Covering All the Angles

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There's no escaping surround sound these days. Music CDs are still being produced in stereo, but the same does not hold true for movies, DVDs, and games. The steady proliferation of affordable home-theater systems means that more and more stereo mixes are leaving listeners staring at four silent speakers. Given recent developments, such as the arrival of surround support in the MP3 spec and surround systems in vehicles, there are fewer obstacles in the way of delivering surround sound to consumers.

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FIG. 1: The preferred speaker arrangement for 5.1 surround sound places all five directional speakers equidistant from the listening position. ITU recommendation 775 specifies the preferred angles for each speaker.

If you've been waiting on the sidelines for the right time to jump into surround production, this may be your chance. One of the first things to address is your monitor system — after all, the number and position of speakers is the thing that distinguishes surround from stereo. With that in mind, I'll attempt to demystify the concepts and terminology of surround-sound speaker configurations to get familiarize you with the new production paradigm.

By the Numbers

The most common buzzword in surround sound has to be “5.1.” Whether you say five point one, five dot one, or five one is less important than understanding that the term refers to five directional channels providing the immersive sound field, and a sixth channel reserved for low-bass elements. There are other configurations in common use, such as 6.1 and 7.1, but 5.1 is by far most common in home-theater systems and DVD production.

The five directional channels of a 5.1 speaker configuration are left, center, right, left surround, and right surround (LCRLsRs; see Fig. 1). Most EM readers are familiar with the time-tested tradition of setting up stereo close-field monitors so that they form an equilateral triangle with your head. It turns out this has become a standard position for the left and right speakers of a 5.1 setup. The center speaker goes — you guessed it — directly in front of the listening position.

One thing that might not be immediately obvious about the center speaker is that it should not be in the same longitudinal plane as the left and right, because that would place it closer to your ears. Events in the center channel would then arrive slightly earlier and slightly louder, causing them to be more evident to you even though they have been set to the same mix level as events in the left and right channels. For that reason, the center is moved back so that it is the same distance from the listening position as the left and right speakers. In fact, it's best to think of all five directional speakers as existing in a circle around your head.

If you are working with video, the vertical position of the center speaker may have to be compromised, because it is competing with the visual display for real estate. If you must move it higher, keep it as close as possible to the same horizontal plane as the left and right speakers. It may even be advisable to turn it upside down to keep the tweeters closer to that plane.

Because we know that the surround speakers must also live on the same circle as the rest, the remaining question is how far behind your head they should go. That is still a topic of some debate, with opinions varying between 90 degrees (directly to the side) and 150 degrees. One prominent standard, codified in International Telecommunication Union (ITU) recommendation 775, is 110 degrees off the center axis, plus or minus 10 degrees. The Recording Academy's Producers and Engineers (P&E) Wing has promul gated an extensive set of guidelines on surround-sound production, including advice to place the surrounds between 110 and 150 degrees, with the optimum range being 135 to 150 degrees. In general, wider angles between speakers yield a better immersive experience, while narrower angles provide better positioning of sounds. Whichever guidelines you choose to follow will represent a considered compromise between optimum localization and the greatest sense of envelopment.

Match Point

You might think that an ideal surround-speaker setup consists of five identical speakers. For the mix environment, that is indeed the preferred setup. One obvious advantage of matched speakers is that timbre, volume, and transient response — all of which can vary noticeably between different models of speakers — will not change as you pan a sound source around the room.

In many playback systems, however, the center channel is different than the rest. That is primarily because of the theatrical origin of the center channel. It was introduced for the purpose of delivering dialog from a position behind the movie screen, thereby anchoring the actors' voices to their images. Many home theaters therefore include center speakers that are optimized for dialog. The downside is that such a configuration can compromise the character of nondialog elements, so music production facilities ordinarily match the center speaker to the right and left speakers.

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FIG. 2: A dipole speaker is a front-to-back array. In home-theater playback systems, it is aimed sideways at the listening position to provide a diffuse surround image.

In movie theaters, the left and right surround channels are ordinarily delivered through an array of speakers along the side and rear walls. The result is a diffuse surround field, and in some home theaters a diffuse surround field is accomplished with dipole speakers (see Fig. 2). Dipoles are the push-me-pull-you of audio — they house a pair of speakers aimed in opposite directions, much like a figure-8 microphone in reverse. The null is aimed at the listening position, so the listener hears very little direct energy. The result is a theater-like diffuse surround field.

In movie soundtracks, the surround field is designed to create a realistic sense of space around the viewer, but mixers are careful never to pull attention away from the movie screen. To do so would cause the dreaded exit sign effect, a big theatrical no-no. Though it's certain that no movie moguls truly believe that if a sound effect causes you to turn your head and you see the exit sign that you will leave in the middle of a movie, it is indisputable that seeing the exit sign or the amorous couple sitting behind you will yank you out of the carefully crafted illusory world you were there to experience.

In game soundtracks, however, it's common and necessary for a character or other element to give you a sonic tap on the shoulder so that you will turn around. If your living room doesn't have an exit sign, that should have no negative effect on your experience. In music-only surround production, it's also quite acceptable to place discrete attention-getting elements in the surrounds. If your work is more like that latter philosophy, you should stick with direct radiators rather than dipoles. The P&E Wing's guidelines strongly advocate five matched full-range direct radiators for music production.

The Lowdown

The “.1” part of 5.1 refers to the low-frequency effects (LFE) channel and the subwoofer through which it is played. The LFE is another part of surround sound's theatrical heritage and was created for the sort of floor-rattling effects moviegoers expect from massive explosions and thundering dinosaurs. Even though music production rarely requires such a dedicated bass channel, most playback systems combine the LFE channel with the low bass from each of the other five channels and send it to the subwoofer. This is a process called bass management, the details of which are beyond the scope of this discussion.

The location of the subwoofer is somewhat less critical than that of the other speakers because of the less-directional nature of low frequencies. Typically, the subwoofer is placed to either side of the center speaker.

As it expands the sonic landscape, so does surround sound expands the variables you must contend with when setting up your studio. Beyond number, type, and position of speakers, one must also consider the acoustics of the room, which are arguably more important in surround mixing than in stereo. You can expect engineers to debate surround monitoring as much or more than they have stereo monitoring, but these guidelines will get you off to a good start.

Brian Smithers is a musician, composer, engineer, and educator in Orlando, Florida.