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Admit It! You came straight to this section! Couldn't resist, huh? Now that you're here, what do we have to titillate and enthral you? Lots, for the really hard core Quad ESL fanatic, there's - Patent History, Mathematics of ESL's and sundry electronics theory necessary to the functioning of Quad Electrostatic Loudspeakers, including the "new" ESL '63. Aaaaaagrh!! I just wanted to fix my speaker - See the Fixit Section. Otherwise, here we go fellow Quad ESL loonies... |
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Developing the Quad Electrostatic Loudspeaker - A Short Patent History |
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o See Patents Page(All relevant patents, in the raw)
Referenced Patents for U.S. Patent 3,008,013 Author of Patent There are three primary patents concerning the Quad Electrostatic Loudspeaker. These are U.S. Patent 3,008,013(filed July 15, 1955) and U.S. Patent 3,008,014 (filed September 12, 1957). Both of these patents were granted on November 7, 1961. The third patent is G.B. Patent 815, 978 [Sheet 2] [Sheet 3] (filed October 19, 1955) and granted July 8, 1959. A brief acquaintance with these dates leads one to observe that the usual attempts to "name" this speaker the "Quad 55" and the "Quad 57" probably stem from these dates. The "Quad Electrostatic Loudspeaker" is what it is called and that is what is stamped on the compliance plates. The two U.S. Patents were issued to David Theodore Nelson Williamson and Peter James Walker, and the Great Britain Patents were issued to Peter James Walker and David Theodore Nelson Williamson. Why the change in precedence of names? - I don't know. Williamson assigned his rights in all three patents to Ferranti Ltd., Hollinwood, England. Ferranti are not so well known these days, but amongst other things, they used to manufacture some excellent phono cartridges. No invention occurs in a vacuum, and this one is no different. In developing this speaker, like any other device which you would like to patent (and produce, and make money from) you need to research previous work in the field. This is, of course to protect your interests, other inventor's interests, and to learn what you can about the field in practice. This way, your most ingenious ideas can be demonstrated to be unique, and not something which someone thought of 50 years ago (if you're lucky). This latter is a situation, which many modern speaker manufacturers suffer from, often claiming, whether they know it or not that their newest speaker breaks all known physical law and is the best transducer since the God put breath into Adam (and usually is anything but). Let us begin then with U.S. Patent 3,008,013 since this was the first filed and has much in common with 3,008,014. The inventors' first reference is to Frederick W. Lee's Patent (U.S. 1,622,039 ) granted in March, 1927. This patent is one of the first, if not the first to discuss symmetrical electrostatic diaphragms in quantitative terms, presenting some mathematical basis for the superiority of symmetrical electrostatic devices. The patent is interesting in that it speaks of a "plurality of diaphragms" in one embodiment of the device, and this is clear in Lee's drawings [Sheet 2].. However, it is very likely that his work on mathematically demonstrating the usefulness of a light, non-elastic diaphragm was of interest to Williamson and Walker. They would have been reading F.V. Hunt's latest analysis at the time ( Electroacoustics, 1954), and that expanded in great part on earlier works such as that of Lee. Lee, amongst other practical aspects of his invention analyses the forces acting on a thin, light diaphragm. He starts by considering the forces acting on the diaphragm. Firstly the Resistance of the air, expressed in the classic Newtonian form:
Then he considers the matter of internal friction in the diaphragm material which is also proportional to the velocity of the vibration, as follows:
Then the simple inertia of the diaphragm and the impedance that it presents, as follows, again, simply applying Newton's Second Law to the particular case:
He then accounts for elasticity in the diaphragm (an undesirable effect in some respects) by using Hooke's Law (slightly modified), as follows:
The applied force, Lee points out is also expressible as a number of harmonic equations, as follows:
Since the expressions in [Exp 1] and [Exp 2] are representing the same set of forces, he then equates:
