Why use Stereo?
Is stereo a valid technique for PA systems? There is little understanding of the whys and wherefores of stereo PA among sound system designers, let alone end users. The vast majority of systems that you will encounter with speakers on either side of the stage, using separate console outputs, signal processing and amplifiers to each, are stereo in name only. Most don't begin to achieve all that is possible in stereo PA. It's quite possible, you've never even heard a well designed stereo PA.
In acoustics there is a metric called Lateral Fraction (LF). It is a comparison of the sound moving side to side, vs. the sound moving front to back, at a given listener position. For music to sound live, rich and spacious, we prefer a high LF. We want sound moving side to side, we want what arrives at the left ear to be different from what arrives at the right ear. In concert halls, this is accomplished by keeping the side walls near, and using the sidewall reflections to accomplish this. Narrow shoebox shapes do this well. Wide fan shapes do not, since for much of the room the side walls are so far away, and the reflections become too low in level and too late in time to be effective.
For speech we desire a low Lateral Fraction, we want the same thing reaching both ears at the same time. We want to reduce the apparent reverberation by keeping sound off of the side and rear walls. We want to design the sound system for high intelligibility and high gain before feedback.
So where does that leave us? A solo instrument is in many ways a mono system, although with complex directivity that can vary with frequency, note played and playing style. Played in a good shoebox shaped music hall, there will be ample Lateral Fraction, with a stereo-like spaciousness no reproduction system can match. A mono reinforcement system in that room, if it allows sound to hit the side walls, will have similar spaciousness.
That same solo instrument played in a wide fan shaped room will have a low LF. Even if the room is reverberant, it will still lack the warmth, presence and intimacy provided by those early lateral reflections. A mono PA will suffer the same fate. The room and sound system may be great for speech.
Using a Stereo PA will increase the LF, even for a single source panned center. It increases even more as different sources are panned left and right. A properly designed stereo PA will have each channel cover the entire seating area evenly, so everyone will hear each instrument or voice regardless of how it's panned. There are also tight constraints on arrival time differences which must be considered at the design stage.
The irony of all of this is that stereo is much more difficult to implement properly in a wide fan room than in a shoebox. And it's much more necessary if music is to sound good. Wide fan rooms are very popular these days, and for good reason. They promote community, they inspire worship as you see other congregants worshiping, they have shorter sightlines, etc. But they place a much heavier burden on the acoustician and sound system designer to produce an excellent product.
Don't give up on stereo. I haven't.
Dale Shirk
Shirk Audio & Acoustics
Terre Hill, Pa.
Sales@shirkaudio.com
