18.4.3 Multidirectional Surround Loudspeakers
Multichannel music is a latecomer to the sound reproduction scene. Professionals on the music side of the industry see surround loudspeakers as functionally equivalent to the front loudspeakers, placed in locations as shown in Figure 15.10a. Home theaters sometimes follow this lead, perhaps embellished with additional loudspeakers of the same kind in locations as suggested in the ITU layout (Figure 15.10b), the Dolby layout (Figure 15.11a) or the author's composite suggestion (Figure 15.11b). These schemes have similar conceptual foundations, and they all work well.
But the movie motivation for multichannel reproduction has a longer history. Its practices have changed with time because of evolving technologies on the professional side of the industry.
Back in the very early days of home theater, we were stuck with a single, limited-bandwidth surround channel in the Dolby Stereo encoded signal. The task of the surround channel in those days was to provide a sense of envelopment and very occasionally to assist in the illusion of a flyover of some sort. There was no ability to direct sounds left or right, channel separation was limited, and sound quality in the surround channel was seriously compromised.
In cinemas, several surround loudspeakers were dotted along the walls and across the back of the auditorium (see Figure 15.3), which when combined with sounds reflected around the large room, provided a pleasant sense of being surrounded by sound, although those in the center of the cinema were treated to the best illusion.
When this experience was replicated in small rooms in homes, using only a single pair of surround loudspeakers, the effect was less than impressive. Inverting the polarity of one loudspeaker helped with the "in-head" localization if one sat in the exact middle, but sitting off-center led quickly to localization of the nearer surround loudspeaker. This is easily explained by the inverse square law propagation loss of conventional small loudspeakers, as illustrated in Figure 16.8b and the fact that the surround signal was mono.
To lessen this effect, Shure and then THX introduced electronic decorrelation into the signal paths of the surround loudspeakers, which eased the localization problem and improved the sense of envelopment (see Section 15.6). However, the problem for off-center listeners, especially those close to a side wall, remained a problem, for the reason explained in Figure 16.8: The perception of envelopment diminishes as the left-right sounds differ in level, and localization of the nearer loudspeaker is probable.
The idea of using bidirectional out-of-phase loudspeakers was introduced by THX as a means of reducing the level of the sound aimed at the listeners and increasing the level of the sounds aimed toward the front and rear. These were called "dipoles." However, a true dipole, or doublet, consists of two sources separated by a very small distance and radiating in opposing phase (Beranek, 1986). Unbaffled diaphragms are approximations of true dipoles. The typical "dipole" surround loudspeakers employ small-box enclosures in which the front-and back-facing drivers are separated by distances that are large compared to the radiated wavelengths, and as a result the directional patterns become very disorderly.
Figure 8.6 shows that sounds arriving at the listening location after reflection from front and back walls are not very effective at generating envelopment. Fortunately the loudspeakers were not true dipoles with sharp nulls, as shown in Figure 8.7; there was substantial "leakage" of direct sound in the direction of the listeners. Nevertheless, the combination was deemed to be acceptable for film sound. Because the record/playback technology of the time did not permit it, there was no intention that individual surround channels be used as stand-alone, localizable sound sources.
Then the situation rapidly improved. First there came several playback algorithms that offered a measure of separation between the left and right surround channels: Dolby ProLogic, Harman/Lexicon Logic 7, Fosgate 6 Axis, and others. Surround channels acquired a full-frequency range. Ultimately, the introduction of digital discrete delivery systems, Dolby Digital and DTS, removed all of the old restrictions.
Suddenly, the original five loudspeaker layouts sounded much better, and inevitably the algorithms were elaborated to provide for six or seven channels. It was a new game. Sound designers for films could themselves control the decorrelation in the surround channels, varying the sense of envelopment, and they took advantage of the discrete surround channels by sending localizable sounds to the left, right, and rear (if available). So the surround channels acquired a new job in addition to providing enveloping ambience. It was time for the playback system to stop "editorializing" and just let the art come through. The motivation for the dipole surround loudspeaker had disappeared.
The residual problem was that it all could work well for those seated in the center of the loudspeaker array, but listeners seated close to the side walls could localize the surround loudspeakers even when they were not intended to be localized - that is, when they were delivering enveloping, ambient sounds. Elevating them helps, but the real problem, as shown in Figure 16.8, has to do with propagation loss. To deliver the correct impressions of the localized sounds, listeners must receive strong direct sounds from the surround loudspeakers so the precedence effect can work. This, as is pointed out in Figure 16.10, requires that the surround loudspeakers have uniformly wide horizontal dispersion (up to ±70° in the example).
Still, dipole surrounds continue to be used by some installers. One of the popular justifications is that they seem to lessen the localization problem. As will be shown in Figure 18.20, the explanation has to do with the greatly attenuated high frequencies in the direct sound, not the bidirectional directivity. Rolling off the high frequencies in the surround channel was used in the first generation Dolby Stereo mixes to prevent audiences from localizing sibilant "splashes" leaking from the front channels because of playback errors in optical sound tracks.
Many manufacturers realized that the "bipole" (bidirectional in-phase) configuration delivered a satisfying set of surround illusions, and that option has become commonplace. New audio jargon began to include descriptions of different surround loudspeaker options based on how "diffusive" they are. Since diffusion is a property of the sound field, what is being referred to is how "dispersive" they are. The ±70° horizontal dispersion requirement found for the sample home theater in Figure 16.7 means that many conventional forward-firing loudspeakers may have difficulty covering a large audience if there are only two surround channels. Adding more surround channels is one solution - certainly if the loudspeakers can be aimed to optimally cover the audience and the added decorrelation contributed by the additional processed channels helps with envelopment.