Here I will try to explain how they work and why they are so good.
You can learn more than you need to know about how ESLs operate by looking at any college physics text book. It all boils down to this- like charges repel each other, opposite charges attract each other. If you place positive electric charges on two objects then bring the two near each other, they will experience forces that push them apart because of the interactions of the electric fields that surround them. If one of the charges is "negative" and the other "positive", the two charged objects will experience a force that pushes them together. The force involved depends on the charges and the distances involved. An ESL uses the force developed by the electric field between charges to move a very thin plastic diaphragm back and forth to produce sound.
The behavior of electric charges is similar to magnets when brought near each other, however the forces developed by electric fields are usually smaller than those we experience with magnets. You can play with electric charges by rubbing a balloon on your head on a dry day. When you lift the balloon, your hair will stand up to try to touch the balloon because the balloon has accumulated one polarity of charge and your hair has accumulated the opposite polarity of charge. This is called triboelectric effect and occurs whenever two dissimilar materials are moved against each other.
ESLs are simply a way to keep electric charges separated but close together so they can do the work of moving a plastic film to produce sound. The usual construction technique is to place a tight plastic film (the diaphragm) with an electric charge between two charged conductors (called stators) on which the charges alternate. At one instant, the diaphragm is driven towards one of the stators, then when the polarity of the charge on the conductors reverses, the diaphragm is driven toward the other conductor. The result is sound.
The charges are applied to the diaphragm and the stators using some simple electronics. A very low current, high voltage power supply is used to charge the diaphragm. Electronic components called transformers are used to apply the alternating polarity charges to the stators. I'll explain more about those, later.
When an electric charge is applied to a metal object, the charge spreads out and the entire surface of the metal rests at a specific voltage (a measure of the quantity of charge divided by the distance separating two opposite charges) depending on the amount of charge applied and the distance to the opposite charge (charges normally come in opposite pairs- you don't usually have a positive charge floating around without an equal negative charge somewhere nearby). Metals as said to "conduct" electricity because electric charge can move around freely on them.
Plastics don't normally allow electric charge to move, so they are called "insulators". An ESL uses a plastic film for the diaphragm because some plastics are very strong and light weight. The diaphragm won't move if it doesn't have an electric charge, so the first problem in making an ESL is how to apply charge to the nonconducting diaphragm.
In between metals which conduct electricity and plastics that don't there are other materials that conduct electricity very poorly. Those materials are said to have high resistance to the movement of electric charge. In an ESL, the diaphragm is coated with one of these materials. An electric charge is applied to the coating and over time, it spreads out over the entire surface of the diaphragm. The very high resistance of the coating prevents the charge from moving around on the diaphragm surface very quickly. The charge is applied with a high voltage power supply that can develop as much as 5000 Volts. All that supply needs to do is supply charge to the diaphragm which is very high resistance, so the current required from the high voltage power supply is measured in microamps. It is not dangerous unless you happen to be in an explosive atmosphere. Remember the triboelectric effect? That is what is responsible for the "static" electric shocks you experience when you walk across a carpeted floor with rubber soled shoes and touch a doorknob. You can easily generate more than 5000 Volts by walking across a room on a dry day.
The speakers most people have experience with use magnetic fields to move a paper, plastic, or metal "cone" back and forth to produce sound. The most common configuration has a coil of wire mounted near the center of the cone, concentric with a very strong permanent magnet. Current produced by an amplifier flows through the coil and sets up a magnetic field that interacts with the field of the permanent magnet and produces force that moves the cone back and forth. The cone is not perfectly stiff, and if it is large, it is relatively heavy. These properties affect the frequencies that can be reproduced and the distortion. A very large, massive cone cannot be moved fast enough to reproduce high frequencies. When the coil is driven with a lot of current, the cone flexes a little resulting in distortion. Low frequency sound reproduction requires movement of a lot of air, so the speaker must be made large, and the cone will be heavy. High frequency sound production requires that the coil and cone be made very light weight. This is why so many speakers have separate drivers for the low, middle, and high frequencies.
Most electrostatic speakers use a single, large diaphragm to produce sound. Even though it is large, it is very light weight because it is made of very thin plastic. The very light weight means it can reproduce high frequencies very easily. It can't move back and forth as far as a magnetic woofer, but what it lacks in distance is made up for in surface area, so it can reproduce relatively low frequencies also. Distortion is very low because unlike a magnetic driver in which the cone is driven only from the coil at the center of the larger cone, the entire surface of the diaphragm is driven by the electric field between the stators. So ESLs are capable of reproducing a wide frequency range with very low distortion.
