The Quad Electrostatic Speaker
The recent appearance of a full-range ESL designed and produced by a well-known manufacturer specialising in high quality reproduction, was bound to cause considerable comment and speculation. This speaker, the "Quad" electrostatic, is now in production, and we are pleased to present our readers with a full review of a specimen drawn from current production. As the method of operation is quite different from that of current moving coil speakers, and it offers certain technical advantages and, possibly even more important, as it is the first ESL in the world to reach this state of development-rather more space than usual is being allotted to this review. Much has already been written on the subject (see Hi Fi Year Books, 1956 and 1957, and several past copies of Hi-Fi News), but a condensed story, sufficiently complete for the newcomer to appreciate the significance of this occasion, is hereby given.
(1) Basic principles stated very simply.
There are many ways of converting electrical energy into sound, but only two main ways have been utilised successfully up to the present. One uses the interaction of magnetic fields produced by a flow of current, and has been developed to a very satisfactory level of performance in moving coil and ribbon speakers. The other uses the attraction and repulsion of electric charges, and though the existence of these forces has been known at least as long as our knowledge of magnetic forces, only quite recently have we been able to exploit it to our satisfaction. The bare essentials are shown in fig. 1. A thin plastic sheet is given an electrically conducting coating and suspended between two rigid perforated metal plates. The plastic diaphragm is charged to a high voltage, and when signal voltages are applied to the outer plates, it will move, pulled one side and pushed on the other. As the signal is an alternating voltage, the diaphragm motion will be to and fro, and will drive air to and fro through the perforated metal plates, this we shall hear as sound.
A Little History
The first public demonstration of ES devices was in 1881, when Dolbear exhibited a complete ES telephone system at the Paris Electrical Exhibition. Between 1920 and 1935 several ES speakers and microphones were made, mainly in Germany. In this country the Primustatic speaker retailed for 25s. in the early 1930s. It consisted of a single perforated aluminium sheet, and a fluted metallised paper diaphragm. The insulation was partly a special lacquer on the perforated sheet and partly the fluted paper, which was waxed. Hairy strands of wool held the fluted diaphragm in Place but allowed movement. The whole was curved to produce wide-angle radiation of the higher frequencies. The speaker was usually connected as in fig. 2B, when the PD between anode and chassis partially collapsed the thin diaphragm on to the fixed plate. Signals increased and decreased this PD, causing more and less attraction respectively, and corresponding movement of the diaphragm and the air surrounding it. It worked very well at high frequencies, but as the programme material in those days had no high frequency content other than microphone hiss and record surface noise, it never became popular. We will appraise its technical capabilities later.
For the next twenty years, the only ES audio devices in general use were condenser microphones. Some of these were superb, and one of them undoubtedly played a vital role in the success of "ffrr" in its early days.
While the mechanical simplicity remained a tantalising attraction to acoustic engineers, the known high distortion when used as a loudspeaker slowed further development. Soon after, the Second World War provided other and more urgent problems.
In 1954, Prof. F. V. Hunt's book "Electroacoustics" provided the key to the distortion problem. He gave a very rigorous mathematical examination of the ESL in various forms. His calculations showed that in push-pull form, like fig. 1, it was capable of handling large signals without much distortion if the diaphragm was given a constant charge instead of a constant potential or voltage. The push-pull form was not new; models were on sale 'm Germany around 1920 ! But the constant charge idea was. new and once more work began in ESL design.
By 1955, P.J. Walker of the Acoustical Manufacturing Co., had produced two laboratory models, and these were demonstrated at the B.S.R.A. Annual Exhibition in May, 1955. One was open back and front and handled middle and high frequencies with amazing clarity. The other covered the whole frequency range with equally good quality and used a large closed cabinet or infinite baffle to maintain its bass response. The completely open type was further developed to cover the whole frequency range, and in March, 1956, it was demonstrated to the Physical Society
in this form. This was the world's first full-range ESL in open or doublet form.
