Once upon a time, when there were only a handful of large-scale loudspeaker system manufacturers, enterprising sound companies typically built their own speaker cabinets, loading them with the drivers and electronic components best suited to the budget and intended application. As in all the best business models, supply expanded to meet demand, and, once system manufacturers had enough orders on hand, economies of scale in both production and R&D tended to outweigh the perceived benefits of custom systems. Today, a manufactured system from a recognized supplier offers a sensible alternative to home-brew solutions, and a wide array of professional-quality off-the-shelf speaker systems are now available at a variety of price points.
With the plethora of manufactured systems available today it's worth stepping back to survey current trends in loudspeaker system design. A major influence on current system design is the computer. Now more affordable and versatile than ever, computers and associated measurement and manufacturing technologies have been used to advance the design and manufacturing process. Another irreversible trend has been toward compactness. Increasingly tight budgets for live productions, which must nevertheless satisfy artists' and audiences' appetites for greater production values, encourage efficient system designs that take up less truck and hanging space.
AMPS IN SPEAKERSRecently, some speaker manufacturers have developed self-amplified, modular, full-range subsystems that require only AC power and a line-level input signal. There are several benefits of self-powered designs. The material costs of amp racks are eliminated, and considerable space can be freed up, both in the truck and at the venue. Since amplifier/speaker load characteristics may be very closely defined, integrated amps and processors may be optimized for specific drivers. The cable runs within an integrated system are, of course, very short, which can greatly simplify the design.
Gain differences, amp flavors and connector incompatibilities are virtually eliminated in self-powered systems, simplifying the cross-rental of inventory. Individual control of each speaker via line-level processors allows levels, frequency response and delay times to be individually manipulated for greater deployment flexibility and array optimization.
Of course, companies that upgrade to a self-powered speaker system must face the problem of recycling a large part of their older, nonintegrated inventory. It may make more financial sense to sell off a used P.A. in its entirety, rather than simply relegating old cabinets to the back of the shop to be slowly stripped of drivers, castors and connectors.
There's also the problem of re-engineering the sound system infrastructure. Though speaker cables are eliminated, line-level signals and AC power now must be distributed to each cabinet. With amplifiers integrated into speakers, adjusting individual channels often goes from inconvenient to impossible, so manual volume controls are eliminated, reinforcing the push toward control and monitoring solutions for amplifiers, along with distributed processing. The hope for a universal standard for computer control by the end of the century may lead us to the Holy Grail of a self-equalizing sound system.
Several speaker manufacturers already make traditional amplifiers along with processors for their own products, and many leading amplifiers have provisions for onboard processing. Since self-powered products compete directly with traditional amps, it would be surprising if many amp manufacturers didn't already have competing designs on their drawing boards. How long can it be before off-the-shelf speaker-mount amp modules become available for proprietary builders?
MORE POWEROver time, increasingly powerful amplifiers have forced transducer manufacturers to rise to the challenge. The materials and geometry of the voice coil, magnet and motor assemblies of transducers have remained unchanged for decades, with minor refinements offering incremental advances in power handling. Ferrofluid, which changes the thermal properties of the voice coil, has been widely accepted as a tool for increasing transducer power handling. The introduction of neodymium magnets has produced some weight savings, and better adhesives, formers, cones and coils have gradually increased power handling in professional products.
Recently, the introduction of a double-voice coil topology in several successful speaker products has created interest from not only buyers of manufactured speaker systems, but also those who use transducers in their own proprietary systems. The idea of using dual differential voice coils goes back nearly a half-century to the days when power-handling in speakers was not a big concern. An AES paper by Doug Button describes how, by designing a magnet with dual gaps of opposing flux and using twin differential voice coils, power handling is doubled or power compression is halved while inductance is lowered.
Significant weight savings are also achieved over the traditional speaker topology by using neodymium and placing the magnet inside the coils. The use of neodymium additionally offers a reduction in flux modulation, translating to lower distortion. Adjusting the spacing of the twin voice coils can provide an increase in linearity without adding the weight of a longer voice coil.
