US 2935575 A
Description (OCR text may contain errors)
L. J. BOBB LOUD-SPEAKERS May 3, 1960 3 Sheets-Sheet 1 Filed Aug. 20, 1957 INVENTOR. 1 1101 0 d 305! ATTORNEY y 1960 I L. J. 5088 2,935,575
LOUD-SPEAKERS Filed AUE- 95 I5 Sheets-Sheet 2 INVEN TOR. 110;0 5065 LOUD-SPEAKERS Filed a- 20, 195'! s Sheets-Sheet 3 INVENTOR. [mm d 5038 therein and to which the spacers 17 are suitably attached. Slight irregularities may be present in the plate surface, but, as a practical matter,-adequate tensioning of the vibratory membrane 12 can be established when the plate 14 is curved, for instance if it has arise of 31,; inch an arc of 3 or 4 inches length. The membrane sleeves 12 can be made of thin, strong, insulating sheet material such as that known as Mylar, a type A polyester manufactured by E. I. du Pont de Nemours and Co., with a suitable conductive coating 12' thereon.
Desirably, and according to known principles, the thickness of the coating 12' amounts only to a small fraction of a mil; that of the entire membrane 12 amounts to a larger fraction of 21 mil; and the thickness of the flat air space B (Figures 3 and 5), which is maintained by the spacers 17 vantages of the present invention. It has been found to be necessary to limit that dimension of said space which is measured between the membranes 12, 12A, to'a value which is generally smaller than the wavelength of the harmonics to be suppressed.
It may be noted that due to the curvature of the singleended, oppositely arranged speakers shown in the drawing,
said dimension or distance varies to some extent, depending on the exact point of a membrane area from which point the said distance is measured. This variation is only of secondary importance for the present invention. The point of major importance is that the distances across the air cushion, gap or space should be smaller, over at least a major part of the membrane areas, than the shortest wavelengths of the audible harmonics to be suppressed.
If the speaker system serves to reproduce both music and speech, the significant harmonics, in this sense, may range up to about 15,000 to 20,000 cycles per second, corresponding to fundamentals of up to about 7,500 to 10,000
cycles. The wavelengths of the fundamentals are in the 'range of about .6 to .9 inch and those of their second harmonics are less than about .3 to .45 inch. For this which has a slight cylindrical curvature incorporated 4 shown, with those of the backplates 14, by conductors 22'. As the two speakers face in opposite directions, the polar izing forces bias the two membranes in opposite directions, relative to the air cushion A.
Signal voltages, in push-pull, are provided by the trans former 25 forming Part of the output circuit of a suitable signal amplifier, and are superimposed on the polarizing voltages by leads 22" connected for instance, as shown, to the conductive membranes (Figures 1 and 5). The electrical signals and their driving effects are schematically illustrated in Figures 6-A to 6-D. Particularly, the curves S, 8-1 (Figures 6A and 6-D) show respectively the ways in which voltages rise and fall between the membranes of sections 10, 11 on the one hand and their respective backing plates, on the other, as each membrane moves from one of its positions (full lines in Figure 5, points X, X-l in Figures 6-A, 6-D) to another (broken lines in Figure 5, points Y, Y-1 in Figures 6-A, 6-D), and then back to the original position (full lines in Figure 5, points Z, Z-l on the curves). Sound is emitted both forwardly and rearwardly (upwardly and downwardly in Figure 5), and of course, suitable, well-known procedures of reflection and/or absorption may be used to utilize and direct the sound, as may be required. The present invention is concerned with the quality rather than with the direction of sound, produced by the system of single-ended speakers. In this connection the following may be noted.
Each individual, single-ended speaker 10 or 11 could be used by itself or both could be used in arrangements dilferent from those according to the present invention. In such cases, however, undesired even harmonics of the signal frequencies, including strong second harmonics thereof, would occur in the acoustic vibration generated by the electrostatic signal, because of the super-position of the signal S or 8-1 on the polarizing voltage P or P-l. In other words, the individual speaker would generate a distorted acoustic output curve D or D1 (Figures 6-B, 6-C), having non-linear response to the signal curve S, 8-1. The second harmonics H, H-l would be among the disturbing influences, superimposed over linear acoustic response curves L, L-1. While this has been known for a long time, I believe the present system is the first arrangement of single-ended, electrostatic speakers which I eliminates the disturbance in an effective and economical reason it will usually be found that the advantages of the "present invention are achieved if the width of the air space between membranes 12, 12A, over a major portion of membrane areas, is less than about .3 to .45 inch; at any rate,- less than about a half inch.
Each single-ended speaker 10, 11, as shown in the drawing, is desirably curved to form an approximately cylindrical segment. As a result of this curvature, and in view of the preferred, mutually facing arrangement of the curved speakers 10, 11 (Figures 2, 5), the system 'asshown has lines of minimum distance between membrane areas 12, 12A, parallel to lines of maximum dis- I tance therebetween and these latter lines extend along the side edges of the system. It is clear that the minimum distances, in a system as shown, are substantially smaller the areas. These minimum distances may be limited to any suitable, small value, for instance to about five or ten mils, in order to facilitate the construction of the speaker.
' In operation, both sections or speakers 10 and 11 havethan either the marginal or the average distances between 4 way.
Prior attempts to eliminate the disturbance, in singleended speakers, were based on the use of a very large and correspondingly expensive speaker unit. This was based onthe ,fact that the distortion was particularly strong when the ratioof signal voltage to polarizing voltage was relatively high. Such ratios were heretofore made as very low, for instance such as 1:10. For this reason all prior, single-ended speakers, and mainly those used for reproducing frequencies extending into a relatively low range, had to be relatively large and expensive.