Rearranging this equality [Exp 3], and solving for the vibrational velocity of the diaphragm, he gets:
The phase relationship is contained in [Exp 4] in the term:
The upshot of all of this, Lee argues, and rightly, as experiment shows, is that if you can arrange to have the diaphragm made of very, very light (near “massless”) material, and that material is not very elastic (doesn't stretch to much in operation), then you can get away with saying that K and S are near enough to zero to be ignored in the practical device, and substitute a value of zero for them in [Exp 5] We know that 6mm polyester film is not "massless", but it is a whole lot closer than the silk and paper that Lee was considering in 1925-27. Neither is it non-elastic, but after heat-shrinking, or appropriate stretching, it is less elastic. In fact, the ideal for a speaker diaphragm is to be near to the tensile limit of the material, although this has an obvious downside with respect to speaker longevity in operational practice. Anyway, K and S are pretty near zero in the diaphragms that are in use in the Quad (well, S ??). Let's set them to zero, and this pretty result emerges:
which expression tidily says that the vibrational amplitude is a function of the resistance only, and that each harmonic is of the same relative value and has the same relative phase displacement towards each other and the vibration of the diaphragm is due (and true) to the "impressed forces" as Lee put it. Also under the above conditions there is no transient coming into the vibration after a given harmonic force comes into action. This helps to explain why electrostatic transducers, in general, are very low in harmonic (and other) distortions. As we say (somewhat colloquially) the light diaphragm gives a lack of distortion and very quick transient response. So, now you know why - partly. Unfortunately for Frederick W. Lee there was no material of practical use in 1925-27 that could fulfil these characteristics in a speaker diaphragm material. That had to wait for polymer films, and specifically the appearance of Mylar® in 1949. Williamson and Walker's second reference is to Walter Hahnemann's U.S. Patent 1,674,683 [Sheet 2 ] granted June 26, 1928. Hahnemann's patent deals mainly with the integration of electrodynamic and electrostatic units into one loudspeaker system. The patent also contains a discussion of electronic crossovers, with Hahnemann speaking of "..a chain series of shunt inductances and condensors which substantially filters out the low frequencies is connected in front of the apparatus associated therewith." This paper was likely consulted as one of the earliest available on crossover techniques, and how to integrate the various drivers that could be used in a speaker system, whether pure electrostatic or hybrid. Our intrepid inventors then took a look at the work of Ralph V. Hartley's U.S. Patent 1,762,981granted June 10, 1930. His patent concerns electrostatic devices and specifically to radiate with uniform efficiency throughout the frequency spectrum. He discusses the theory of symmetrical speakers, and the principle of a "tensionless" diaphragm, which he proposed be made of foil (0.0005" aluminium, in fact). His proposed electrostatic speaker was probably the first to rely on charged grids of wires, pre-dating Arthur Janszen significantly, in this respect. Most importantly, for later inventors, Hartley gives a good discussion of the theory of reducing second harmonic distortions and the importance of the unit charge on the wire grid used. This points to Hunt's later elucidation of the "constant charge" principle. Hartley presents a fair bit of mathematics, but it is not particularly interesting as a general study. A number of "experimental factors" are involved in his equations which model a specific device (the object of the patent), and he quotes specific dimensions proceeding from experiments he carried out. It is not, therefore, worth pursuing these practical models in detail here, since they apply primarily to the device under patent. That having been said, it is interesting to note that Hartley observes - "In order that the electrostatic element be a highly efficient device, two things are necessary; first that Z, the mechanical impedance per unit area of the diaphragm with its air load is primarily the radiation resistance to the air, and second that the reactance of the capacity C be large compared to the impedance ZA/K2". The first part of this statement regarding the matching of the diaphragm to the air and its effect on efficiency is referred to (rather obliquely) by