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Surround Sound Philosophy 101
 by: Eli Aloisi

There has been a long evolution in commercial movie theater sound. During the first two and a half decades of movie theater presentations, a piano, organ, orchestra, sound effects man, or actors reading the dialog comprised the sound of movies. Electronic sound appeared in 1926 with the coming of the "talkies." Theaters were wired for sound, and a big speaker perhaps set behind the center of the screen did it all. This was the monophonic sound era. Then stereo sound arrived in the 1940's with left and right channels. Additional sound channels were added in the 1950's. (Somewhere in there, a center channel was added to anchor the dialog to the center of the screen.) Finally surround sound came on the scene in the 1980's in various versions adapted to the acoustic challenges of the commercial movie theater. (The advent of DVD's brought affordable surround sound to our home theaters.)

Home theater sound can follow any of these commercial movie theater approaches, or it can move beyond to new levels of sonic realism and effects.

One important difference between movie theaters and home theaters is that movie theaters must present acceptable sound to a (hopefully) large group of people sitting at every location of a large room. In contrast, a home theater usually serves a much smaller group of people sitting in a much more limited part of the home theater space.

The limited size of the usual listening/viewing location in a home theater can work to the advantage of home theater owners due to the nature of sound reproduction.

To understand how sound reproduction bears on this discussion, let's start by considering stereo sound.

In stereo systems, if a listener is closer to the left speaker, all the sound apparently comes from the left speaker. If you have a stereo, turn it on and try this: sit in a location equidistant from the two speakers and listen to a good stereo recording with your eyes closed. Note the spread of locations the sound appears to come from. Now move a few feet to the left of and then to the right of center and notice how the sound which was spread across from left to right collapses into "all left" or "all right". This failure of the stereo illusion is unavoidable when you use just two speakers. This means there's always a "sweet spot" (where the stereo effect works best) located on a line centered between the two speakers in a stereo system.

By the way, purchasing more expensive speakers cannot overcome this effect, as the failure of the stereo effect ONLY has to do with both the differences in loudness between the two speakers (due to being closer to one than to the other) and the difference in the time when the sound arrives at your ears from each of the two speakers.

The center speaker in movie or home theaters is an attempt to override this problem by placing a speaker in the middle of the screen for dialogue and other sounds which the film maker wants to make sure comes from the center of the screen, no matter where you sit in the theater. The center channel solves the problem of stabilizing the dialogue but alas, any stereo sound being provided by the front left and front right speakers will still seem to collapse to one side or the other if a person sits well to the left side or the right side of the theater.

So, now let's consider the surround speakers. In movie theaters, the sound system designers are really stuck in a dilemma. Some audience members are often sitting right under or right next to one of the surround speakers, which means there's no hope of the person hearing the other surround speakers' output at the correct volume and at the right time to get any sort of stereo effect from the surround speakers. This is probably why the older Dolby Pro-Logic system rear surround was only monophonic.

Instead, sound system designers for movie theaters apparently threw up their hands and designed and arranged the surround speakers to:

1. Really lag in time, (so the surround sound wouldn't arrive BEFORE the sound from the main speakers, no matter where you sat, and)

2. Arranged for those speakers to smear their sound all over the back of the theater to mask the problems caused by the great variety of audience/surround speaker time and distance relationships.

Now, along comes home theater.

Most home theater users don't fill the room with audiences, but the philosophy of earlier commercial theater design is still being applied. You will observe how some home theater rear surround speakers are designed to project sound in multiple directions and how the set up manuals will often direct that the speakers be placed to project their sound away from the main viewing location.

Here's the thing - if you want to reproduce the movie theater listening experience, use the surround speakers which try to spread sound all over and position those speakers to aid that goal.

But, if you want to enjoy the more accurate sound source positioning (the sound appears to come from some exact location behind you, to your left, right, or even overhead!) made possible by Dolby Digital or DTS, a different approach should be used.

In this approach (labeled "Holosonic Sound" by Gary Reber and the gang at Widescreen Review magazine [www.widescreenreview.com]) the rear speakers are placed behind the viewers at about the same distance from the main listening position as the front speakers. They are usually somewhat further apart than the front speakers. These surround speakers should be:

1. Well matched to the sound quality (timbre) of the front and center speakers.

2. Direct radiating, and pointed at the prime listening position.

3. Capable of handling at least one-third to one-half the power that the front speakers can handle.

4. Located at a height at or slightly above the height of the ears of the audience.

(To prevent sound from the rear speakers from being blocked by seatbacks, they might have to go a bit higher. The viewer's ears must be able to directly "see" the surrounds.)

Home theater owners whose seats are right back against the wall will have to cope by placing the surrounds on the back wall facing the seating, but spaced well away from the viewers (same distance from the viewers as the distance from the front speakers to the viewers, if possible) to minimize the collapse of the rear stereo effect if an audience member is not sitting exactly between the two rear surround speakers.

Movie makers today are releasing films on DVD with sound that is designed so that home theaters arranged to produce accurate stereo sound from good rear surround speakers will really give you the feeling the you are inside the action, with actors sometimes speaking behind you, and sounds moving right out of the screen over your head.

How do you reliably adjust and test your home theater for the kind of performance we're talking about here?

Easy! Order the AVIA disk from Ovation Software's website. (www.ovationmultimedia.com)

Study the materials presented on the AVIA DVD, and then follow the instructions on the disk, or hire a pro to do the job after watching the DVD has helped you to understand the outline of what has to be done. The audio portions of this disk will assist you mightily in tuning up your system if you do it yourself. It contains "circulating" audio test signals that circle around the room and if you set up your theater for accurate surround sound, that test will show you how well surround sound can work in your home theater.

It can be very satisfying to have better surround sound than the commercial movie theaters.

About The Author

Eli Aloisi is one of the many knowledgable staff members that encompass the PlexHomeTheater.com community. For more great articles check out www.PlexHomeTheater.com.

eli@plexhometheater.com

This article was posted on February 17, 2007

 

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