Self-empowerment: Fully contained speaker systems meet the demands of immersive surround formats
The film exhibition industry is understandably upbeat about the new immersive audio formats, Dolby Atmos and Barco Auro 11.1. Both systems offer exhibitors the means for gaining a competitive edge relative to home systems and across-town rivals. Granted, operators grumble about choosing one or the other in “format wars.” Regardless of the outcome, however, the adoption of immersive sound requires a substantial investment in new amplifiers and loudspeakers. And it’s not just a matter of a greater quantity of loudspeakers. To realize the audience-exciting potential of immersive sound, it is important for exhibitors to consider the demands these new formats place on the loudspeakers, particularly the surround components.
A Different Role for Surrounds
Driving the need for high-quality surround loudspeakers is the capability to incorporate “object-oriented” sound design into soundtracks. This is critical, because instead of having multiple speakers all along one wall carrying the same ambient sounds, “object-oriented” technology places and moves a discrete sound source (helicopter, cannon shot, shouting voice) in and out of single loudspeakers. Movie sound designers match these effects with the picture to bring the audience into the story. If the sound is not consistent when moving from a screen channel into a surround channel, the effect falls flat.
It’s not simply a matter of relative loudness. Realizing the optimum effect also demands superior dynamic range, flat phase and frequency response, and linear reproduction across all operating levels. That’s a tall order for a two-way system using a passive crossover at the end of a long speaker cable. It’s an assignment far more suited to technology widely adopted in other professional applications: self-powered loudspeaker systems.
What’s the difference between self-powered and conventionally powered loudspeakers? Superficially, self-powering simply means that the amplifier and associated processing are located inside the loudspeaker cabinet, not in a distant equipment room. But it’s not simply the packaging. In high-quality systems, the internal amplifier and processing components are meticulously designed and calibrated to work with the specific transducer (woofer or tweeter) to which they are connected. These fully self-contained systems make it possible to deliver predictable and repeatable results. Also, all quality self-powered systems are internally bi-amplified: Separate amplifiers drive the woofer and the tweeter to achieve optimum performance. Most conventional surround systems instead use a single amplifier channel to drive both the woofer and the tweeter through a passive crossover, which inevitably compromises sonic accuracy.
Long Loudspeaker Cables Eliminated
Typical surround systems also have long cable runs between the amplifier and the loudspeakers. As hi-fi enthusiasts know, you must either use very thick (and expensive) speaker cable or suffer losses in both high-frequency accuracy and bass transient attack. In contrast, the line-level input to a self-powered loudspeaker exhibits no signal degradation within even the largest screen room. (Losses between the amplifier and speaker in a self-powered system are not measurable, as the cable is barely inches long.) In addition, self-powered systems include equalizers and phase-correction circuits for linear frequency and phase response, optimized active crossover design, and limiter circuits that provide reliability and protection of components.
Self-powered systems first achieved widespread acceptance in the 1990s as recording studio monitors, where recording engineers quickly discerned their increased accuracy and spatial imaging. Today, the transition is nearly complete in that market: Most recording studios have adopted self-powered near- and mid-field studio monitors. The transition has since moved into film post-production, where facilities like Skywalker Sound, Wildfire Post-Production Studios, and London’s De Lane Lea now rely on self-powered monitoring systems. Self-powered systems are also increasingly preferred in sound-reinforcement systems for live performance venues and concert tours.
Then why have self-powered systems lagged behind in film exhibition? In the past, self-powering introduced added costs that were not justified. The approach of using passive surrounds on long cables began with Dolby Surround and evolved through 5.1, 6.1 and today’s 7.1, offering good backwards compatibility. Despite known sonic compromises, they worked well enough to meet the relatively limited demands.
This Is Different
However, with the new immersive surround formats, everything has changed. Now every surround loudspeaker must have its own amplifier channel and a cable run back to the booth, regardless of the technology employed. These formats can accommodate up to 64 channels, which means a lot of expensive loudspeaker cable runs in conventionally powered systems.
Consequently, self-powered systems have become far more cost-competitive, particularly in light of recently introduced low-voltage, self-powered surround systems. No longer is it necessary to have AC power available at each surround location. With the new low-voltage systems, internal amplifiers are powered by 48 V DC from a remote power supply, rather than from a local AC outlet. Power and the line-level audio signal are carried by a single, lightweight five-conductor cable that does not require Class I conduit. There are no additional installation costs, and the total amount of copper used is less than needed for an appropriate loudspeaker cable connecting a passive system.
Low-voltage, self-powered surround loudspeakers are now available in different cabinet sizes. They can be paired with self-powered screen-channel loudspeakers and subwoofers. These larger systems do require local AC power, but this adds no extra cost in new construction, as the electrical circuits are simply put in a different location.
Linearity and the Moviegoing Experience
A key concept that governs high-quality cinema sound reproduction is linearity. The basic idea is simple: The acoustical sound that comes out of a loudspeaker should accurately represent the electrical signal that went into it, regardless of volume, and regardless of the nature of the sound.
High-quality, self-powered systems are inherently better suited to linear performance. Linearity enables better matching of screen and surround loudspeakers, as well as surround speakers of different sizes. Linearity also means improved sonic imaging between multiple loudspeakers, enhancing the illusion of sonic movement in space.
Both conventional and self-powered systems can amplify sound. But the difference in linearity is measureable, and it is clearly audible in A-B comparisons. Audiences may not measure it. For them, greater linearity translates into a heightened sense of realism, immediacy and impact that is not experienced elsewhere—not at home, and not with the conventional loudspeakers across town.