As wonderful as ESLs are, they have some limitations. The large surface area means the speakers tend to dominate whatever room they are placed in. They usually are not as sensitive as "normal" magnetic speakers so they won't play as loudly. The frequency response of ESLs is a function of their size. The larger you make them, the lower they can go, but it is almost impossible to get flat response down below 50-60 Hz because the front and rear of the speaker are out of phase. At high frequencies the radiation pattern narrows to a tight beam. This results in rising response- about 6 dB per octave, and a "sweet-spot" that is suitable for one listener only.
The response issues can be overcome by electronic equalization. The only way to overcome the low frequency response limitation is to crossover to a magnetic driver in a box at some low frequency. This will allow the speaker system to play loudly and remain very low in distortion. If you aren't concerned about flat response below 50-60 Hz (most magnetic speakers can't go lower than that either), you can use ESLs with no crossovers at all. They sound amazingly lifelike!
Tuesday, February 23, 2010
Electrostatic advantages and disadvantages
Advantages of electrostatic loudspeakers include levels of distortion one to two orders of magnitude lower than conventional cone drivers in a box, the extremely light weight of the diaphragm which is driven across its whole surface, and exemplary frequency response (both in amplitude and phase) because the principle of generating force and pressure is almost free from resonances unlike the more common electrodynamic driver. Musical transparency can be better than in electrodynamic speakers because the radiating surface has much less mass than most other drivers and is therefore far less capable of storing energy to be released later. For example, typical dynamic speaker drivers can have moving masses of tens or hundreds of grams whereas an electrostatic membrane only weighs a few milligrams, several times less than the very lightest of electrodynamic tweeters. The concomitant air load, often insignificant in dynamic speakers, is usually tens of grams because of the large coupling surface, this contributing to damping of resonance buildup by the air itself to a significant, though not complete, degree. Electrostatics can also be executed as full-range designs, lacking the usual crossover filters and enclosures that could color or distort the sound.
Since many electrostatic speakers are tall and thin designs without an enclosure, they act as a vertical dipole line source. This makes for rather different acoustic behavior in rooms compared to conventional electrodynamic loudspeakers. Generally speaking, a large-panel dipole radiator is more demanding of a proper physical placement within a room when compared to a conventional box speaker, but, once there, it is less likely to excite bad-sounding room resonances, and its direct-to-reflected sound ratio is higher by some 4–5 decibels. This in turn leads to more accurate stereo reproduction of recordings that contain proper stereo information and venue ambience. Planar (flat) drivers tend to be very directional giving them good imaging qualities, on the condition that they have been carefully placed relative to the listener and the sound-reflecting surfaces in the room. Curved panels have been built, making the placement requirements a bit less stringent, but sacrificing imaging precision somewhat.
Disadvantages include a lack of bass response (due to phase cancellation from a lack of enclosure (bass rolloff 3db point occurs when the narrowest panel dimension equals a quarter wavelength of the radiated frequency for dipole radiators, so for a Quad ESL 63 at 0.66 meters wide this occurs at around 129Hz so is comparable to many box speakers. speed of sound taken as 343 m/s) and the difficult physical challenge of reproducing low frequencies with a vibrating taut film with little excursion amplitude, however as most diaphragms have a very large surface area compared to cone drivers only small amplitude excursions are required to put relatively large amounts of energy out), and sensitivity to ambient humidity levels. While bass is lacking quantitatively (due to lower distortion than cone drivers) it can be of better quality ('tighter' and without 'booming') than that of electrodynamic (cone) systems. Phase cancellation can be somewhat compensated for by electronic equalization (a so-called shelving circuit that boosts the region inside the audio band where the generated sound pressure drops because of phase cancellation). Nevertheless maximum bass levels cannot be augmented because they are ultimately limited by the membrane's maximum permissible excursion before it comes too close to the high-voltage stators, which may produce electrical arcing and burn holes through it. Recent, technically more advanced solutions for perceived lack of bass include the use of large, curved panels (Sound Lab, MartinLogan CLS), electrostatic subwoofer panels (Audiostatic, Quad) and long-throw electrostatic element allowing large diaphragm excursions (Audiostatic). Another trick often practised is to step up the bass (20–80 Hz) with a higher transformation ratio than the mid and treble.