COMPOSITE CABINET MATERIALSThe physical properties of speaker cabinets themselves have remained unchanged the longest. Thicker walls, denser hardwood plys and sturdier bracing have all been used to minimize the efficiency losses caused by energy escaping from the back and sides of the speaker cabinet, but all these methods add weight. For years studios have resorted to concrete construction to eliminate resonances in large-format control room monitors, but this approach is not practical for most portable applications or installations. However, materials with higher stiffness-to-mass than wood are now becoming available, and these materials can improve performance and reduce weight, while also reducing the cabinet's resonance so that it acts more like a perfect enclosure with all its energy delivered in the intended direction. (It's surprising how often the total polar response of a sound system is ignored.)
Carbon-fiber composite materials were originally developed for use in aerospace applications where weight, strength and rigidity are of paramount importance. Civilian applications of carbon-fiber composite materials now include speaker cabinets that weigh less than half as much as similar cabinets built from wood. Speakers in non-resonating enclosures provide up to 3 dB more output than the same transducers in a traditional wooden box, and, because there is no energy coming off the back of the speakers, monitor cabinets made from these materials are an engineer's dream.
An extra benefit of carbon-fiber enclosures is the extremely durable nature of the material. There is no need for paint or carpeting, and they require little maintenance. Though carbon-fiber composite cabinets cost at least twice as much to build as traditional enclosures, the extra output and lighter weight offer an attractive payback.
LINE ARRAYSThe speaker design archives are littered with good ideas whose development was limited by the technology of their day. Modern electronics, materials and manufacturing methods now make it feasible to revisit some of these older concepts. For example, the positive coupling exhibited by neighboring speaker components that operate in the same bandwidth is well understood. Long before efficient horn-loaded designs were developed for touring systems, speaker manufacturers discovered that the combined response of a vertical line of closely coupled transducers offered improved performance, better than the sum of the individual drivers. Many speaker enclosures for public address featured this design, and engineers of a certain age will no doubt remember the Shure and WEM columns that predated horn-based systems.
As a practical matter, concert engineers soon learned that transducers and cabinets arranged in a column behaved differently when the same system components were piled in a block. This led to the common arrangement of drivers being aligned vertically, both within cabinets and throughout modular speaker arrays. Coupling between cabinets has long been accepted as a fact of life in concert sound reinforcement.
Several years ago, a landmark AES paper co-authored by a couple of French scientists showed that when adjacent speakers are close enough, the array behaves like a single sound source having the same shape. This effect is, of course, frequency-dependent, which is why it has been observed in the low and midrange frequencies of many systems for years. The paper further established that this coupling also occurs when the total area of radiating sources is at least 80% of the total area of their arrangement. Practical demonstration of the paper's contents can be found in the V-DOSC[superscript]TM system. The vertical alignment of the tall, thin openings of the unique V-DOSC high-frequency wave-guides allows the coupling to extend into the highs to match the propagation at lower frequencies. The entire array behaves as a single sound source, without beams or seams, and with well-defined coverage.
Several speaker manufacturers have already taken note of the success of the V-DOSC line array and have fielded similar systems, while others are working on designs that have yet to see a trade show. (The V-DOSC high-frequency wave-guide is patented and cannot legally be a part of other designs unless it is licensed.) No doubt there are those frantically looking for new ways to couple super-tweeters. Old road dogs are reminded of the days when metal radial horns were stacked up together in a column.
This short overview of current trends in loudspeaker system design is not necessarily a roadmap for the future. Though future loudspeaker products may incorporate some or all of the new technologies outlined above, traditional configurations will continue to serve in a wide range of applications. Though compact, self-powered line arrays made of carbon-fiber composite materials and including new, higher-powered dual-coil drivers may offer users a competitive advantage in certain circumstances, there will be situations in which the most technologically advanced solution may also be the most expensive. Successful sound system operators realize that the decision to purchase new equipment must be based on sound business reasons, as well as the desire to provide the best tools for the job. Until the return on investment of sound systems based on new technologies rivals that of more traditional approaches, the best speakers for the job may be the ones already in inventory.