Floor Loudspeaker Reviews

Sidebar 2: Planar Imaging

I find planars to be the only speakers inherently capable of properly reproducing image size. I'm just as impressed as the average card-carrying audiophile with the soundstaging capabilities of a good minimonitor. Such a point-source speaker can be adept at fleshing out a convincing soundstage with excellent width and depth. However, its portrayal of image outlines tends to be too specific. Thus was born the cliché of "pinpoint imaging." Ironically, this is bandied about as a positive attribute, when in fact it's totally artificial.

[Actually, DO is only right in the broader sense, in that real-life images are larger than pinpoint ones. But those images have to be squeezed through the distorting lens of the microphones and recording process. When you consider what imaging information is carried on the two channels of a stereo recording—amplitude and timing differences—the "pinpoint" imaging produced by point-source loudspeakers is actually an accurate representation of that information. What is lost by all current stereo recording systems is the intensity characteristics of the original soundsources; as DO points out next, large planar speakers synthesize reproduced intensities that are closer to the originals.—Ed.]

Looking at the issue from a technical viewpoint, conventional speakers may be said to distort the radiation pattern and surface loudness density of many large musical instruments. If you divide a piano's significant acoustic power output by its radiating area, you arrive at the instrument's surface loudness density. This figure is considerably different for a trumpet, where you have a large output concentrated over a small radiating surface. These differences in loudness density provide important clues as to image size, and help define the timbres of particular instruments—from the piercing cry of a trumpet to the stately demeanor of a piano. The impossible problem for a conventional speaker, then, is this: how to convince you it's a grand piano when the instrument's full acoustic power is being squeezed through an 8" woofer.

By its very nature, a loud instrument possessing a low surface loudness density implies a planar acoustic wave launch. A theoretical point source of sound emits a spherical wave, meaning that, in free space, its intensity falls off as the square of the distance: double the distance, and intensity is reduced fourfold. In real life, musical instruments emit sound from a well-defined surface, such as a sounding board, or from the resonating body of the instrument itself. In the instrument's nearfield, the wave launch is planar (with little curvature), meaning that intensity falls off much more slowly with distance. Small head movements near a planar sound source do not produce as sharp a difference in intensity as they do near a point source. The practical meaning of all of this is that the transducer needs to possess these very characteristics if it's to successfully evoke realistic image outlines.

Some readers have questioned the premise that planars are in fact needed to realistically reproduce a piano. Yip Mang Meng of Singapore wrote me to ask whether "5'-diameter microphones [are] in use," his point being that, after the signal is squeezed through a 1" mike capsule, later reproduction through an 8" woofer should have no effect on image size. It's important to realize that a mike samples only a single point in the soundfield of an instrument. The stereo mike feed contains localization cues that allow us to map the soundstage, but lacks the sort of cues that allow us to determine image size.

Listening live, small and involuntary head movements are used to gauge the spatial extent of a sound source. These are the types of cues that planar speakers synthesize for us in the listening room. Information lost during the recording process is re-created for us by virtue of a planar's radiation pattern and surface loudness density. When I close my eyes and try to conjure up an image size, a planar speaker reproduces piano, double-bass, cello, and human voice with much more spatial conviction than is possible with a conventional dynamic speaker. It's true—a planar makes a recorded piano sound more like a real piano.

Wendell Diller captured the essence of these concepts with the remark that the MG-20s continue in the listening room the wavefront launched originally by the musical instrument. Because the MG-20 is a three-way design, it roughly re-creates the proper range of surface loudness densities. The sounds of higher-pitched instruments are radiated from a small surface area, while large-bodied instruments get the full treatment from the planar woofer and midrange unit.—Dick Olsher