ELECTROSTATIC RESEARCH - FUNCTIONALITY

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.

ELECTROSTATIC RESEARCH - DISADVANTAGES

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 RESEARCH - ADVANTAGES

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.

KEY FACTORS

JUSTIFICATION VIABILITY OF SOLUTION+ STAKE HOLDER CONCERNS

STAKE HOLDER CONCERNS

The brief of making electrostatic speakers had never been given before in this class. This means Electrostatic speakers had not yet been made in the history of this class. The idea of electrostatic speakers was new so it was of great concern to the stake holder, Terry Hawkings, that the speakers may not work. We were to test the theory of the electrostatic to see if it actually worked. We used design theory's which we found through research to build the different components of the electrostatic extension. The electrostatic did work in the end so the stakeholder was happy as the brief to build electrostatic speakers had been completed.

JUSTIFICATION VIABILITY OF SOLUTION

I wanted to complete the brief in building electrostatic speakers. This is because through research I found that the sound quality of electrostatic is much better than normal speakers. This is because the frequencies are dispersed better. The electrostatic makes the sound crisper and disperses a larger range of frequency. The cost of electrostatic speakers to buy retail was also much larger which meant my speakers would be more valuable if they were electrostatic. We concluded that it would be viable to make electrostatic speakers. It is viable as the extra Perforated metal needed to turn our regular speakers into electrostatic is approx 50 dollars. With the correct usage of the material we were able to create electrostatic speakers which were a few thousand dollars worth more than they were previously without the perforated metal addition. It also was viable as through research we found that previous electrostatic speakers had been made through adding small costs to the normal speakers. This means that to build the speakers we could spend approx about 10 percent of what the speakers were worth. This is clearly economically viable.

BRIEF

At the start of my project I was given the Brief:

Design and complete set of functioning electrostatic speakers. The speaker cabinets must be of original shape ie not rectangular. You must design a functioning crossover to be used by your speakers. You must show the design process.


Outcome

The alignment of the woofer, tweeter and port is important to fulfill a quality technological outcome. Having the woofer and tweeter on different axis and correct space in between the two allows for the sound quality to be better if the mass is calculated correctly. I did put the woofer and tweeter on different axis but did not calculate how far apart they should be from each other. This was corrected by tuning the port length to compensate the loss of quality lost by me not calculating the mass of the cabinet to speakers.

The electrostatic is to help the frequencies to disperse better. Usually without electrostatic the frequencies are dispersed outward from each of the speakers but with electrostatic the frequencies are dispersed up the large sheet of metal and outwards from the metal. The electrostatic disperses a larger range of frequencies than regular speakers so makes the sound quality better and suits a genre of music that uses a wide range of frequencies like jazz.