Something Old, Something New

Loudspeaker systems have used cone and dome transducers almost since their inception in the late 19th century. We've seen advances in speaker technology,
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Loudspeaker systems have used cone and dome transducers almost since their inception in the late 19th century. We've seen advances in speaker technology, of course, such as the development of compression drivers, electrostatic-planar designs, and Kevlar. But in many respects, speaker transducers have remained unchanged since General Electric researchers Chester Rice and Edward Kellogg patented the moving-coil, direct-radiator speaker in 1924.

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FIG. 1a: The MRS diaphragm includes ribs and notches (gussets) to minimize breakup modes.

Now, a new driver design from Infinity (www.infinityspeakers.com) adds an interesting alternative to the mix. As part of the Harman group of companies (which also includes JBL, Lexicon, dbx, and other names familiar to electronic musicians), Infinity is a well-regarded manufacturer of speakers for the consumer market. The company's engineers were given a mandate to develop a new driver that would accommodate low-profile cabinets to accompany flat-panel video displays without sacrificing sound quality. Almost two years and 8,000 man-hours later, the resulting driver is called the MRS (Maximum Radiating Surface) flat-panel transducer.

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FIG. 1b: Two elongated voice coils apply uniform force to the entire surface.

MRS is said to combine the best attributes of traditional cone and planar drivers, using a flat, rectangular diaphragm measuring 7 × 3.5 inches that provides the same radiating surface area as a 6-inch cone. The diaphragm is made of Infinity's proprietary CMMD (Ceramic Metal Matrix Diaphragm) material, which consists of a stiff aluminum core anodized with layers of alumina ceramic on both sides to provide a good balance between stiffness and mass.

The diaphragm is formed with beveled edges, raised ribs, and notches (called gussets) around the perimeter (see Fig. 1a). The beveled edges add rigidity, and the ribs and gussets are spaced to minimize breakup modes; in fact, in the company's first commercial MRS products, the first major breakup mode is around 10 kHz, far above the 2 kHz crossover frequency. The motor structure uses three high-energy neodymium magnets and two 5-inch-long cigar-shaped voice coils (see Fig. 1b) that apply force uniformly over the entire surface. As a result, all parts of the surface move in phase, with very little breakup.

The diaphragm is mounted in a low-profile, self-contained module that is vented in the rear to dissipate heat and reduce standing waves. The rectangular surround is attached to the module in the same plane as the voice coils to minimize so-called “rocking modes,” and the corners of the surround are rounded with a carefully calculated radius to allow for maximum movement without collapsing. According to Infinity, a larger model would need a second suspension, which can be added without increasing the overall depth.

All of those factors result in a shallow mounting depth, as with planar drivers; good mid- and low-frequency response, like that of cone drivers; and a peak excursion that is more than 25 percent of the overall driver depth (compared with 10 percent for planar and 12 percent for cone). In addition, the modules are designed for mounting horizontally or vertically and very close together to form a line array.

In the initial line of speakers, MRS transducers are mated with a 1-inch CMMD dome tweeter mounted in a constant-acoustic impedance (CAI) wave guide that imparts the same directionality characteristics as the MRS diaphragm and provides some gain, improved dynamic range, and lower compression and distortion. Those speakers are intended for use with a subwoofer crossed over at about 80 Hz, but a larger MRS driver might not need a subwoofer to hold up the bottom end.

During the press introduction to the new line, I got to hear these speakers in several different listening environments, and I was impressed. The MRS transducer is aimed at the consumer market for now, but it has the potential to affect all segments of the speaker industry, and studio monitors may not be far behind.