This relative lack of loud bass is often remedied with a hybrid design using a dynamic loudspeaker, e.g. a subwoofer, to handle lower frequencies with the electrostatic diaphragm handling middle and high frequencies. Many feel that the best low frequency unit for hybrids are cone drivers mounted on open baffles as dipoles transmission line woofers or horns, since they possess roughly the same qualities (at least in the bass) as electrostatic speakers, i.e. good transient response, little box coloration, and (ideally) flat frequency response. However, there is often a problem with integrating such a woofer with the electrostatics. This is because most electrostatics are line sources, the sound pressure level of which decreases by 3 dB for each doubling of distance. A cone speaker's sound pressure level, on the other hand, decreases by 6 dB for each doubling of distance because it behaves as a point source. This can be overcome by the theoretically more elegant solution of using conventional cone woofer(s) in an open baffle, or a push-pull arrangement, which produces a bipolar radiation pattern similar to that of the electrostatic membrane. This is still subject to phase cancellation, but cone woofers can be driven to far higher levels due to their longer excursion, thus making equalization to a flat response easier and they add distortion thereby increasing the area (and therefore the power) under the frequency response graph, making the total low frequency energy higher but the fidelity to the signal lower.
The directionality of electrostatics can also be a disadvantage in that it means the 'sweet spot' where proper stereo imaging can be heard is relatively small, limiting the number of people who can fully enjoy the advantages of the speakers simultaneously.
Because of their tendency to attract dust, insects, conductive particles and moisture, electrostatic speaker diaphragms will gradually deteriorate and need periodic replacement. They also need protection measures to physically isolate their high voltage parts from accidental contact with humans and pets. Cost-effective repair and restoration service is available for virtually every current and discontinued electrostatic loudspeaker model.
Since many electrostatic speakers are tall and thin designs without an enclosure, they act as a vertical dipole line source. This makes for rather different acoustic behavior in rooms compared to conventional electrodynamic loudspeakers. Generally speaking, a large-panel dipole radiator is more demanding of a proper physical placement within a room when compared to a conventional box speaker, but, once there, it is less likely to excite bad-sounding room resonances, and its direct-to-reflected sound ratio is higher by some 4–5 decibels. This in turn leads to more accurate stereo reproduction of recordings that contain proper stereo information and venue ambience. Planar (flat) drivers tend to be very directional giving them good imaging qualities, on the condition that they have been carefully placed relative to the listener and the sound-reflecting surfaces in the room. Curved panels have been built, making the placement requirements a bit less stringent, but sacrificing imaging precision somewhat.
Disadvantages include a lack of bass response (due to phase cancellation from a lack of enclosure (bass rolloff 3db point occurs when the narrowest panel dimension equals a quarter wavelength of the radiated frequency for dipole radiators, so for a Quad ESL 63 at 0.66 meters wide this occurs at around 129Hz so is comparable to many box speakers. speed of sound taken as 343 m/s) and the difficult physical challenge of reproducing low frequencies with a vibrating taut film with little excursion amplitude, however as most diaphragms have a very large surface area compared to cone drivers only small amplitude excursions are required to put relatively large amounts of energy out), and sensitivity to ambient humidity levels. While bass is lacking quantitatively (due to lower distortion than cone drivers) it can be of better quality ('tighter' and without 'booming') than that of electrodynamic (cone) systems. Phase cancellation can be somewhat compensated for by electronic equalization (a so-called shelving circuit that boosts the region inside the audio band where the generated sound pressure drops because of phase cancellation). Nevertheless maximum bass levels cannot be augmented because they are ultimately limited by the membrane's maximum permissible excursion before it comes too close to the high-voltage stators, which may produce electrical arcing and burn holes through it. Recent, technically more advanced solutions for perceived lack of bass include the use of large, curved panels (Sound Lab, MartinLogan CLS), electrostatic subwoofer panels (Audiostatic, Quad) and long-throw electrostatic element allowing large diaphragm excursions (Audiostatic). Another trick often practised is to step up the bass (20–80 Hz) with a higher transformation ratio than the mid and treble.
This relative lack of loud bass is often remedied with a hybrid design using a dynamic loudspeaker, e.g. a subwoofer, to handle lower frequencies with the electrostatic diaphragm handling middle and high frequencies. Many feel that the best low frequency unit for hybrids are cone drivers mounted on open baffles as dipoles transmission line woofers or horns, since they possess roughly the same qualities (at least in the bass) as electrostatic speakers, i.e. good transient response, little box coloration, and (ideally) flat frequency response. However, there is often a problem with integrating such a woofer with the electrostatics. This is because most electrostatics are line sources, the sound pressure level of which decreases by 3 dB for each doubling of distance. A cone speaker's sound pressure level, on the other hand, decreases by 6 dB for each doubling of distance because it behaves as a point source. This can be overcome by the theoretically more elegant solution of using conventional cone woofer(s) in an open baffle, or a push-pull arrangement, which produces a bipolar radiation pattern similar to that of the electrostatic membrane. This is still subject to phase cancellation, but cone woofers can be driven to far higher levels due to their longer excursion, thus making equalization to a flat response easier and they add distortion thereby increasing the area (and therefore the power) under the frequency response graph, making the total low frequency energy higher but the fidelity to the signal lower.