Between these two events, H. J. Leak had demonstrated a very successful ES tweeter (above 1,000 c.p.s.) and in America several ES tweeters appeared. By April, 1956, at the Audio Fair three ES units were being demonstrated; the newcomer was another fullrange model by Goodmans. The journey from laboratory model to factory production is seldom easy, and this one is no exception, since it covers completely new ground. There are no one else's mistakes to benefit from, and no one else's success to copy from ! It is good going to reach production at acceptable high-fidelity standards in a mere 4 years-the moving coil speaker has had a 30-year start !
What Does The ESL Offer?
Engineers are sometimes upbraided for undue enthusiasm over the advent of the ESL. (The writer is still unrepentant !) It is not without just cause. For years, acoustic engineers have been trying to move air with devices loaded with lumps of metal-iron (not recently), copper (very often), and aluminium (in the better ones). The thing that actually pushes the air (the diaphragm or cone) is rather heavy too. It has to be stiff enough for at least most of it to move together, though it is only pushed at one place. Now the moving parts of the ESL can be pretty well as light as we care to make them. The driving force is equal all over the diaphragm, so it does not have to be stiff enough to be rigid. It need be little thicker than just sufficient to support itself. This gives it a tremendous advantage as far as high note reproduction is concerned, and, what is probably even more important, transient response.
Transient response concerns the ability of the system to respond with the minimum time delay to the onset and the termination of a signal. If the moving parts are heavy, considerable damping is necessary to bring it quickly to a standstill. Even if one bit of the system stops, there is no guarantee that all the rest of the moving parts stop at the same instant. If any or all of the system carries on after the signal has stopped, then the sound is blurred and coloured by the characteristic sound of that vibration.
In fairness to the moving coil speaker, the better samples we enjoy have reduced these unwanted effects to such small dimensions that they are not obvious, not even to the experienced ear. However, the ESL having such a fantastically small weight per unit working area, just doesn't suffer from this trouble or at least has no need to ! (As far as the HF response is concerned, the ribbon speaker, too, can give similar results.) Even the bass response can be equally superior. If we can use a very large diaphragm, no cabinet is necessary and therefore no cabinet resonances and consequent colouration can occur. This is not very convenient with a single MC speaker, since the necessity to retain a cone shape in the interests of rigidity, results in a depth which is usually greater than half the diameter. A more suitable alternative is to use several speakers of more acceptable size mounted on a flat baffle, and a very successful design of this type has appeared quite recently.
Large Amplitudes: Little Distortion
However, the ESL again scores heavily, since even very large amplitudes of movement are possible with very little distortion. The moving coil, unless an extremely costly magnet design is used, tends to "run out of field" at extremes of displacement. This leads to distortion and also to intermodulation, which is more serious and more noticeable. The driving force on the ESL diaphragm is undistorted for all deflections short of hitting the "stops". At very low frequencies there will be a little distortion due to the stiffness of the diaphragm necessary to prevent collapse. This is strictly comparable with the non-linearity distortion of the MC due to its surround and centring. The ESL is, however, practically free from intermodulation since the driving force for the higher note component is still intact, whereas for the MC, the driving force falls for large displacements as the coil is part way out of the gap.
Summing up, we can expect clearer results and less colouration.
A further advantage arises with the choice of the doublet type speaker, i.e., open back and front and radiating equally. At low frequencies, where its width (and height) is small compared with a wavelength, it only radiates energy in two directions-forwards and backwards in two very broad beams. (At higher frequencies the beams are not so wide, but never become narrow as the radiating width is reduced). No energy then is radiated upwards, sideways, or downwards, resulting in far less colouration due to the room. It particularly reduces trouble due to eigentones.
The Constant Charge Principle and Distortion
(a) The simple ESL: Consider first a simple ESL consisting of two elements only, e.g., Primustatic.
The steady force V2 / d2
Now apply a signal of e volts peak.
The maximum force (V + e)2 = V2 + 2Ve + e2
The minimum force (V - e)2 = V2 - 2Ve + e2
The increase is 2Ve + e2
The decrease is 2Ve - e2
This latter is smaller and represents distortion, mainly second harmonic. Worse than this, when the force increases, the distance d decreases, further increasing the force. Results can only b acceptable, i.e., distortion small if V is very much greater than e at all times and diaphragm movements are small, i.e., small signal or as a tweeter.
