Time alignment is a very important yet often overlooked aspect of system setup and tuning. A correctly time-aligned system has many benefits, including more even coverage where two sound sources overlap and a more even response across acoustical crossover points. It can give the cheapest of systems a couple decibels more in the area where engineers tend to like them most: bass frequencies. So let's take a look at subwoofer time alignment.
Two ways to quickly achieve this are, first, by using a sine wave, and second, by delaying the P.A. back to the kick drum. Each approach will have its own advantages in different situations depending on the size of your gig, time you want/have to work on it and just how much you care.
With the more frequent use of digital crossovers, system controllers, amps and consoles, it has become easier than ever to add delay to multiple signals, whereas years ago you would have to eat up an entire rack space and insert cabling for just one channel of delay.
This is intended to be a minimalistic, quick way to time-align your system, so only a few pieces of gear will be required. If you choose the sine wave approach, you will only need a sine wave generator, the capability to delay an output signal and to reverse the phase of it. To delay the P.A. back to the kick drum all you need is a channel of delay and your "golden ears."
For this article, we will assume that the sound system we are working with is a standard front loaded "stack" configuration. This means there is a subwoofer (producing sub frequencies from roughly 100 Hz and down) ground-stacked on the floor with a top box (producing roughly 100 Hz and up) directly on top of it. We will also assume that both subs and tops are currently producing the same polarity and facing the same direction (azimuth/splay angle).
The Sine Wave Approach
Usually an engineer would not want to use polarity to cancel out a signal. But that is the whole concept behind this technique.
First, it is important to find the acoustical crossover frequency between the subs and tops. A measurement device is the most accurate way to do this. But if you don't have one there is an easy way to rough it in. Assuming you don't have a measurement device, flip through the pages of your crossover to see what the crossover frequency is and use that (for this article it will be 100 Hz.).
Let's get back to the concept. We will take two similar sound sources, sub and top, that are playing a 100 Hz sine wave, and flip one out of phase. As illustrated in Fig. 1, we can see that the crossover frequency is the point at which both sound sources overlap and start to fade off from one another. But more importantly they will still reproduce 100 Hz, because that is the beginning of the crossover filter on each source.
By physically looking at the speaker cabinets and knowing where the drivers are in the boxes themselves, you will be able to determine which signal to add delay to. Typically in the club world, top boxes are placed a little behind the front of the sub. That way if the top box falls, it will fall back and not onto the drunk audience. So, assuming this is the case, delay should be added to the sub. While the 100 Hz is playing through the sub and top, start increasing the delay to the subwoofer signal. Eventually the two signals will start to cancel out and the total SPL will reduce substantially. Find the point at which most cancellation occurs and leave it at that. Change the signal that's out of phase back into phase and you should have summation at the crossover point.
Some crossovers have an "on" and "off" function for the delay. A good way to check if summation is occurring is to flip the delay on and off to hear the difference.
This whole process can sometimes be done with music, preferably with driving, kick drum heavy music. However it can be hard to distinguish what is canceling and summing in the crossover region with full-range music playing. A quick fix on a digital console would be to throw a low-pass filter on the music channel.
Back to the Kick
Delaying the P.A. back to the kick drum is very simple. It works very well in the small "hole-in-the-wall" biker bar, but once you get into a bigger venue, it can become a bit harder to hear the difference. Smaller venues that have lots of reflective surfaces and 90 degree walls create standing waves. This makes it almost pointless to spend the time to get out a measurement device – especially when the club owner wants you to be done setting up before the first road case is even in the building.
Since this technique works better in small venues, we will assume your P.A. is in a small venue and set up in front of a band. Even though it's still surprising to club owners and patrons, we all know how loud a drum set can be in a small room. That is why typically most engineers won't even turn up all the drum mics, or just not mic the whole kit.
So the drums are roughly five feet behind the P.A. Think of the kick drum as another speaker. If you are sitting in the audience, would you want to hear the main P.A. with another set of P.A. five feet behind it? Probably not. So if you are sitting in the audience listening to a band, you probably would not want to hear the kick drum once through the P.A. and then again five feet later.
Most drummers do sound check routines that are relatively similar. They will hit each drum individually with quarter notes at a medium pace until you ask them to switch to the next drum. When you get to the kick drum, get a rough sound in on the channel strip and start tweaking the delay back. Add a little bit of delay at a time. You will start to hear a change in tone and depth. Once you get to the "sweet spot" and are happy with the sound, that's it!
Things to Consider
In the digital world, delay can also be known as "latency." This refers to processing time involved in AD-DA conversion, and filter processing. Both of these can contribute to delay already in the signal chain before any is intentionally added.
Another thing to consider is the kind of speakers that are set up. Are they front-loaded, horn-loaded, band-pass? Where are the tops and subs physically placed in respect to one another?
All of these factors will contribute to what will actually need delay added to it.
The biggest advantage of a properly time aligned system is a substantial increase in SPL at the acoustical crossover point. As shown in the illustration, time aligning gave the system 6dB more gain at this point. Now that is something we could use these days, free gain!