In the operation of the present speaker unit, by contrast, the thin air cushion A, between two speakers 10 and 11 driven in push-pull, serves as a close acoustic coupling medium, which on the one hand causes the separately driven speakers 10 and 11 to vibrate in unison, in direct response to the signal voltages, while on the other hand cancelling the disturbing even harmonics. From an inspection of Figures 5 and 6-A to 6-D it will be clear that this unique effect is entirely due to the close acoustic coupling A between the singleendedly constructed, double-endedly arranged and operated speakers 10 and 11. For instance, as the electro' static attraction of the membrane 12-A increases from level X toward level Y, tending to produce the first part of non-linear acoustic response D, the simultaneous decrease of electrostatic force, from X-l, to Y-l, precisely by tending to produce another disturbed acoustic reaction D 1the two disturbances being equivalent to harmonics H and 11-1 in opposite phase-cancels the ties therein.
'cally tensioned, double-ended unit, =the cost of which would be many times greater and the life expectancy of which would be substantially shorter than that of the present pair of speakers. *Even more favorable is the comparison of this present pair of speakers with prior systems of single-ended speakers. Those systems, as noted above, had to utilize a low ratio of signal voltage to polarizing voltage and a correspondingly large membrane area, and they were still unable to fully eliminate'the' harmonics.
In the present speaker unit, by contrast, the strength of the signals, relative to the polarizing voltages P, P- l, can be and desirably is kept rather high. For instance, the polarizing voltage may be maintained at a level such then have peak amplitude such as 100 volts or more, causing signal oscillations between the limits of 250 and 350 volts. correspondingly the present speaker unit can be made relatively small, even when used to reproduce frequencies which extend from the high range, conven tionally supplied by electrostatic tweeters, to a substantially lower frequency range, such for instance as 1000 or 500 cycles per second.
The greatest fidelity in the production of music, along with an extremely wide range of sound volumes, is obtained by the'present speaker if the width of the major as 300 volts and the signal voltage curve, as such, may" portion of the air space A between one membrance sec tion 12A, and the other, 12B, is substantially smaller than the wavelengths of the higher acoustic frequencies to "be reproduced. For instance, this space may some times be made smaller than one-quarter of the wavelength of the highest fundamental note to be reproduced.
This is readily achieved. As explained above, the desirable spacing, along the lateral margins of the membranes, may amount to a few hundred mils, and the spacing along the center lines of the membranes, as shown, may equal about five or ten mils. Such distances can be established with ease and they can be maintained for a practically indefinite period of time by spacers 23 and frames 13. v i p The fidelity achieved by these expedients is greatly superior to that of the best, conventional, single-ended speakers. Those latter speakers can, at best, reduce the even harmonics somewhat, and that only if the ratio of signal voltage to biasing voltage is made small and the speaker correspondingly large, expensive and cumbersome.
Double-ended speakers of conventional design, either large or small, have heretofore provided a fidelity which initially was comparable to that maintained by the present system, but this was possible only at much higher a cost. In addition, the fidelity once established is retained much more safely and over a much longer period of time metal from which the plate 14 is formed has irregulari- In conventional double-ended speakers,
such retention of fidelity has been lacking, even if extreme care was used in originally producing a highly regular backplate surface.
The speaker of Figures 1 to 5 can be modified many respects, so long as the paired, single-ended speakpush-pull drive.
and membrane 12, to apply pressure to the membrane in a less concentrated way, or on the other hand simplifying the system 19 (Figure 8) by replacing it with conventional spring means 50, secured to the egdes of sheet-type membranes 51 by clamps 52. It will be understood that the several speakers illustrated and described herein can be modified in many other respects. The invention dis- 1 closed herein is therefore claimed as follows:
I claim! 1. Loud-speaker apparatus for reproducing sound with high fidelity and for maintaining such fidelity for a long time, said apparatus comprising: a pair of electrostatic loud-speakers, each of single-ended construction, each comprising a slightly arcuate, almost fiat, electrically conductive plate, a series of membrane spacers extending generally along the convex surface of said plate, and electrically conductive membrane means insulated from said plate and tensioned over said spacers sothat the membrane means generally follows the slight arcuate curvature of said plate; means mounting the speakers of said pair in spaced relation, facing one another, and closely" air coupled to eliminate second harmonics; and electrical conductors, individually connected to the conductive plates and membranes of said pair of loud-speakers, for driving the entire membrane areas of both speakers in push pull. I 2. In apparatus as described in claim 1 the feature that said electrical conductors are connected individually to said plates and to said membrane meansin such a way as to drive the membrane means in push-pull at frequencies within a predetermined audio range; said mount-- ing means having said plates, with said membrane means,
mounted thereon with close acoustic air coupling therebetween as to all audible second harmonics of sound of said predetermined audio range. v
4. In an electro-acoustic transducer, a pair of electrically conductive, substantially" rigid, slightly arcuately curved backplates; a pair of groups of parallel membrane spacers, one group extending along the convex side of each of said backplates; a pair of thin, electrically conductive membrane sections, one tensioned over said spacers of each backplate in such a way that the section generally follows the curvature of said convex side; mounting structure, having said pair of backplates mounted thereon, one opposite the other, with said membrane sections thereon and with a narrow, substantially unobstructed air space therebetween; and electrical conductors, separately connected to said backplates and membrane sections for feeding electrostatic signals to the entire area of each backplate and to the entire area of each membrane section and for thereby electrostatically vibrating the membrane sections on the spacers in pushpull, whereby the transducer is capableof such acoustic fidelity "as is provided by electrostatic push-pull speakers and of such maintenance of acoustic performance as is provided by single-ended electrostatic speakers.
References Cited in the file of this patent UNITED STATES PATENTS Great Britain Oct. 17,