LOUDSPEAKERS

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go to: Contents | Features | Bookshelf, Stand-Mount and Desktop | Floorstanding | Editors' Choice Awards Book Excerpt: Loudspeaker Types and How They Work which means “two poles.” Fig. 4 shows the radiation patterns of a point-source loudspeaker (left) and a dipolar loudspeaker. Another great advantage enjoyed by ribbons is the lack of a box or cabinet. An enclosure can greatly degrade a loudspeaker’s performance. Not having to compensate for an enclosure makes it easier for a ribbon loudspeaker to achieve stunning clarity and lifelike musical timbres. A full-range quasi-ribbon loudspeaker is illustrated in Fig. 5. The large panel extends the Figure 3 system’s bass response: when the average panel dimension approaches half the wavelength, front-to-rear cancellation reduces bass output. Consequently, the larger the panel, the deeper the low-frequency extension. Ribbon loudspeakers are characterized by a remarkable ability to produce extremely clean and quick transients—such as those of plucked acoustic guitar strings or percussion instruments. The sound seems to start and stop suddenly, just as one hears from live instruments. Ribbons Figure 4 Point-source radiation pattern (left) and di-polar radiation pattern (right) sound vivid and immediate without being etched or excessively bright. In addition, the sound has an openness, clarity, and transparency often unmatched by dynamic drivers. (Incidentally, these qualities are shared by ribbon microphones.) Finally, the ribbon’s dipolar nature produces a huge sense of space, air, and soundstage depth (provided this spatial information was captured in the recording). Some argue, however, that this sense of depth is artificially produced by ribbon loudspeakers, rather than being a reproduction of the actual recording. Despite their often stunning sound quality, ribbon drivers have several disadvantages. The first is low sensitivity; it takes lots of amplifier power to drive them. Second, ribbons inherently have a very low impedance, often a fraction of an ohm. Most ribbon drivers therefore have an impedance matching transformer in the crossover to present a higher impedance to the power amplifier. Design of the transformer is therefore crucial to prevent it from degrading sound quality. From a practical standpoint, ribbon-based loudspeakers are more difficult to position in a room. Small variations in placement can greatly change the sound, due primarily to their dipolar radiating pattern. This dipolar pattern requires that the ribbon loudspeakers be placed well away from the rear wall, and that the rear wall be acoustically benign. Low-profile, ribbon-based loudspeakers with the ribbon top at the same height as the listener’s ears will have a radically different treble balance if the listener moves up or down a few inches. That’s because ribbon loudspeakers have very narrow vertical dispersion, meaning they radiate very little sound above and below the ribbon at high frequencies. If you sit too high or listen while standing, you’ll hear less treble. Some ribbon loudspeakers have a tilt adjustment that allows you to set the correct treble balance for a particular listening height. Ribbons also have a resonant frequency that, if excited, produces the horrible sound of crinkling aluminum foil. Consequently, the ribbon must be used within strict frequency-band limits. In addition, ribbon drivers are tensioned at the factory for optimum performance. If under too much tension, a ribbon will produce less sound; if under too little tension, a ribbon can produce distortion that 20 Guide to High-Performance Loudspeakers www.theabsolutesound.com previous page NEXT page
go to: Contents | Features | Bookshelf, Stand-Mount and Desktop | Floorstanding | Editors' Choice Awards Book Excerpt: Loudspeaker Types and How They Work Finally, some loudspeakers use a combination of dynamic and ribbon transducers to take advantage of both technologies. These so-called hybrid loudspeakers typically use a dynamic woofer in an enclosure to reproduce bass, and a ribbon midrange/tweeter. The hybrid technique brings the advantages of ribbon drivers to a lower price level (ribbon woofers are big and expensive), and exploits the advantages of each technology while avoiding the drawbacks. The challenge in such a hybrid system is to achieve a seamless transition between the dynamic woofer and the ribbon tweeter, with no audible discontinuity between the drivers. Figure 5 sounds like the music is “breaking up.” This is most noticeable on piano; the transient leading edges sound “shattered” and distorted. A sudden increase in ambient temperature can cause a ribbon driver to lose some of its tension and introduce the distortion described. If you hear this sound from your ribbon loudspeakers, contact the manufacturer for advice. The solution may be as simple as turning a few tensioning bolts half a turn. Ribbon drivers don’t necessarily have to be long and thin. Variations on ribbon technology have produced drivers having many of the desirable characteristics of ribbons but few of the disadvantages. The Electrostatic Driver Like the ribbon transducer, an electrostatic driver uses a thin membrane to make air move. But that’s where the similarities end. While both dynamic and ribbon loudspeakers are electromagnetic transducers—they operate by electrically induced magnetic interaction—the electrostatic loudspeaker operates on the completely different principle of electrostatic interaction. No discussion of electrostatic loudspeakers would be complete without mentioning the classic electrostatic loudspeaker, the Quad ESL-57, created in 1957 by Peter Walker. The ESL-57 revolutionized the standard for transparency upon its introduction, and still holds it own more than 50 years later. Many listeners’ first exposure to high-quality audio was through an ESL-57. A large number of contemporary loudspeaker designers still have a pair of ESL-57s on hand as a reference. The ESL-57 doesn’t have much low bass, won’t play very loudly, and produces a very narrow sweet spot, but when operated within its limitations, it’s magical. In an electrostatic loudspeaker (sometimes called an ESL), a thin moveable membrane— usually made of transparent Mylar—is stretched between two static elements called stators (Fig. 6). The membrane is charged to a very high voltage with respect to the stators. The audio signal is applied to the stators, which create electrostatic fields around them that vary in response to the audio signal. The varying electrostatic fields Figure 6 Courtesy MartinLogan generated around the stators interact with the membrane’s fixed electrostatic field, pushing and pulling the membrane to produce sound. One stator pulls the membrane, the other pushes it. This illustration also shows a dynamic woofer as part of a hybrid dynamic/electrostatic system. The voltages involved in an electrostatic loudspeaker are very high. The polarizing voltage applied to the diaphragm may be as high as 21 Guide to High-Performance Loudspeakers www.theabsolutesound.com previous page NEXT page
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