(b) The push-pull ESL: If the diaphragm is central when the polarising charge V only is applied, no movement occurs. Now apply the signal voltages and the nett force becomes:
Now as long as the diaphragm remains central, d1=d2=d say, this expression simplifies to Nett force 2Ve/d2 i.e.. it is proportional to the signal voltage e. This means no-distortion as long as its diaphragm doesn't move ! Still it was an improvement on the simple ESL, but not by any means good enough.
(c) The constant charge ESL: Referring back to fig. 4, when the diaphragm is displaced to one side, the capacity of that side increases whilst that of the other side decreases. A simple check will show that the increase is bigger than the decrease. Therefore. the nett capacity has increased, and some extra charge would have to flow on to the diaphragm to maintain its voltage at V volts. If we prevent this flow, then its voltage will fall and the driving force will not rise so much due to deflection-in fact, not at all ! Stated simply, the force on a fixed charge Q anywhere between the plates is only proportional to the PD (e volts) between them. This, then, is truly distortionless. In practice the "constant charge" has to be maintained against leakage via a very high resistance. This is made so high that the charge on the diaphragm doesn't vary appreciably during a half cycle of the lowest frequency to be handled. The diaphragm, though conducting, must be of very high resistance to prevent the charge "walking about" on it. Thus not only must the charge be constant, but the charge per unit area must also remain constant.
The Acoustical Quad Electrostatic L.S.
Description: All domestic equipment has to be a compromise involving size, weight, cost, technical complexity and performance, shape, finish, decor, etc. The moving coil systems, over a period of years, have settled down to fairly stable designs-there cannot be very much real difference in good speakers. With this new ESL speaker there is no design to follow, to improve on, etc., etc. The designer has had to decide his own selection of compromises and the general approval it has already received at its various public demonstrations, has shown the wisdom in the present choice. Time will tell whether we settle for this style, or some other. Meanwhile, it is definitely in the front rank of the best loudspeakers, on all scores.
The diagrams show most of the design features. High-note distribution is achieved by progressively reducing the active width of diaphragm with rise Of frequency. In this speaker it is done in two main steps. The central 6in. strip at 1,000 c.p.s. has a polar distribution that will match that of the whole area at low frequencies. Above about 7 Kc/s only the central 1+ ins. radiates, again helping to keep the beam reasonably wide.
In the vertical plane the HF strip is the full height and, if flat, would produce a very narrow beam. The full length is needed to radiate the necessary energy, so adequate beam "height" is produced by curving the speaker.
The EHT unit is a small 50 cycle ladder-type circuit delivering a few microamps, at a few thousand volts for charging the diaphragm. It is therefore quite non-lethal, even if exploration with a knitting needle were attempted ! The electrical consumption is negligible, barely more than the transformer iron losses, a mere few watts. The heaviest part of it is the matching transformer. This is a fairly high ratio step-up transformer having very low loss and distortion, and a bulky low-capacity-sectionalised secondary winding-probably the most difficult bit to design !
Inspection with a torch shows the "rigid perforated plates" to be very thick indeed-as they should be if they are to remain effectively rigid at large amplitude low frequencies. The whole acoustic unit is sealed between thin plastic films (having negligible acoustic effect) to prevent ingress of dust and moisture.
Overall height is 3l ins., width 34.5 ins., and overall depth is 10.5ins. Weight is 35 Ibs (16 Kilos) and it is thus easily moved by any adult member of the household. The external finish of the present model has been designed by Christopher Heal.
As this speaker presents a rather different load to the amplifier from that of a moving coil type, certain requirements must be met. At frequencies above 8 Kc/s it presents a capacitative load, just about the reverse of a moving coil unit, which is inductive, as a rule, in this region.
It was designed to work with the Quad II amplifier, but any equivalent amplifier can be used.
Specific Amplifier Requirements
Performance: The reviewer has heard this speaker on many occasions previously, in a wide variety of surroundings, including a pair, giving stereo reproduction. The opinion formed, was that it did perform as the very convincing theory suggested. Not until it was actually on the "home pitch", however, was it realised how far this speaker is ahead.
