It’s About Time…
It’s about time. It’s about a sometimes misunderstood and/or overlooked but hugely important element of live sound. In reality, it can make a big difference between a poor, an average or a great sounding mix. It’s about a very important tool in our kit.
This is a crash course in basic sound system time alignment/correction, MacGyver-style!
If you’ve never used Smaart Live, SIM, or had any software based acoustic measurement training, relax. You’re not alone. If any of the aforementioned items simply aren’t in your budget (or classes within your ability to attend in the near future) yet, relax. You’re not alone.
It’s not all that complicated. In fact, many of us already have the tools we need to make decently accurate measurements without the use of either complex and/or expensive acoustic analysis tools/software. Add to this the fact that most of us mix engineers spend the majority of our time mixing entirely in the digital domain. A few relatively affordable items, a little basic math, a little extra time and we’re mix engineers equipped to both measure and apply very basic time alignment to our PA systems. This will directly result in both a much better sounding mix and superior listening experience for the audience.
Let me start by saying that this is a “101-basics” overview in sound system time alignment. I won’t mention the word “phase” here because it is yet another often misunderstood and/or misused term. With regard to PA speakers, sound system measurement and time alignment, let’s just say that the word “phase” is a term that might be reserved for the more mathematics/physics inclined geniuses among us.
Not to mention our beloved system techs, PA speaker designers and their likes. No offense intended, LOL!
This is the “how to get it in the ballpark fast” crash course. We aren’t all on mega-tours with “A” rigs or playing super-size festivals with killer “state of the art” rigs manned by first-rate, top-of-their-game system techs. No, we’re in the trenches. We get a different PA every single day. Some bigger, some smaller, some better, some worse. We are at the mercy of what is given us on a daily basis.
Disclaimer #1: “MacGyver-Style” applies generally well to front loaded speakers and subs. Horn loaded and manifold type sub speakers require additional measurement!
So lets start at the beginning. You’ve got a 2 or 3-way active PA speaker or line-array speaker. It’s got multiple drivers. The drivers are all different sizes and their respective voice coils are located at different distances from the front of the cabinet.
A digital crossover/processor will compensate (with delay) for the differences in distance from the voice coil(s) to the front of the cabinet. This is to ensure that sound from each of the individual elements (ie; high, mid and low drivers) all leaves the front of the PA box at the same time. (See, I didn’t use that word that I’m not going to use here!)
See Fig. 1 (To access the figures, click on the keys in the image at the right labeled Fig. 1, Fig. 2, etc)
This applies to both conventional and line array style PA speakers.
Lets take it up a notch and add a sub speaker on the ground with a PA speaker in the air. Here are three typical examples of what you might find in any given venue:
See Fig. 2
Disclaimer #2: Some processor manufacturers use combinations of IR and FIR filters in their processing signal path, which introduce latency (delay) in the throughput of the processor (possibly not known to us). This could potentially offset our “MacGyver-Style” measurement results. This is because more latency (delay) is required to correct a full range speaker enclosure than a single sub cab as an example. This would be good info to know going in. The amount of latency/delay (if known) must be subtracted from your final output delay calculation of the PA speaker in question. (Thanks to Bernie Broderick of EAW for that reminder!)
By simply looking at the speaker placement, we can easily predict which speaker is most likely to arrive either early or late in the absence of pre-existing accurately measured delay compensation. Hint: Basic geometry. Triangles.
We have two options available to us where time correction/alignment is required. In order to have both the PA speaker in the air and the sub speaker on the floor arrive at the same time for the listening audience, we can apply delay via either
1. The available digital crossover/processor input and/or output.
2. Apply the same digital delay directly via the output(s) of your digital
Disclaimer # 3: Any latency inherent in the output processing of your digital console is also not taken into account here and may skew your final results too. This specific amount of latency/delay (if known) must also be subtracted from your final output delay calculation.
The Main L-R PA mix output (or stereo Matrix mix output) to compensate for an early flown PA (speaker) arrival time. A mono or stereo Aux mix output to compensate for an early sub speaker arrival time. Or Matrix output, for that matter.
Let’s see what might happen when we add a third speaker, a front fill.
See Fig. 3
Notice that the measured distance to each of the three PA elements changes with each listening position in the venue! This is why it’s important to visualize an “average” point from which to measure distance in advance of final measurement.
Once again in the absence of pre-existing measured delay compensation, we have two options available to us where time correction/alignment is required. In order for the PA speaker in the air, the front-fill speaker at the front of the stage and the sub speaker on the floor arrive at the same time for the listening audience, we must apply delay via either of the following methods:
1. The available digital crossover/processor inputs and/or outputs.
2. Apply the same digital delay directly via the output(s) of your digital console.
Notice any trends? Hint: The “zero delay point” is usually the farthest speaker or element away from the single point of distance measurement. All other elements should be delayed back to this “zero delay point”. Doing so will ensure equal arrival time to the audience of each separate speaker in the sound system. This time alignment will result in vastly improved clarity and cohesiveness of our mix.