The directionality of electrostatics can also be a disadvantage in that it means the 'sweet spot' where proper stereo imaging can be heard is relatively small, limiting the number of people who can fully enjoy the advantages of the speakers simultaneously.
Because of their tendency to attract dust, insects, conductive particles and moisture, electrostatic speaker diaphragms will gradually deteriorate and need periodic replacement. They also need protection measures to physically isolate their high voltage parts from accidental contact with humans and pets. Cost-effective repair and restoration service is available for virtually every current and discontinued electrostatic loudspeaker model.
Electrostatic speaker design research
An electrostatic loudspeaker (ESL) is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field.
The speakers use a thin flat diaphragm usually consisting of a plastic sheet coated with a conductive material such as graphite sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids. For low distortion operation, the diaphragm must operate with a constant charge on its surface, rather than with a constant voltage. This is accomplished by either or both of two techniques: the diaphragm's conductive coating is chosen and applied in a manner to give it a very high surface resistivity, and/or a large value resistor is placed in series between the EHT (Extra High Tension or Voltage) power supply and the diaphragm (resistor not shown in the diagram here).However the latter technique will still allow distortion as the charge will migrate across the diaphragm to the point closest to the "grid" or electrode thereby increasing the force moving the diaphragm, this will occur at audio frequency so the diaphragm requires a high resistance (megohms) to slow the movement of charge for a practical speaker.
The diaphragm is usually made from a polyester film (thickness 2–20 µm) with exceptional mechanical properties, such as PET film. By means of the conductive coating and an external high voltage supply the diaphragm is held at a DC potential of several kilovolts with respect to the grids. The grids are driven by the audio signal; front and rear grid are driven in antiphase. As a result a uniform electrostatic field proportional to the audio signal is produced between both grids. This causes a force to be exerted on the charged diaphragm, and its resulting movement drives the air on either side of it.
In virtually all electrostatic loudspeakers the diaphragm is driven by two grids, one on either side, because the force exerted on the diaphragm by a single grid will be unacceptably non-linear, thus causing harmonic distortion. Using grids on both sides cancels out voltage dependent part of non-linearity but leaves charge (attractive force) dependent part[1]. The result is near complete absence of harmonic distortion. In one recent design, the diaphragm is driven with the audio signal, with the static charge located on the grids (Transparent Sound Solutions).
The grids must be able to generate as uniform an electric field as possible, while still allowing for sound to pass through. Suitable grid constructions are therefore perforated metal sheets, a frame with tensioned wire, wire rods, etc.
To generate a sufficient field strength, the audio signal on the grids must be of high voltage. The electrostatic construction is in effect a capacitor, and current is only needed to charge the capacitance created by the diaphragm and the stator plates (previous paragraphs referred to as grids or electrodes). This type of speaker is therefore a high-impedance device. In contrast, a modern electrodynamic cone loudspeaker is a low impedance device, with higher current requirements. As a result, impedance matching is necessary in order to use a normal amplifier. Most often a transformer is used to this end. Construction of this transformer is critical as it must provide a constant (often high) transformation ratio over the entire audible frequency range (ie large bandwidth) and so avoid distortion. The transformer is almost always specific to a particular electrostatic speaker. To date, Acoustat built the only commercial "transformer-less" electrostatic loudspeaker. In this design, the audio signal is applied directly to the stators from a built-in high-voltage valve amplifier (as valves are also high impedance devices), without use of a step-up transformer.
The speakers use a thin flat diaphragm usually consisting of a plastic sheet coated with a conductive material such as graphite sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids. For low distortion operation, the diaphragm must operate with a constant charge on its surface, rather than with a constant voltage. This is accomplished by either or both of two techniques: the diaphragm's conductive coating is chosen and applied in a manner to give it a very high surface resistivity, and/or a large value resistor is placed in series between the EHT (Extra High Tension or Voltage) power supply and the diaphragm (resistor not shown in the diagram here).However the latter technique will still allow distortion as the charge will migrate across the diaphragm to the point closest to the "grid" or electrode thereby increasing the force moving the diaphragm, this will occur at audio frequency so the diaphragm requires a high resistance (megohms) to slow the movement of charge for a practical speaker.