Walker in his Wireless World articles in the 1950's. It is also one reason why the Quad Electrostatic Loudspeaker has those felt mats in the back, especially the treble panels. Thirdly, it confirms in the experimental world, the propositions made in mathematical form by F.W. Lee, noted earlier. The next patent examined by Walker and Williamson was that of Hans Vogt, granted U.S. Patent 1,881,107 [Sheet 2 ] on October 4, 1932. Vogt's patent is the first to mention selectively coated stators, with an overlying insulative layer, which is very much in the vein of that construction used in the Quad Electrostatic.Vogt also speaks of : "pressing an insulating material into a plate having a layer on one side, and a plurality of depressions extending into the plate on the other side, and removing the layer from the plate to a thickness or depth to just cut through the depressions whereby the plate will be provided with the perforations." This sounds remarkably similar to the method of production of the Quad stators. Vogt's patent is significant from a point of view of practical manufacture more than its theoretical bent. Again, however, at this time, Vogt, and others were defeated in the manufacture of a practical speaker of any size by the lack of suitable materials. Onward the trail leads... The next (chronologically speaking) patent consulted was that of Jurgen S. High of New Jersey, an employee of Westinghouse Electric, Pennsylvania. He was granted U.S. Patent 1,930,518 [Sheet 2] on October 17, 1933, assigning the rights to Westinghouse. High's patent, in part, examines the problem of stretching the diaphragm while in operation and the consequent likelihood dielectric breakdown as the diaphragm to stator distance decreased. This patent also shows an attempt to solve the "problem" of directionality through the use of a curved radiating surface. High also hints at high resistance diaphragm coatings (but no more) as a partial solution to the "hot arcing" problem in electrostatic speakers. This is not a particularly significant paper with respect to the Quad ESL, but it does show that thinking along the lines of solving the problem of arcing started early in the piece. Since High's device had an aluminium diaphragm (common for the period) it is a novel departure to note him talking of semi-conductive diaphragms. Now we come to one of the most significant patents in the history of electrostatic speakers and particularly in the history of both the Quad Electrostatic and the ESL '63. U.S. Patent 1,983,377 issued to Edward Washburn Kellogg, an employee of General Electric on December 4, 1934 forms the basis of the thinking that went into the design of the Quad ESL '63, and in the improvement of electrostatic speaker efficiency in general. Walker revisited Kellogg's work in great detail when he set about the design of the ESL '63. However, for the moment, we are concerned with the original Quad Electrostatic Loudspeaker, and the significance of Kellogg's patent in that context. Kellogg speaks extensively of the matching of one speaker element to another with “appropriate inductances”. Apart from this matter, which is not small in itself he considered "the improvement of distribution of sound in the auditorium "through utilisation of "the phase differences between the several sections of the diaphragm". This, of course, would be separate diaphragms in the Quad Electrostatic, but in the '63 there is only one diaphragm. His paper also talks of modelling the speaker behaviour in terms of "the shape of the wave front of a sound wave leaving the speaker, and by utilising the principle that sound is propagated normal to the wave front." Very much in the style of the lecture Walker gave at the AES in June, 1979. Back, again to the Quad Electrostatic (original) and we also find, "...that it is practically necessary for best results that the parts of the diaphragm opposite the several sections of stationary electrode, shall be capable of substantially independent vibration. This is for the reason that the forces due to the electrostatic field differ in phase between the several sections and each part of the diaphragm should be capable of vibrating in phase with the force applied to it. This mechanical independence of the diaphragm panels may be secured by providing mechanical supports for the diaphragm at the edges of the panels. Thus, if the electrical subdivision is of the stationary electrodes, the diaphragm, while not necessarily electrically subdivided, is preferably mechanically subdivided." This