The same rule applies regardless of which speaker is the farthest away. (The sub, the main PA speaker or the front fill)
So what are those affordable tools for “Ballpark MacGyver” measurement I mentioned earlier? Here are a few suggestions that will get you in the game: (Remember that most digital crossovers/processors and/or digital consoles allow you to choose your method of delay compensation ie; feet, meters or milliseconds. You can also easily convert between them onboard in the preferences as well!)
1. Measuring tape and gaff tape.
First, mark a single spot on the floor or chair or place a mic stand out in the audience as your average point of measurement. Start by attaching the end of your measuring tape to the center of the front grill of the farthest speaker away from your chosen average point of measurement with gaff tape. Then, walk back to your exact average point of measurement spot. Write that distance down on a piece of paper. Or use your phone. Just make a note of it. This will become your “zero delay point”. Repeat this step with each of the other closer speakers in turn. Keep in mind that you may need a ladder or lift for the flown PA speaker.
2. Laser range finder.
The more money you spend here, the better equipped they come ie; multiple measurement memory, choices of measurement methods, maximum distance and granularity (level of detail) etc. Warning: Less expensive models tend to work better or even only indoors. The same technique applies here. Measure the distance to all speakers from a single chosen “average” point. Start with the farthest speaker (zero delay point) and finish with the closest.
3. Standalone delay finder App.
Now available for some cell phone devices at a very reasonable cost. I haven’t tried this one yet myself however the same methodology applies here too.
4. The old-fashioned “Cricket Pulse” or “Pop” generator trick for testing driver polarity.
Route the generator though the outputs of either your digital console and/ or your digital crossover/processor and out to all of the individual PA speakers simultaneously. You can acoustically verify the accuracy of your delay calculations using your ears. Any gross misalignment(s) will become immediately apparent to your ears. If your ears are pretty darn good you can adjust your individual output delay settings until the pulse becomes a single, coherent, uniform “Pop” at your measurement position. This can be actually quite fun to try if you’ve got time.(Thanks again to Mr. Broderick for reminding of this one!)
5. The old-fashioned “polarity-reversal” sub timing trick.
You can accomplish this using either pink noise or a sine wave at the exact crossover frequency of the sub speaker to your full range enclosure (for example: 80Hz). You’ll also need a vocal mic and a meter on your console. First remove the low-pass filter from the “low” output of your full range speaker. Then reverse the polarity of the sub speaker output. Mute the mids and highs of the full range speaker and then match the output levels of both remaining low and sub speaker with either pink noise or a sine wave at the crossover frequency. Place the vocal mic at your chosen point of measurement. Now adjust the delay forwards (or backwards) and observe the metering of the vocal mic input. When the meter level dips to it’s lowest point, you’ve found the greatest amount of cancellation at your measurement point.
Now flip the sub speaker output back into correct polarity and you’ve achieved maximum low frequency summation at your point of measurement! Don’t forget to reapply the low-pass filter to the low output of your full-range PA speaker. Fun? Wow! (Also compliments of Bernie Broderick!)
It’s about time…The dreaded math. See Fig. 4
Here are few good things to watch out for before you start any of this…
1. I always politely ask the house tech or systems person upon arrival if the crossover/processor settings are factory preset and if the PA has been measured and time aligned correctly for the application. Depending on the answer I’ll also ask which method was used and then further specifically ask to see the delay data. When the answer is a clear no, you’ll know what you have to do.
2. When you find it necessary to “ballpark” your own measurements for time alignment (and I think it’s a good habit to do this daily regardless), make sure to double check that all crossover/processor input/output delay times are reset to 0 (zero). (Excluding any manufacturer factory preset internal speaker delay compensation settings!) It’s a good idea to not mess with a manufacturers factory preset for a multi-way speaker cabinet. The manufacturer spent a lot of time and money on those settings, you can assume they’re correct. They’re usually locked out (or should be) anyway.
3. Double check to make sure that you’ve reset the output delays to 0 (zero) on your console, too, should you find it convenient to make any delay adjustments there instead.
That’s how to get yourself into the ballpark when you’ve got only yourself to get in the game, MacGyver-style. Without the benefit of more complex measurement tools.
In conclusion, I highly recommend taking a course or class offered by any of the measurement software/analysis and/or PA speaker manufacturers sometime in the future. When you have time. Some are free to attend and others require a modest fee. The resulting technical education, real-world measurement and alignment skills garnered and genuine understanding of loudspeaker physics will open doors to a whole other world. It’s an investment in both yourself and your future.
Not to mention a much better sounding and coherent mixing experience for everyone!
It’s about time.