The diaphragm is usually made from a polyester film (thickness 2–20 µm) with exceptional mechanical properties, such as PET film. By means of the conductive coating and an external high voltage supply the diaphragm is held at a DC potential of several kilovolts with respect to the grids. The grids are driven by the audio signal; front and rear grid are driven in antiphase. As a result a uniform electrostatic field proportional to the audio signal is produced between both grids. This causes a force to be exerted on the charged diaphragm, and its resulting movement drives the air on either side of it.
In virtually all electrostatic loudspeakers the diaphragm is driven by two grids, one on either side, because the force exerted on the diaphragm by a single grid will be unacceptably non-linear, thus causing harmonic distortion. Using grids on both sides cancels out voltage dependent part of non-linearity but leaves charge (attractive force) dependent part[1]. The result is near complete absence of harmonic distortion. In one recent design, the diaphragm is driven with the audio signal, with the static charge located on the grids (Transparent Sound Solutions).
The grids must be able to generate as uniform an electric field as possible, while still allowing for sound to pass through. Suitable grid constructions are therefore perforated metal sheets, a frame with tensioned wire, wire rods, etc.
To generate a sufficient field strength, the audio signal on the grids must be of high voltage. The electrostatic construction is in effect a capacitor, and current is only needed to charge the capacitance created by the diaphragm and the stator plates (previous paragraphs referred to as grids or electrodes). This type of speaker is therefore a high-impedance device. In contrast, a modern electrodynamic cone loudspeaker is a low impedance device, with higher current requirements. As a result, impedance matching is necessary in order to use a normal amplifier. Most often a transformer is used to this end. Construction of this transformer is critical as it must provide a constant (often high) transformation ratio over the entire audible frequency range (ie large bandwidth) and so avoid distortion. The transformer is almost always specific to a particular electrostatic speaker. To date, Acoustat built the only commercial "transformer-less" electrostatic loudspeaker. In this design, the audio signal is applied directly to the stators from a built-in high-voltage valve amplifier (as valves are also high impedance devices), without use of a step-up transformer.
Electrostatics research
Electrostatics is the branch of science that deals with the phenomena arising from stationary and/or slow-moving electric charges.
Since classical antiquity it was known that some materials such as amber attract light particles after rubbing. The Greek word for amber, ήλεκτρον (electron), was the source of the word 'electricity'. Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law. Even though electrostatically induced forces seem to be rather weak, the electrostatic force between e.g. an electron and a proton, that together make up a hydrogen atom, is about 40 orders of magnitude stronger than the gravitational force acting between them.
Electrostatic phenomena include many examples as simple as the attraction of the plastic wrap to your hand after you remove it from a package, to the apparently spontaneous explosion of grain silos, to damage of electronic components during manufacturing, to the operation of photocopiers. Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with nonconductive surfaces.
Since classical antiquity it was known that some materials such as amber attract light particles after rubbing. The Greek word for amber, ήλεκτρον (electron), was the source of the word 'electricity'. Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law. Even though electrostatically induced forces seem to be rather weak, the electrostatic force between e.g. an electron and a proton, that together make up a hydrogen atom, is about 40 orders of magnitude stronger than the gravitational force acting between them.
Electrostatic phenomena include many examples as simple as the attraction of the plastic wrap to your hand after you remove it from a package, to the apparently spontaneous explosion of grain silos, to damage of electronic components during manufacturing, to the operation of photocopiers. Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with nonconductive surfaces.
Monday, February 22, 2010
Sunday, February 21, 2010
Brief
At the start of the project I was given a brief for my project.
The brief was to design and make a pair of high quality functioning electrotatic speakers, and a functioning crossover.
The conditions of the brief were that the speaker cabinets had to be an original shape, and that we must have to show the design process.
Outcome:
Having the tweeter and woofer on a different axis means that the sound quality is improved. Generally you do calculations to find the correct distance between the woofer and tweeters for the optimum quality , we didn't do this but Terry tuned the port differently to account for this.
Having the long sheet of Aliminium metal turns the speakers into electrostatic speaks, as the sound waves travel up the sheet of metal and a dispersed more evenly, creating a higher sound quality for all frequencies of sound wave.
The brief was to design and make a pair of high quality functioning electrotatic speakers, and a functioning crossover.
The conditions of the brief were that the speaker cabinets had to be an original shape, and that we must have to show the design process.
Outcome:
Having the tweeter and woofer on a different axis means that the sound quality is improved. Generally you do calculations to find the correct distance between the woofer and tweeters for the optimum quality , we didn't do this but Terry tuned the port differently to account for this.
Having the long sheet of Aliminium metal turns the speakers into electrostatic speaks, as the sound waves travel up the sheet of metal and a dispersed more evenly, creating a higher sound quality for all frequencies of sound wave.
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