latter remark indicates exactly the situation prevailing in the Quad Electrostatic, and perhaps why the speaker is particularly phase pure, since it employs both mechanical and electrical subdivision of the frequency spectrum as proposed here in one form by Kellogg. To finish up, for the moment, we have Kellogg stating: "An incidental advantage of my invention over the usual type of electrostatic loudspeaker is that the reduced capacity per section reduces the destructiveness of a spark should a spark occur between the electrodes. With a large capacity charged to a high potential, a spark may readily be hot enough to burn a hole in through the diaphragm which must be of very light material. With the small sections a much less intense spark occurs for the same voltage. The adjacent sections may discharge through the arc but to do so must build up currents through intervening coils, and this slows down the discharge and permits the energy to be largely dissipated in the coils instead of in the spark." This would be of interest to Williamson and Walker in the first instance, since they were producing a speaker of "several sections" and in the second instance since the delay lines in the Quad '63 contain a series of "coils" very much as Kellogg describes here. You could not design an electrostatic speaker in the 1950s without seriously consulting the work of Arthur Janszen. Williamson and Walker were no different, and no doubt had a careful look at U.S. Patent 2,631,196 [Sheet 2] [Sheet 3 ] issued on March 10, 1953. Janszen's aims, as stated clearly in this patent included: "protection against electrical breakdown and having satisfactory frequency response, power output, and polar energy distribution characteristics..". He tried to achieve these using an array of tensioned wires as the stator, giving his design the state of the art (for the day) amount of free space in the stator itself. At least far better than perforated plates up until that time. The wire diameter was 0.015”, the PVC insulation on the wires was 0.040”, the air gap was 0.04” and the diaphragm was 0.0004” thick. The diaphragm was made of “Pliofilm” which Janszen describes as a “rubber chloride” compound. with a diaphragm this thin, he was able to achieve the best high frequency response of the day, and in a world of pedestrian moving coil speakers, this would have sounded very good indeed. One odd (in retrospect) feature of the speaker was the means of dividing the moving element into random large and small areas with what appear to be strips of dielectric material stuffed between the wire stator and the diaphragm. There is nothing in Janszen's work that has direct bearing on the manufacture or design of the Quad Electrostatic, and it is probable that Williamson and Walker were just ensuring that their design was not treading on the toes of a potential major rival in the electrostatic loudspeaker wars. Of course, in hindsight, (always a nice way to look at things), we can see that they had no real need for alarm. The last of the U.S. Patents referred to by Williamson and Walker in their Patent 3,008,013 is from William E. Kock, an employee of Bell Telephone Laboratories, awarded U.S. Patent 2,796,467 on June 18, 1957. This patent does not apply (strictly) to loudspeakers at all, but microphones. However, the principle of reciprocity tells us that what is useful in a microphone, can, in reverse, be used in a loudspeaker. The main interest in this patent probably stemmed from Kock's mode of "stress free mounting" of the diaphragm, and some of his proposals for coating stators, which were undoubtedly novel for the time. There remains one patent of non-U.S. origin consulted by the designers, and that was issued to Etablissements S.E.M. in G.B. Patent 610,297 on October 13, 1948. the one oddity of the patent is one of the proposed modes of charging the diaphragm, as follows: "Claim 3. As a minor, but not totally trivial footnote to the above, Williamson and Walker referred to "Fibres, Plastics and Rubber" by W.S. Roff, Butterworth's Scientific Publication, London, 1956, pages 22, 27, 77 and 312. Presumably in the search for stator, diaphragm and coating materials of a suitable nature to use. As a final note on this patent (U.S. 3,008,013) it is very interesting to note the drawing on sheet 4 of 4 of the published patent. ESL '63 owners will no doubt see some similarities between this and the speaker they thought began its conceptual life in 1963 - remember this drawing was made in July of 1955, and the patent granted in 1961.
