Should We Run Our PA System In Mono Or Stereo?April 24, 2009 6:00 am Live Sound
We’ve all gotten used to hearing recorded music in stereo by now. Only real old-timers (like me) can remember when all records were mono and a “hi-fi” system only needed one speaker. Stereo arrived in the mid-60s, and following a brief period when each record was available in separate mono and stereo versions, often with very different mixes (see Pepper, Sgt.), we finally reached the point where all records, all cassettes, all CDs are now in stereo.
Lots of bands that play live have PA systems with two main speakers. Since home stereo systems also have two speakers on the left and right, the question arises, should we create a stereo mix of the PA signals to play through the “stereo” PA speakers? My answer is basically no, but before I go into why, let’s quickly review exactly what stereo is in the first place. (Audio engineers can skip the next section.)
Before there was stereo (short for “stereophonic”) there was mono (short for “monophonic” or “monaural”). In a mono music system, there is one audio signal containing all of the instruments and vocals. If you play the mono signal through a single speaker, you hear everything.
In a stereo music system, there are two audio signals, designated left and right. The left and right signals together contain all of the instruments and vocals, but the two signals are different from each other in a special way that creates the familiar left-to-right stereo “spread” in front of the listener (or inside her head if she’s using earphones!).
Instruments that are panned all the way left or right appear in only one of the two signals and thus are heard full volume from that speaker and not at all from the other one. Instruments that appear at an equal level in both the left and right signals will seem to be centered, coming from a “phantom” source midway between the speakers.
Please note that if you send a mono signal to two speakers at once, it is still mono, not stereo. Stereo means “two signals,” not “two speakers.”
So, Mono Or Stereo PA?
Once again: since our band uses a stereo mixer with L-R pan controls, and we have two main speakers to the left and right of the stage, should we create a stereo mix of the PA signals to send through the speakers? Well, except for one special case, I would suggest sticking with mono. Set all the pan controls all the way to the left or right and use only that output for the mains. But why? Well, here are some of the problems with using stereo:
1. Many (or most) audience members will hear an unbalanced mix.
With a stereo mix, some instruments and vocals will come mostly from the left speaker while others are mostly from the right speaker. (Otherwise it wouldn’t be a stereo mix!) People sitting quite close to the left speaker will hear the “left” signals quite loudly and the “right” signals much less or not at all, with the reverse being the case for the poor sods right in front of the right speaker. Only the part of the audience in the “sweet spot” (centered, partway back) will hear the intended stereo mix.
2. The sound man is probably not in the “sweet spot” himself.
At typical gigs, the sound man is off to the side or right up by the band, making it impossible to create a stereo mix that sounds right in the centered “sweet spot.” Sure, he can use earphones to monitor, but then he’s not hearing the contribution of the onstage amps, plus that makes it hard to judge overall room volume. Also, the audience is not using earphones!
I guess you could go nuts and set up a stereo mic at a centered monitoring point in the audience and listen to that in earphones, but I would question whether it is worth the effort (see Problem 4).
3. An additional amplification channel is required.
Most power amplifiers that get paired with stereo mixers in live settings have two separate sections, which are generally used for the main speakers and the onstage monitor speakers. (See my article A Basic Live Sound Setup Diagram for typical connection info.) For a stereo mix, you will need to use one side for the left speaker and one for the right speaker (remember, the signals are different). This means you will need to buy an additional power amplifier for the monitors. Naturally, the additional equipment and wiring will increase your transport and setup time.
4. It’s just not worth it.
If you do go with stereo, you’re going to have to keep everything panned fairly close to the center to avoid Problems 1 and 2. So it’s going to be almost mono anyway. Plus, the way band sound richochets around inside a typical club or venue, do you think anyone will even notice your subtle stereo mix? Not bloody likely. It’s hard enough to produce and maintain a decent mono mix. Why complicate your life?
I mentioned a special case where stereo sound makes sense. This would be a concert type situation, perhaps in an auditorium, where the band and the speakers are way up front and the audience is confined to a fairly narrow area between the left and right speakers and somewhat back from them. In this one case, assuming the sound man is centered, you can present an impressive stereo PA mix to the audience, since most or all of them are in the “sweet spot.”Tags: equipment, live mixing -->