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Technology, Theory and Design

The Zen of Loudspeaker Placement

At Surround 2002, I asked Tomlinson Holman about his thoughts regarding a topic of continuing controversy in both the pro and consumer worlds: the placement of loudspeakers in a multi-channel playback system. Specifically, I brought up the setup approach circulating around the Internet, driven advocacy by ‘Widescreen Review’ magazine, for an unorthodox “6.1” setup – essentially a classic quad-style “X” with front left and right speakers each at 45 degrees from a center, mirrored in the rear with surrounds and single rear center. “That would be typical of the internet,” he answered, “It’s all wrong!

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45 degrees might evoke nostalgia for the days of quad, he pointed out, “…but it was thoroughly debunked at the time by the BBC, among others.” The problem with such a configuration, Holman insisted, is that “it leaves huge gaps from 45 to 180 degrees, so side imaging – and by the way envelopment – is out!” Holman agrees with long-established research into lateral phantom imaging that “Envelopment is best at 90 degrees, and rearward imaging better at 135 degrees when there is no rear channel (other angles haven’t been tested to my knowledge in a rigorous test).” Given the better envelopment gained by sticking with speakers placed close to the sides (either at 90 degrees or 110 degrees per ITU specifications), Holman said, “the best thing you can do is a center back channel.”

The use of a center back channel to fill in the rear soundfield gap naturally raised the problem of front-back reversal along the centerline axis of the head, in which the listener can perceive sounds originating in the rear as coming from the front, or vice versa. “I’ve heard it,” Holman acknowledged, “though it’s very contextual. If you’re expecting a sound to come from the front, if it’s played in the rear you’ll still hear it coming from the front. It’s most likely to happen with voices, but it’s still very rare.”

Reversal issues are a major reason why home 7.1 speaker setups recommended by THX, Dolby, and DTS all utilize dual rear speakers for their matrix-extracted mono rear channel. (Manufacturers of surround processors with proprietary decoding such as Lexicon and Meridian also utilize 7.1 speaker setups). Nevertheless, Holman is not convinced that the potential problem warrants the use of valuable speaker resources. “Another channel would not be harmful,” Holman said, “it then might be split centered on 180 degrees. There are a lot of ways to approach it, but I think it’s crazy to put more emphasis on imaging in the rear than in the front.”

Holman is convinced there are more gains to be realized by adding more front channels, citing three reasons: (1) there are fewer natural discrete reflections of importance in the rear region, whereas abundant literature on the design of concert halls indicates that the points of first reflections, at about 60 degrees, are the most important. (2) psychoacoustically, the poorest region for finding the minimum audible angle is straight overhead, to the sides and behind; and (3) the preponderance of sources in the real world are located in front of us. “This is even more true in home theater,” he pointed out, “where there is a picture only at the front, where for verisimilitude the picture and sound images should match.”

Referencing Theile’s research findings that four front channels beat three on imaging, Holman pointed out that, when defining a loudspeaker setup for home theater, “We nevertheless had to have an odd number because all films in inventory had been made with a hard center channel – no phantoms with their problems allowed for center!” That meant a frontal array of number is three or five, and three was the only practical answer in those early days.

However, if given seven channels to work with, Holman said he’d rather see a front center, left and right fronts at ±30 degrees, left-wide and right-wide at ±60 degrees (for early reflections), and left surround/right surround at ±110-120 degrees as the best allocation of resources. “Of course it is possible to have other opinions,” he conceded “but each of these has documented reasons for being, and my order of them makes physical and psychoacoustic sense.” In his experimental 10.2 system, covered in SMR’s CES 2001 report, Holman uses exactly this basic configuration, augmented with a pair of height channels, a rear center, and two (enormous) subwoofers. However, Holman’s system is not an encoding scheme but a reproduction platform for his own proprietary demonstration recordings – and very much at odds with the current state of commercial home theater. “I can’t help what the marketplace does,” he shrugged. “Sometimes I think we are in the ping pong stereo era of multi-channel sound!

Ever the iconoclast, Holman took two more noteworthy shots at orthodoxy during his densely detailed session on recording techniques for the Miami New World Symphony. Despite conventional wisdom against adding delays to spot mics used in close proximity to the instrument, Holman reported that he achieved greater clarity in both level and time of spot-miced instruments with a 15ms delay relative to their main microphone signal.

The second innovation Holman discussed in his lecture was a novel solution to the ineffectiveness of traditional cancellation techniques to filter out background noise. Holman discovered that wrapping a microphone in a condom and placing it in the air conditioning duct yielded an ideal cancellation signal. At TMH Labs, the sonic prophylactics of choice are Trojans. []


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Show report last updated: 2nd February 2003.

 

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