Referenced Patents for U.S. Patent 3,008,014 (chronological order) Author of Patent Joseph Masolle It will be immediately appreciated from reading the above list that the reference trail for U.S. Patent 3,008,014 is very similar (as one might expect) to that of its sister patent. The references to the previously mentioned patents are exactly the same in this document. The prior references are noted above and the additional / replacement references are described below. Why a second, very similar patent? The second patent deals in more detail with the manner of construction of the actual panels that were eventually embodied in the Quad ESL, and contains the only mention, in a U.S. Patent of the "...strip of conductive material in contact with and extending around the peripheral portion of said diaphragm electrode through which a polarizing voltage may be applied to said electrode." This is a significant, and in the case of the Treble Panel, a mysterious and somewhat disguised feature of the panel. Now, to the first reference cited by the designers for U.S. Patent 3,008,014. This reference was to the work of Joseph Massolle recorded in U.S. Patent 1,550,381 issued on August 18, 1925. A patent in fact for an electrostatic telephone! Co-applicants and authors of the patent include Hans Vogt and Dr. Josef Engl. This patent is mainly of interest as one of the earliest practical electrostatic devices of any kind in the literature, barring that of Chilowsky and Langevin - note the word practical. The designers no doubt were studying the field and this patent gives a description of means of suspension and plate construction of a single-ended electrostatic device. Since the device was single-ended it would be of little interest in the field of high fidelity reproduction, as it is well known that single-ended electrostatic speakers have severe problems related to restoring force on the diaphragm. However, when there are no text books on the subject, where else can you study, but from the practitioners of the art and science? The next prior patent referred to for this patent was that of Frederick W. Lee. John Depew's U.S. Patent 1,631,583 of June 7, 1927 shows details of a means of ribbed suspension of an electrostatic speaker diaphragm. This is one of the earlier references to a design which could be directly applied to a "large device" such as a loudspeaker. Depew speaks of: "...,the diaphragm being subdivided into a number of smaller diaphragms, and its freedom to respond to the more delicate impulses brought to bear upon it by the condeser plates is greatly improved." So, here is the earliest mention of the equivalent of an electrostatic tweeter, or the great grandfather of the Quad Electrostatic Treble Panel. The patent also shows a means of insulating the diaphragm using additional spacers between it and the stators, which was an initial idea adopted by Williamson and Walker, but later able to be superceded by the better insulative nature of the plastic diaphragms of their era. A series of patents was referred to for this patent which contain identical work as for U.S. Patent 3,008,013; as follows: The U.S. Patent (U.S. 2,855,467) [Sheet 2] of Paul Curry was the next to be consulted by the designers for this particular patent that involves work not examined for Patent 3,008,014. Curry's ideas cover the use of two conductive, flexible diaphragms on either side of a central insulating one, and moreover that the diaphragm was "...secured to the dielectric sheet at their edges only and left free over their surfaces between the edges". The latter structure is novel, to say the least, and of doubtful facility in practice, and is seemingly ignored by Williamson and Walker. Closer to their own ideas on building "folded" electrostatic speakers, for compactness were Curry's propositions on folded diaphragms, as follows: “....wherein the assembly of said diaphragms and said sheet is repeatedly folded upon itself to provide accordion-like pleats throughout a substantial portion of its unrestrained surface area, whereby the ratio of sound generating area to the effective sound-projecting area is increased.” Practical manufacture of such devices by Quad, or anyone else, has never been attempted to the author's knowledge. It is likely that Williamson and Walker had to consult this patent in case of infringement of its protected work by their own proposals regarding folded (zig-zag) diaphragms. An "incidental" then, in the Quad ESL's development. The last prior reference to U.S. Patents for this patent is to the U.S. Patent 2,864,899 of Henry W. Parker of Flushing, New York, issued on December 16, 1958. Parker's papers include some very interesting and critical analysis of diaphragm tension, and this alone may be why it was examined by the Quad designers. It is worth a closer look, as follows: Parker treats tension in the diaphragm as equivalent pressure (dynes/cm2 in the ensuing discussion, and this is very reasonable, since one is trying to alter the air pressure, after all. The equation shown in [Exp 1] expresses the tension per square centimetre in terms of the voltage difference V, and the air gap separation distance s, between (in his device) partially inserted interdigitated electrodes measured normal to the diaphragm.
Parker continues: “For high fidelity reproduction of sound, a linearity between pressure and the applied signal voltage is required, because the intensity of sound depends on the pressure of the sound wave squared, as shown in [Exp. 2] where pm is the maximum value of the "excess pressure" of the sound wave in dynes per square centimetre; c is the celerity of the sound wave taken as 3.31 x 104 centimetres per second; r is the air density at sea level taken as 1.27 milligrams per cubic centimetre and I is the intensity of the sound expressed in ergs per square centimetre.”
and, “The maximum tension that can be supported in air at sea level is determined by the maximum volts per centimetre allowable. At 3000 Volts per centimetre, the maximum tension is 400 dynes per square centimetre, which on substitution in [Exp 2] shows about 0.2 milliwatt per square centimetre of sound radiating surface. This power can be improved by certain electrical insulation processes which allow a greater field strength.” On the subject of the diaphragm polarising voltage, Parker writes the following expression expressing the linear dependence of pressure on the applied signal voltage.
