Much ado is made about the time alignment (or more accurately, phase alignment) between the mains and the subwoofers in a sound system. Despite the large amount of “PR” dedicated to this topic (to which I inadvertently contribute by writing this), I maintain that this topic can generate more buzz than it warrants.
The truth of the matter is that in most circumstances, it’s simply not as critical as we seem to think. The inherent geometry of combining flown mains and ground-stacked subs creates a situation where the sub-main alignment will not hold over the space. As I wrote previously (Don’t Phase Me Bro), in most venues, if you don’t like the sub-main alignment, try changing seats. This isn’t as dire as it seems.
I’ll admit this can look pretty scary on an acoustic prediction – no self-respecting system tech likes to see the deep “blue river” nulls jutting through the house in section view. However, the prediction can be misleading. Predictions show the energy of a small frequency range over the whole space, which is the opposite of what a listener (or a reference microphone) would experience – the full frequency spectrum at a single location. And once wall reflections are considered, it’s a different story entirely…
However, all of those potentially mitigating factors are, in my opinion, secondary to a more interesting and pertinent question: how bad can it be? If the stars align in the worst way, how much damage can be done to the audience experience by having a poorly aligned main-sub crossover?
The situation to pay attention to is when the subs are flown in close proximity to the mains – either at the top of the same hang, or directly behind or adjacent. In this scenario, the close placement results in a more “coupled” summation behavior that will be far more consistent over the space – because moving throughout the space doesn’t change the relative arrival times between the sources nearly as much as it would for ground-based subs.
Thus, a good alignment has the potential to “hold” more uniformly over the space. But so does a bad alignment. It follows that this is the situation that can cause the most trouble if we don’t handle it properly. But what does “trouble” look like? Is it a show-ruining experience, a minor inconvenience, or perhaps somewhere in between?
Getting The Picture
During a recent (Rational Acoustics) Smaart training class, I had the opportunity to answer that question. The venue we used for the training had a new PA system installed – but not yet tuned. Each side consisted of seven boxes hung beneath a pair of 18-inch subwoofers. This main-sub relationship is about as “coupled” as one can get, and at the wavelengths in question, we can feel relatively confident that we can observe a stable relationship between the two throughout the space.
We deployed three measurement microphones at the top (Figure 1A, blue), middle (Figure 1B, purple), and bottom (Figure 1C, red) of the house right seating area, on axis to the array, and measured the responses of the main and the sub at each location. Figure 1 shows that in all cases, it’s well-aligned through the crossover region around 88 Hz – thanks to the coupled configuration for this well-behaved alignment.
Now that we have a picture of the best-case scenario, let’s flip the polarity of the sub and see how bad things become. Since it’s the degradation of the summation that’s of interest, Figure 2 shows the full sub-main summation at A, B and C, plus the same summations with the sub polarity inverted (yellow). The traces are vertically offset for visual clarity.
Note that this is a situation in which the effects of a bad alignment are going to be far more pronounced than usual, and we’re intentionally making the alignment as poor as possible. Is there a measurable effect here? Yes, absolutely. Is it a show-ruiner? I vote no. (So did all 25 of the class attendees.)
Remember that with an uncoupled system configuration (subs on the ground, mains in the air) we can expect these types of variations as a matter of course throughout the seating area. It’s possible for the main-sub time offset to vary by a 10 millisecond (ms) window or more in a large venue, equivalent to a full cycle at a 100 Hz crossover frequency, meaning that the summation situation will run through the gamut of possibilities over the space. Anyone who considers this unacceptable should fly the subs.