Advances in Loudspeaker Technology--A 50-year Perspective (TAS 196)

A 50-year Perspective

We begin our journey circa 1957, the dawn of the stereo age, with the introduction of the first full-range electrostatic loudspeaker (later known as the ESL-57) by Quad Electroacoustics. Although the principles of electrostatic transducers were already well established, it was the lack of suitable diaphragm materials that constrained its commercial development. The advent of Polyethylene Terephalate (PET) films made the Quad’s nearly massless moving diaphragm possible. (Nowadays almost everyone is familiar with such films as Mylar, DuPont’s original trade name for this material.)

Peter Walker’s pristine “window” onto the orchestra did justice to Quad’s motto: “The Closest Approach to the Original Sound.” And it quite remarkably remained in production for 28 years. I’m not sure whether it was a question of Quad having set the bar too high in the midrange or the industry’s infatuation with the frequency extremes that focused development effort for the next 50 years on woofers and tweeters. Certainly, from a marketing standpoint, the new notion of high-fidelity sound argued for extended frequency response.

As with the Quad ESL, massless tweeters also became a commercial reality in the late 1950s. An improved version of Dr. Sigfried Klein’s ionic tweeter was commercialized in 1956 by Dukane in the U.S. as the Ionovac. An RF oscillator ionizes a small air mass, creating a plasma, which can then be modulated by the audio signal transferring vibrational energy directly to the surrounding air. No moving parts, no breakup resonances, and an extended bandwidth—truly the ingredients for a superb (but costly) tweeter. Another approach to creating an ionic tweeter is represented by the corona wind transducer. Here a high voltage applied to needle-like electrodes creates a corona discharge, ionizing nearby air molecules. I even recall a couple of “full-range” corona wind speaker prototypes that made it to trade shows, but neither of these was released commercially. Probably the most promising was Nelson Pass’ Ionic Cloud, a prolific ozone producer that nearly did Nelson in. Efficiency of ionic tweeters is generally low and they are usually coupled to a horn for that reason. An exception to the rule, and my favorite plasma tweeter with a reach down to around 1kHz, was Dr. Alan Hill’s Plasmatronics helium-plasma transducer. Helium gas was used to minimize ozone production. I should know, being a former owner (in the mid-80s) and having wrestled lab-size helium bottles across the house to feed the Hill Type 1 loudspeaker. Plasma tweeters are still a commercial reality today and are incorporated in systems offered by two German firms, Lansche Audio and horn-specialist Acapella.

Another tweeter well ahead of its time was the Decca ribbon. Similar in conception to a ribbon microphone, the diaphragm is a thin (think fragile) aluminum ribbon centered between the poles of a magnet. The signal is typically fed to the ribbon element through a transformer in order to maintain decent impedance at the amplifier output terminals. The Decca used horn-loading to improve efficiency.

As you can see, all basic transducer mechanisms were well known by the end of the 1950s, and since the laws of physics have not changed in the interim, advances in the art since then have been mainly technological in nature, aided by a few conceptual breakthroughs. Many of these advances were materials-based. Nowhere is that more evident than in the evolution of the dome tweeter. The 1” soft fabric or plastic dome has adorned the front baffle of countless two and three-way box speakers over the years. It isn’t particularly fast or resolving, but its one saving grace is a smooth response, as it breaks up in a controlled manner without significant peaking. The ideal dome should have low mass for good efficiency and sufficient stiffness to push break-up resonances well above the audible bandwidth. Early light-metal domes such as aluminum and titanium had much faster sound velocities (i.e., excellent transient speed) relative to plastic, but audible resonances were a spoiler. Two fairly recent high-end tweeter developments aimed at scaling stiffness to mass utilize magnesium and exotic beryllium. Significant research and development were also expended on developing ceramic dome and cone materials. The payoff is excellent impulse response with internal damping to prevent ringing. Molding sintered ceramics to sufficiently thin profiles to reduce mass and maintain good efficiency is a challenge. Accuton, an industry leader in this area, produces a very thin concave aluminum oxide diaphragm in a sapphire matrix. Accuton has outdone itself recently with the introduction of the world’s first true diamond diaphragm. About five times harder than Accuton’s own ceramic membrane and an exceptional heat conductor, it demonstrates that diamonds are not just a girl’s best friend.