Examination of [Exp 3] shows that the polarising voltage and the air gap must be substantially constant and that the signal e on one electrode and the inverse signal on the opposite electrode should be exactly that - equal and opposite in value and also 180o out of phase with each other. Under these conditions the "excess pressure" that Parker refers to is a linear function of the signal and a true high fidelity transducer is the result. Parker then goes on to model the movement of the diaphragm by reciprocity, using the longitudinal model of sound waves as a basis. This model is expressed, most compactly, in the following:
Very nice. So, what is he saying at the end of the day? Basically this - if we choose a low inertia material for the diaphragm, (he suggests a few milligrams per square centimetre) then the diaphragm amplitude will approach A in the limit. This would give you a relatively efficient and high fidelity device. All of this is pertinent to the Quad ESL as we know it. Parker went on to patent a device which, although it would have been very efficient as electrostatics go, was nevertheless probably too mechanically fussy to be economical to produce. That completes the U.S. Patent references for the second of Williamson and Walker's U.S. Patents. There are three patents issued in Great Britain that they also consulted. These include the patent by Etablissements S.E.M. referred to in the prior patent and two other G.B. Patents issued to Albert Rauser, et alia. Albert Rauser and Wilhelm Steuer were issued G.B. Patent 348,573 on May 12, 1931. The patent is directly concerned with "...electrostatic loudspeakers, dielectric diaphragms and conducting electrodes". The key interest in this older work for Williamson and Walker probably revolved around Rauser's discussion of "high ohmic resistors" or as we would call them "charging resistors". It is apparent from the comments of the Quad designers, who had the advantage of reading Hunt's analysis many years after Rauser, that the external resistor was considered neither desirable, nor necessary, if the diaphragm was of suitably high and constant resistance itself. This reduced the "quadratic forces" predicted by Hunt and thus harmonic distortion from the device in general. The second G.B. Patent ( G.B. 370,248 ) issued to Rauser and Steuer on April 7, 1932 was also consulted by Williamson and Walker in which the interest lay in Rauser discussing the replacement of foil (actual metal foil) diaphragms by semi-conductive diaphragms (no, not semi-conductors as we no them, just that they were not "well-conducting"). This was a significant early attempt to reduce the damage caused by sparking and the "great disturbances to the hearing perception" that take place because of them. Rauser also discusses obtaining the polarising voltage from the windings of a transformer, instead of the "usual battery". Most significantly, and only mentioned right at the end of the patent in the claims section is Claim 5, as follows: "An electrostatic loud-speaker according to any of the preceding claims, characterised by the feature that the diaphragm is coated with a protecting coating of a material which is non-hygroscopic or as little hygroscopic as possible - e.g. varnish, rubber or gutta-percha." I wonder if this got the Quad designers thinking about hydrophilic membrane coatings, as they state in their own patent. They eventually chose Nylon for its ability to retain a molecular film of moisture close to the membrane, and it is this molecular layer that conducts on the original Quad membrane, not the Nylon itself.
A General Note on G.B. Patent 815,978 A complete review of all three patents issued to Williamson and Walker for the original Quad is clearly not complete without mention of the Great Britain Patent (GB 815,978) they were issued on July 8, 1959, fully more than two years before their patents in the U.S. This patent contains all of the U.S. Patent work in one patent with the significant omission of the final method of panel construction shown in Fig. 6. of U.S. Patent 3,008,014 [Sheet 2]. This omission means that the final embodiment of the "sandwiches" of the Quad Panel were never fully patented in Great Britain. There endeth the lesson in respect of the Quad Electrostatic Loudspeaker's patent history. For now, you may like to browse other sections of Hard Core where the structure and performance of Quad panels is discussed, or the general operating principles of electrostatics in general are discussed, or, follow on here to the Patent History of the ESL '63.
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