometrical shapes. A segmentation of the type shown in Fig. 8 is satisfactory and has the advantage of permitting connections to be made to the sides of the segments. In this form the corner segment 40 is the radiating segment for the high frequencies, while the

segments

42 and 44, which partially surround the corner segment, are added electrically at the lower frequencies. Network connections are made as in Fig. 1.

The impedance characteristic of the speaker is illustrated in a general way in Fig. 4, wherein the logarithm of the impedance is plotted against the logarithm of frequency. If it were not for the electrical division of the speaker into different zones, the theoretical impedance frequency characteristic would be a straight line, indicated by the dash line. However, by reason of the characteristics of the network, the high-frequency impedance is increased, and the characteristic assumes the full line position shown in Fig. 4. The diaphragm may be electrically divided into any number of areas and the network characteristics may bedetermined according to familiar principles of network theory, whereby widely varying impedance characteristics may be obtained. It has, however, been found that the use of three zones as shown in Fig. 1 is usually.

adequate for satisfactory reproduction.

An alternative network for the speaker is shown in Fig. 8, wherein the.

resistances

30 and 32 are connected to form a ladder network, by which a similar impedance characteristic may be obtained.

A modified, and in some respects preferable, form of the invention is shown in Fig. 9. This form of the invention acts to maintain a substantially constant pressure response while providing effective non-directional action at high frequencies. In general, this result is attained by varying the acoustic power output per unit area of the diaphragm, preferably by slightly dishing either the diaphragm or the back plate, so that the high frequency segment has a closer spacing'than the outer annuli. This arrangement is analogous to the type of segmentation shown in Fig. 1. For an arrangement analogous to the segmentation of Fig. 7 it is only necessary to use a flat diaphragm and a flat back plate which are angled to be relatively close together at the upper left corner and farther apart elsewhere.

In Fig. 9, the lateral spacing between diaphragm supports preferably varies with separation. Accordingly the spacers 5B in Fig. 9 have a closer lateral spacing in the central portions than at outer extremities. Thus the lower frequencies are excluded by mechanical filtering action, from the central portion of the diaphragm.

The frequency distribution over the surface of the diaphragm may also be accomplished by reducing the acoustic transparency of the back plate in selected areas, as by placing the wires 6 very close together at one or more portions; this has the effect of increasing the high frequency response relative to the low frequencies in the areas of reduced transparency, and gives a frequency-selective network, in which account is taken of the effects of mechanical stiffness which is added by the entrapped air.

The speaker may be of any desired size, and is preferably operated at a high polarizing potential, which may be about 3500 volts. The frequency response characteristic i highly satisfactory as compared with conventional speakers since it presents fewer sharp peaks and hence results in an improvement in respect to phase distortion,

While the preferred form of the invention has been described it will be understood that it may be varied in respect to detail. The principal features of the invention are, first, the acoustically transparent back electrode which is insulated in such a way as to permit the operation of a large air gap at or near the ionization potential of the air while at the same time it permits the appearance of a high percentage of the A. C. signal voltage across the air gap; second, the me chanical division of the diaphragm into irregular and non-symmetrical areas of non-uniform size for the spreading of resonant frequencies; and third, the electrical division of the diaphragm into zones or areas whereby the high frequency impedance is increased and the directive characteristics of the speaker are improved.

Having thus described my invention, I claim:

1. A loudspeaker comprising a stationary electrode, a thin diaphragm separated from the stationary electrode by an air gap, dielectric material completely enclosin the, stationary electrode. while maintaining it.acoustically.transparentand having part thereof. between the stationary 91GB: trode and the air gap, and means for applying a polarizing voltage between the stationaryelec: trode and the diaphragm, the dielectric material having a high dielectric constant and a. moderate leakage resistance, the polarizing potential being sufficient to maintain the air gap at a po, tential which is at or near the ionizing potential of the gas, the greater part of the signal volt age appearingacross theair gap;

2. A loudspeaker comprising a stationary electrode of mesh-like structure whichis acoustically transparent, a thin diaphragm, spacersfor sepj arating the diaphragm from the stationary electrode by an air gap, dielectric material completely enclosing the mesh structureand having part thereof between the stationary electrode and the air gap, and means for applying a polarizing-voltage between the stationary electrodeandthe diaphragm, the dielectric material having a high dielectric constant and a moderateleakage resistance, the polarizing potential being sufficient to maintain the air gap at a potential which is at or near the ionizing potential of the gas, the greater part of the signal voltage appearing across the air gap.

3. A loudspeaker comprising a stationary electrode consisting of a number of closely spaced but separated wires, a diaphragm separated from the stationary electrode by an air gap, sheaths of dielectric material completely enclosingthe individual wires without filling the spaces be.- tween them, and means for applying a polarizing voltage between thestationaryelectrode and the diaphragm, the dielectric material havinga high dielectric constant and a moderate leakage resistance, the polarizing potential being sum? cient to maintain the airgap'at a potential-which is at or near the ionizing potential of the gas, the greater part of the signal voltage appearingacross the airgap.

4. A loudspeaker comprising a stationaryelectrode, a thin diaphragm separated-from thestationary electrode by an airgap, dielectric material completely enclosing the stationary electrode while maintaining it acoustically transparent and having part thereof between the stationary electrocle and the air gap, and means for'applying' a polarizing Voltage between thestationary elec-- trode and the diaphragm, the dielectric material having a high dielectric constant and a -moderate leakage resistance, whereby the airgap may be maintained under a polarizing potential in the neighborhood of the ionizing potential and the greater part of the signal voltage appears across the air gap, and means for establishing a; non-uniform frequency distribution overthediaphragm..

5. A, loudspeaker comprising a stationary .elec: trade, a thin diaphragm separated from the sta: tionary electrode, by an air gap, dielectric mate-.- rial. completely enclosing the stationary electrode while maintaining it acoustically transparent and having part, thereof between the stationary elec-.- trode and the air gap, and means for applying a, polarizing voltage between the stationary electrode and the diaphragm, the dielectric material having: a high dielectric constant and a moderate leakage resistance, whereby the air gap may be maintained under a polarizing potential in the neighborhood of the ionizing potential and the greater part of the signal voltage appears across the air gap, and means for confining high frequenciesto portions of the diaphragm.

6. A loudspeaker comprising a stationary electrode, a thin diaphragm, and a plurality of spacing means for separating the stationary electrode and the diaphragm to form an air gap between them, said spacing means being arranged to provide less separation over some portions of the total area than over the remainder of the diaphragm and having closer lateral spacing in the region of less separation, whereby some portions having a small air gaphave a relatively small free diaphragm area and other portions have a wider air gap and a correspondingly larger free diaphragm area.

7'. A loudspeaker comprising a stationary electrode,v a thin diaphragm, a conducting coating for the diaphragm, the diaphragm being electrically divided into a small zone and at least one larger zone, network means for substantially confining high frequencies to the small zone, and spacing means for separating the-stationary elec-- trode and the diaphragm, said spacing means being variable to provide a smaller separation in the high frequency zone than elsewhere, said spacingmeans also having closer lateral spacing in the high frequency zone.

A. JAN SZEN.

GES CIT ED The following references are of record in the file of this patent:

D A E AT N S