Flat panel loudspeakers--A primer

This guide is intended to be an introduction to the various kinds of flat-panel loudspeakers available. Flat panel loudspeakers do not use a conventional "box" to house the sound-making devices. They may or may not use conventional cone drivers--generally not. Those that do use cone drivers (generally for bass notes) are considered "hybrid" speakers.Ribbon, Quasi-ribbon, and Magneplanar drivers all work on the same principle. The difference is how much of the driver surface is also the electrical conductor. In each case, a thin membrane which may be aluminum foil, high-grade Polyester or Kapton (but if you imagine it as household aluminum foil or plastic wrap, you'll have a good mental image) stretched and anchored in some kind of frame. The frame also secures permanent magnets close to the conductor. The aluminum foil is conductive on it's own, the plastic film has a foil or wire conductor glued to the surface in some kind of pattern. The conductor (the sheet of aluminum, or the foil or wire gridwork on the plastic film) begees magnetic because of the electrical current of the amplifier signal, and wiggles in the magnetic field produced by nearby permanent magnets. The wiggling moves the air, and so produces sound. So, to recap: There is a stable magnetic field supplied by a non-moving set of permanent magnets. The musical signal from your amp is supplied to the flexible conductor, which moves to produce sound. An electrostatic speaker looks very similar, but uses a different operating principle. The musical signal from the amplifier is directed through a special transformer. The voltage is greatly increased, and then delivered to non-moving conductors called "stators" in such a way that when one stator is driven , the other stator is driven -. The two stators sandwich a diaphragm much like the permanent magnets sandwich the ribbon. A stable electrostatic charge is delivered to the flexible diaphragm. The electrically-charged diaphragm is attracted to first one, then the other stator as the musical signal alternates. Thus, the diaphragm wiggles back and forth to produce sound. Note that each of these designs--ribbon, quasi-ribbon, Magneplanar, and electrostatic--tend to be dipole speakers. Dipoles put out exactly, or almost exactly, the same sound to the rear as to the front, but no sound is deliberately delivered to the sides. Of course, some sound from the front and the rear "wraps around" the side, but this is really quite small. Because the sound output to the rear of the speaker bounces off the rear wall of the room before making it's way to your ears, it's both reduced in volume and out-of-phase with the sound waves geing from the front of the speaker. It's perceived as an increase in the "spaciousness" of the sound. As a bonus, because there's so little energy (sound) radiated to the sides of the speaker, nasty interactions with the side walls of your listening room are greatly reduced.RibbonA ribbon driver is a flexible sheet of conductive material--aluminum foil for example--that is anchored on it's edges and passes the amplifier's output signal. The action of conducting electricity makes the conductive sheet an electromagent. Because it is located within the magnetic field of some permanent magnets, it is attracted toward and repelled away from those magnets, depending on which direction the alternating current of the amplifier's output is flowing through the conductive sheet. Because it wiggles back and forth, it produces sound of the same frequency as the amplifier signal. The entire sheet is conductive, so the entire sheet--within it's mechanical limits--is attracted and repelled by the magnetic forces. Because ribbons don't handle lots of electrical current very well, they are generally used only as high-frequency drivers--tweeters. A ribbon driver assembly can be mounted in a conventional box speaker, there are several gemercially available loudspeakers that use a ribbon tweeter or supertweeter.Quasi-RibbonA quasi-ribbon driver uses a sheet of flexible plastic, onto which a thin, flat, conductive foil has been glued. The foil conducts the amplifier's output signal, and creates the electromagnetic field that is attracted to, and repelled away from the permanent magnets located close by. Because the foil covers a great deal of the total surface area of the flexible plastic, it mimics a ribbon driver, but since there is some area of the plastic sheet that isn't conductive, it is not a true ribbon driver. A Quasi-ribbon can be less expensive to make than a true ribbon driver.MagneplanarIf a Quasi-ribbon is "almost" a ribbon driver, then a Magneplanar is "almost" a Quasi-ribbon. In this case, the thin flexible plastic sheet has a metal wire glued to it. The wire passes the amplifier signal, and the wire is attracted to and repelled from the magnets located nearby. The wire is capable of handling more current than the conductive foil of the quasi-ribbon, but it does not cover as much of the flexible plastic sheet as the conductive foil of a quasi-ribbon. These drivers will handle quite a bit of current, and can be used into the bass regions. Jim Winey invented the Magneplanar driver, and built his Twin Cities, Minnesota-based gepany--Magnepan--to manufacture Magneplanar/Ribbon and Magneplanar/Quasi-ribbon speakers.ElectrostaticAn electostatic driver is similar to the ribbon, quasi-ribbon, and Magneplanar drivers, in that a large, thin diaphragm is driven over it's entire surface. A difference is that a static electrical charge interacting with the amplifier signal, rather than magnetism is doing the attraction and repulsion. A conductive film carries an extremely high constant voltage, this conductive film is sandwiched between two perforated metal plates called "stators". The stators conduct the amplifier's output once the amplifier output has passed through a transformer to greatly boost the voltage while reducing the amperage. The stators "see" the amplifier signal, but at much higher voltage than the amp can put out on it's own. As the stators are driven by the alternating current of the amplifier/transformer signal, they attract and repel the conductive film located between them. The vibrating of the conductive film makes the sound. If you try to play these speakers too loudly, the center conductor can touch the stators, which results in sparks, localized heat, and therfore melted spots in the conductive diaphragm. Some stators are therefore electically insulated so as to eliminate or minimize the sparking. An electrostatic speaker will have a power cord that plugs into the wall socket. This feeds the step-up transformer that provides the high voltage (but very little amperage) to the diaphragm.Hybrid speakersBecause flat-panel speakers need to be very big to produce much bass, many designs have been tried where a flat-panel midrange and tweeter are gebined with some kind of conventional cone bass drivers. The bass drivers may--or may not--be called a "subwoofer" but they usually operate higher in the frequency range than a true subwoofer. Note that I intend the term "Hybrid" to mean more than just another box speaker using a ribbon tweeter--the biggest part of the speaker will be the flat panel part, not the box. An example here would be Martin Logan's electrostatic speakers which have a cone woofer section, or even Carver's "Amazing" Ribbon/Cone speakers from '86 to about '97 or so when they were discontinued.If this guide has been helpful, please give me a "Yes" vote by clicking the button below. 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