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dinsdag 24 januari 2017
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  Electrostatic Loudspeaker

Quad ESL 57, the famoust?

How to build your own ESL system

DIY ESL Kits, Electronics and Parts

Electrostatic Principles

The underlying theory of electrostatic speakers couldn't be more simple. The laws of physics teach that "like" electrical charges repel one-another and "unlike" electrical charges attract one-another. Using this principle, let's construct a conceptual electrostatic speaker. First, let's stretch a thin plastic membrane on a rigid frame. Then, let's coat it with a low-mass electrically-conductive substance.

Next, we fabricate two stiff, flat electrodes, called the stators. They are insulated to prevent electrical discharge. We give each stator the same area as the membrane and place one on either side of it. The membrane is placed exactly equidistant between the two stators.

Finally, the stators must have holes in them to permit sound to pass through. We now have a primitive electrostatic speaker.
To make our simple electrostatic speaker work, a dense population of electrons is forced onto the membrane using a power supply.
The audio signal voltage from the amplifier is connected to the two stators, but in a special manner: the signal applied to each stator is identical with the exception of phase, one signal is 180 degrees out of phase with the other.
Thus, as the signal voltage on one stator increases positively, it attracts the negative electron charge on the membrane. The signal voltage on the other stator is increasing negatively and, hence, repels the charge on the membrane.
Thus, a "push-pull" force is exerted on the membrane. When the audio signal reverses, the push-pull force also reverses. Since the membrane is compliant, the push-pull electrostatic force applied to it causes it to move.
Thus, air is moved and sound is created in the image of the electrical driving force.

Advantages of electrostatic loudspeakers include the extremely light weight of the diaphragm, and exemplary frequency response (both in amplitude and phase) because the principle of generating force and pressure is not as prone to resonances as in the operating principle of the more common electrodynamic driver.
Musical transparency can be better than in electrodynamic speakers because the radiating surface is much lighter mass than most other drivers and so more responsive to the applied signal.

Disadvantages include a lack of bass response (due to phase cancellation from a lack of enclosure, and the difficult physical requirement to reproduce low frequencies with a vibrating taut film with little excursion amplitude), and sensitivity to ambient humidity levels. While bass is lacking quantitatively, 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).
Speaker Setup
1. Dipole speakers like breathing room behind them. Use as much space as you can afford. If the speakers must be placed directly against the back wall (the worst possible condition for a dipole device) life doesn't end. Simply toe-in the speakers to reduce direct reflections from the back wall. The toe-in angle can be large, on the order of 30 to 40 degrees. Use as large an angle as necessary.

2. Room modes "generally" cause a low-frequency "suck-out" in the geometric center of the room. This is "generally" no the best place to put your couch. I say generally because some rooms like to make me appear incompetent by not following my rules-of-thumb. Just use this advice as a starting point.

3. Low-frequency energy seems to socialize near walls. Therefore, if you wish to increase bass frequency energy, put your couch near a wall. By judiciously placing the couch, you can get the best spectral balance that pleases you the most.

4. Side-wall placing is not particularly critical with this speakers. This is because an energy null exists at the sides of the speakers and little energy reaches the side walls. The exception is low-frequency energy, which goes everywhere with careless abandon. Acoustical devices to control "lows" must be huge to really effective, since the wavelengths are so long. From long, hard experience in trying to induce bass energy to go where I want it I have found it less frustrating to go where it is camping by moving either the couch or the speakers (or both in some cases).

5. As a starting point, set the speakers about 3 feet from the back wall and toe them in such that if imaginary perpendicular lines were to extend from the center of each speaker, they would cross at your head position as you sit at your favorite listening spot. Initially, place your couch about 12 to 16 feet from the speakers. Separate the speakers approximately 8 feet apart, center-to-center. The closer you sit to the speakers, room influence on the sound is less.