There is no doubt music sounds very nice, more like the real thing than ever before, speech, too, has lost all trace of boom and that's not due to lack of bass. Another interesting feature is the complete lack of any fatigue effect, unless, of course, the programme quality is bad. All sensation of listening to a loudspeaker is lost.
To make sure that the reviewer's judgement was not influenced by the thrill of the technical novelty, a series of searching tests with familiar records, BBC FM, white noise, rapid switchover to other excellent and familiar speaker systems was performed to try and find where it excelled, where it did not, and, if possible, why.
Without doubt, the transient response was superior to anything yet experienced. Several moving coil systems, notably a horn loaded one, could reproduce the transients, but not with the complete lack of colouration that the ESL could achieve. Well, this was expected, and now it has become actual fact. Again, in fairness to the MC types, some of the best produce such slight colouration of transients that it was only detected with certainty after direct comparison with the ESL. The "threat" by the ESL has certainly produced remarkable progress in the MC field, so don't rush to throw them all away just yet !
Record surface noise is noticeably lower than with any MC system with comparable HF response, again demonstrating the freedom from HF resonances. (A horn-loaded speaker of good design will also produce less surface noise, but unless the horn design is very good and the crossover correctly balanced, it will still colour this noise.)
When listening on or near the axis, the effect is as of an open window looking directly on to the performance. It is a clearer, wider view than anything hitherto, and the apparent distance away of the performer varies with the microphone placing at the studio. Variation of the volume control will also affect the apparent distance, of course. A few degrees off the axis is often better, as any distorted (slightly) top in the sound is less obtrusive.
Probably the most striking phenomenon is the fact that the resultant sound sounds less like a loudspeaker than any previous experience has provided. This is due to two things, first, the lack of colouration anywhere in the frequency range. It has no noise of its own. Secondly, due to its doublet form the room participates far less than usual. The listener is virtually transported to a front balcony seat.
With slightly poorer quality signals, the speaker is still not wasted, though the high frequency content would sound almost as good from a much cheaper speaker. The lower register is still clean and uncoloured-"unobstructed" is a word that comes to one's mind when listening to it. For some time now, the reviewer has become aware of a peculiar sensation when listening to many loudspeakers. It was as if a pillar, or similar obstruction, was situated between the performance and the listener. This effect was practically absent in one particular speaker, and is completely absent when listening to the ESL. The phenomenon was explained by the realisation that diffraction of sound round a solid object produces an uneven frequency response. Some frequencies are boosted a little, some are attenuated-about as much as a fairly well damped resonance would do. No resonance, flat response, no apparent obstacle ! This is one of the main reasons for the expression "clear view" involuntarily coming to mind.
Readers will have gathered by now, that this is not a device for impressing the casual listener with one's hi-fi equipment. The top doesn't sparkle all the time, and the bass doesn't shake one's tummy all the time. It merely does what it's told, no more, no less. The bass response below 45 c.p.s. is down somewhat compared with many current loudspeakers, but it does not spoil enjoyment of organ tone, rather the reverse. It does not make every pedal stop sound like a big fat Bourdon ! Every orchestral instrument is treated perfectly, especially the large bass drum - it is no longer in a big felt-lined box !
Various speaker positions were tried, and while the room does not appear to participate so much, its position did have a considerable effect. Fig. 8, shows very roughly five positions tried. A and B were poor-and the published leaflet warns thus. C was much better, D, close to but not parallel to a low open (full) bookcase, was very good. E, similar to C, but closer, was also very good. The lady of the house will probably insist on helping to decide where it is placed, but never mind, it is not heavy to move !
To sum up: The "Quad" ESL is slightly ahead of all dynamic types on transient response and freedom from colouration; it is a loudspeaker designed to be listened to and enjoyed in the home; it will produce adequate sound level for realistic results; it is not suitable for attempting to show off "larger-than-life" impressive noises. It will not suit all tastes, but the reviewer can confidently recommend it as an excellent domestic speaker. As Mr. Briggs has recently demonstrated really convincing stereophonic reproduction with two completely dissimilar speakers, it is seriously suggested that one of the listener's pair of speakers should be a full range electrostatic - the perfect excuse. R.L.W.
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