The Care and Feeding of Surround Channels

Last month, as part of my current work involving surround sound, I wrote about the use of subwoofers, from the viewpoints of both audio professionals and consumers. This month, I'll tackle the surround channels.
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Last month, as part of my current work involving surround sound, I wrote about the use of subwoofers, from the viewpoints of both audio professionals and consumers. This month, I'll tackle the surround channels.

To my way of thinking, these channels are extremely important simply because they provide the "surround" in surround sound. Even more important, they enable a sensation of envelopment that, when done well, is both subliminal and tremendously powerful and engaging.

WHY DO WE WANT SURROUND?

Let's talk about envelopment briefly. When we listen to normal acoustic sounds in the real world, those sounds present to us both information about the sound source and also information about our environment.

The various reflections of a sound that arrive at our ears within the first 50 milliseconds are integrated with the direct sound to form a singular holistic perceptual construct. This construct includes identification and localization of the sound source, which is presented to our conscious minds. In addition, this construct includes information about the size of the space we're in, as well as location of the walls, floor and ceiling and the nature of the materials used on those various surfaces.

This latter information is not directly presented to our consciousness, but is pervasive, compelling and essential for our mental perception nonetheless. It is particularly important to processing spatial information outside of our visual field.

Surround sound, bless it, provides us with very real sonic cues about such environmental information in a way that stereo can only hint at. To me, this is the most attractive aspect of surround sound as a format, and the most useful element of what usually is sent to the rear speakers. It is powerful medicine indeed.

SURROUND IN FILM & VIDEO

For film and video, such envelopment information is used sparingly and intermittently. Because of rapid shifts in camera perspective, not to mention scene and context, it is difficult to meaningfully represent such environmental information effectively.

So the surround channels seem to be mostly used for "static" ambience and/or reverberance (where the scene is fixed or constant, such as crowd noise in a sporting event or music concert), for panning off-screen effects or transitions from off-to-on screen or vice versa.

(click thumbnail)Figure 1
SURROUND IN MUSIC

In music production, we typically view the surround channels as a source of reverberance from a concert hall. In fact, that is only appropriate for the limited range of concert music. I have found that small-club ambience and other sorts of treatments can be extraordinarily effective. Further, in multitrack work, a variety of other possibilities arise, including putting various rhythm instruments in the surround channels; wrapping lateral left/right stereo placements beyond the left and right speakers by using the surround channels (check out the Ray Charles "Duets" SACD for a good example of this); and profound envelopment by using loops and surround ping-pongs that don't correlate to anything in the concert or acoustic music realm.

SURROUND IN THE HOME

The end user is little baffled by all this, of course. There is a tendency to cut costs on the surround speakers on the assumption that "all" they do is add a little reverberance. Further, sometimes the speakers are treated as bipoles, where direct radiation to the listener is suppressed.

While in production, we call for identical speakers for all channels; for the end users that is seldom the case. Further, there is a problem with positioning. The ITU has come up with a surround layout topology derived from cinematic usage.

Fig. 1 shows the ITU recommended layout for a five-channel surround scheme, (from my book "Total Recording," KIQ Productions). I haven't included the subwoofer here.

(click thumbnail)Figure 2
Note that in this layout, speakers are all equidistant from the listener, and the surrounds are more like side than rear speakers to correlate at least somewhat with movie theater practice. Such a wide spread means that rear phantom images are practically impossible, and while sounds from the side are a little better, mostly there are no sounds from the rear or sense of total envelopment, which to my way of thinking is a pretty significant loss.

A more typical setup might resemble what's shown in Fig. 2, which shows a common sense room-friendly surround topology, with a normal stereo array in front and surrounds at approximately 130 and 230 degrees respectively (again, from "Total Recording"). This layout gives fair side and rear information and is generally pretty satisfactory.

None of these setups can be counted on, of course. Our beloved end users stick the surrounds wherever they can, subject to the usual round of domestic squabbles, negotiations and friendly constructive discussions about what will yield the greatest good for the greatest number of family members and friends. Right!

WHAT DOES IT ALL MEAN?

The surround channels can provide immense sensory impact. Ironically, at its best, such a sensory quality isn't directly perceived, and often listeners are not aware that the surround channels are on. Often, the presence and effect of surround channels is most clearly heard when they are muted and the surround envelopment collapses. Further, in production, we tend to restrict the surrounds to audible artifacts, such as crowd noise, hall reverberance, SFX and the like, instead of playing to their real strength, subliminal ambience. (For a really good example of the latter, listen to Junior Wells' "Come On In My House" on Telarc, in either DTS or SACD--the ambience is brilliantly done by Michael Bishop).

Meanwhile, it is important to very carefully tend to surround channel levels. I have found them to be really touchy, and fairly sensitive to listener position. Too loud, and the listeners often feel like they are in a barn. Too soft, and they feel like they are in their boring old living room with no surround sound. It's a fine line, and we're just learning how to tiptoe along it.

Thanks for listening. Next month I'll consider the center channel.

Dave Moulton is surrounded by bill collectors, while trying to maintain a phantom image of wealth. You can complain to him about anything at his Web site, www.moultonlabs.com