In his January 2010 FOH "Theory and Practice" column, Brian Klijanowicz discussed the concept of subwoofer time alignment. If you haven't read that article (or even if you have) you should revisit the January issue for a refresher. Brian pointed out the fact that venues often have physical constraints requiring less-than-ideal placement of top boxes relative to subwoofer boxes.
For example, tops may be flown above the stage from rigging while subs are ground stacked in front of the stage. The result is that the subs are closer to the audience than the tops. This variation of distance to the listener means that the tops and subs are not phase- coherent. Depending upon the distance between the "acoustic centers" of the boxes, we could experience some pretty serious phase cancellation. This phase cancellation will be most noticeable in the low frequencies where you need the most power, and where phase cancellation robs you of said power.
PA vs. Backline
In smaller venues such as clubs and theaters, we also need to address the fact that the PA system is never phase-coherent with the backline. Why? Well, the PA system is closer to you than the backline. Let's look at the bass rig as an example. For the moment, let's say that there's a DI on the bass patched to a channel on the console (we'll get to the miking amp in a minute). When the bassist plays a note, sound emerges from the bass amp, and it emerges from the PA speakers at the same time via the DI channel through the console. If you are listening in the audience area, sound from the PA reaches you earlier than sound from the amp. How much earlier depends upon the depth of the stage, the rigging of the PA hang (or stack) and the band's preference for how far upstage the backline is set. Ten to 15 feet is typical in a club, while 15 to 20 feet typical in a theater.
Simple Math
Let's take a look at the math involved in this. I promise not to hurt you.
The low "E" string on a bass has a frequency of approximately 41 Hz. If we use the formula below, we can calculate the length of this sound wave:
Wavelength = Speed of Sound ÷ Frequency
We know that the speed of sound in air is roughly 1,130 feet per second in an environment of standard temperature and humidity. (You can look up the variations due to temperature and humidity.) We also know the frequency of the sound in question (41 Hz).
So if we plug our numbers into the equation, we get the following:
Wavelength = 1,130 ÷ 41
Wavelength = 27.56 feet
What this means is that one complete cycle of a 41 Hz wave is 27 feet long.
Now imagine this 27-foot long wave coming out of the PA and the bass amp. Think in slow motion. The bass player plucks the string. The note comes out of the bass amp and the PA at the same time, but the PA is closer to you so the sound from the PA reaches you first. Fractions of a second later sound from the amp also reaches your ears. Does it matter?
Yes, it does. Think about the distance between the PA and the backline. Suppose the PA is 13.5 feet in front of the bass amp. That is exactly one-half of that low E wavelength. The PA is perfectly out of phase with the bass am, so you get destructive interference between the two. Net result: weak bass.
Delay the PA
What can you do? These days, there are many possibilities. One option is to delay the PA to the backline. Every modern drive processor on the planet has a signal delay for the left/right output. Some will let you view and adjust this delay in feet so you don't even need to do any math. Just measure the distance from the bass amp (try to approximate the position of the voice coil inside the cab) to the PA (ditto). There are even one or two processors that allow you to enter the temperature and humidity, and they automatically compensate for the variation in the speed of sound. Once you have a rough idea of the delay time, you can fine-tune by ear.
Rocket science is all well and good, but is the difference audible? In a decent-sounding room, yes. I have found some rooms where aligning the PA to the backline makes a stunning difference in low end clarity, especially when the room is small and the band is loud (in such a situation the stage sound represents a larger proportion of the "mix" heard by the audience). In really large venues the difference is not so drastic because the stage sound is negligible relative to the SPL from the PA (though you might hear a slap on the snare drum). To be honest, I have also experienced a few small rooms where the acoustics are horrible and applying this technique makes no difference whatsoever.
Bass Mic vs. Bass DI
Next, we add a mic on the bass amp for some tonal variation. It's paramount that you check the phase between the bass mic and the bass DI. Why? When combined, these two signals are always in question, for the simple reason that the DI signal goes to the PA through wire (infinitely fast) and the bass mic receives a signal that has traveled through the air from the bass amp's speaker (much slower than the signal from the DI). Even when the mic is placed right on the amp grill, it is delayed relative to the DI. In the Mesozoic era, we'd simply switch the polarity reverse button and hope that it corrected the problem, but this assumes that the mic is perfectly out of phase with the DI (not always the case. Sometimes the mic is partially out of phase with the DI). So I hope at the very least – whenever you are mixing a mic on the bass amp with a DI – you are comparing the sound of the two with the phase "normal" and "inverted" on one of the channels. (Don't waste your time switching phase on both channels.)
Digital Options
Now that we have digital consoles, you can dial in a very small delay on the bass DI channel to compensate for this difference. A millisecond or two can be like the parting of the Red Sea, time-aligning the two waves (one from the mic and one from the DI) so that they crest and fall at precisely the same moment. If you want to go nuts, you can do the same with multiple mics on the kick drum (e.g. SM91 inside and D6 at a hole in the front hear) too. If you are mixing on an analog desk you can try Little Labs' IPB or IPB Junior or Radial Engineering's Phazer, all of which allow you to adjust phase infinitely between 0 and 180 degrees (or between 181 and 360 degrees if you flip the polarity switch).
