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Understanding Home Theater Audio

Aug 11, 2010 10:58 AM, By Bob McCarthy

Putting large-scale and small-scale audio in perspective.


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Figure 2. Compare and contrast the surround experience in the home with wall reflections in the arena at 100Hz. Top: The home surrounds create lateral sound from a small number of sources, creating a nondiffuse interaction at low frequencies and easily localized in the midrange. Bottom: The arena walls create diffuse reflections in the LF and create a spread spatiality over a wide frequency range. See a larger image.

Stereo rescaled

When you arrive at the arena concert, you see loudspeaker stacks on either side of the stage and immediately recognize the familiar stereo configuration of your home. This is a mirage. The reason is scale. The stereo experience does not translate into the big room because only one of the two vital parameters for stereo experience survives the change of venue: level. The time component does not rescale, and our binaural perception is incapable of neglecting that aspect. Stereo is a horizontal plane experience. To experience the smooth continuous panoramic placements of the recording studio mix, we must be in a position to have only one changing variable between the channels, and that position is dead center. As soon as we move off-center, the panoramic horizon becomes distorted and compressed. The time offset between the loudspeakers has a strong effect on localization, a factor that competes with and often dominates the level component. Time offsets of a few milli­seconds are sufficient to overcome even relatively strong level panning. It will take 8dB of level pan to match just 5 milliseconds of time offset. In the home environment, we can stay close in time even when we sit a bit off to the side. The upscaled world of the arena, however, has very few seats within a 5-millisecond window of the left and right loudspeakers. Stereo separation is much more expansive in the studio world, but in the arena, we seldom find much more than a few tom drums and keyboard effects hard panned to either side. In cases where important instruments are moved apart, lots of people wonder why the mixer is only letting them hear half the show.

Surround scaled in reverse

Did somebody say flop? Betamax? No, that was video. For audio, flop is spelled “quadraphonic.” Back in the day, some folks thought the demise was due to format wars, but really it was all about totally missing the point. Surround in a small space was an attempt to reverse the direction of audio scaling—hoping to get the big hall qualities into the small space. Adding two loudspeakers in the rear corners did not fool anyone into thinking they were in a concert hall. It sounded like, for lack of a better explanation, two easily localized loudspeakers in the corners.

The modern 5.1 cinema-style surround system is able to take things a step closer. Sound images can be moved to the outer limits, but creating a seamless circle of sound requires an array of loudspeakers spread along the surfaces. This has practical limits in the home. Cinemas and musical theatrical productions use surround loudspeakers for special effects, but this is still a far cry from the complex multifaceted, multidirectional, spread-over-time reverberation that occurs in an actual large space. Once again, scale rears its head. In a large space, the full range of wavelengths are in a very complex weave, even the large low frequencies have sufficient space and decay time to envelop our ears with spatial clues. Meanwhile, in a small room, it is a challenge to localize the lows in any event, whether they are coming from one source or 10. The timing of the reflections is simply too close to discern separation. There are technological solutions that can recreate large-room acoustics in small, or acoustically dead spaces. These are reverberation enhancement systems that use densely spaced loudspeakers and recirculating microphones to replicate standard room acoustic responses. These are even used in rehearsal rooms that are no bigger than your living room and start at around $250,000.

Rooms big and small. Wavelengths big and small. Ears that stay the same size for all of them. Next time you see an ad that promises a true concert-hall experience in your home, you might consider this. Next time you hear that the concert will be mixed in stereo, you might pay the extra money to get a seat on the center aisle—after all, prices are scaled accordingly.

Bob McCarthy is president of Alignment and Design. McCarthy specializes in the design and tuning of sound reinforcement systems and conducts trainings around the world. His book, Sound Systems: Design and Optimization, was named “2007 Sound Product of the Year” by Live Design. Visit his blog at bobmccarthy.wordpress.com.



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