UPDATE – Aug 27 2010:
An example of a line array tuning using thr ABC method described below – with 19 SIM data screens and some photographs can be found here.
Every time I teach a seminar it is simply a matter of time before the question arises: How do I tune a line array? Typically it takes less than 10 minutes of the 4 day seminar to elapse before this comes up. If it were a simple matter, then I would not waste 3.5 days working up the physics of speaker and room interaction to set up the answer to this very important question. I would also not have spent two years writing a book to get to the same answer. So how about we cover it in three paragraphs in a blog? No problem.
The first step is to remove from the table the idea that new physics have been invented, and that new rules apply. This is an adaptation, not a revolution. What is the big difference between tuning a 12 element coupled point-source array that has boxes spread horizontally and vertically (we call those “point-source systems” or “conventional systems” or “old systems”) and a 12 element point-source array that is only a single box wide and is spread vertically (we call these “line arrays”)? The difference is that the “line array” has a lot more slices of vertical coverage, and a lot fewer slices of horizontal coverage.
The following items still hold true in either case:
1) We need to define the target coverage area for every element in the array. Does it make sense to you to tune the top boxes that are pointed into the balcony with a mic down on the floor? Not to me – which does not mean I have not seen it done.
2) Once we define custody areas for the room we can proceed with tuning to make the areas match. By custody I mean: these speakers are assigned to these seats, and these ones are assigned to these seats.
3) Unique areas require unique tuning. Do you think the same acoustical conditions are found on the first 30 rows on the floor of an arena as the last 30 in the upper deck? This is absurd. Do you think then that a one-size-fits-all tuning solution makes sense? Not me. That is why we have been dividing up the tuning in such systems by the ABC method for over 25 years. This “old” approach can be applied to the “new” technology to provide uniformity over the space.
4) The HF response is the most amenable to zonal separation, and the LF response is the least. This is true in both “old” point sources and “new”. Therefore the tuning approach must incorporate the spearatio0n of high and the overlap of lows into its process.
The ABC tuning method (also –known-as Papa Bear, Mama Bear and Baby Bear)
The principal the ABC tuning is to define coverage area for each element. Element A (the longest throw and most dominant subsystem) has a center of coverage and two off-axis edges, Element B has a center and an edge that transitions to element A. This A-B transition is the custody change which we term the “crossover point”. Next we move on to Element C with its area of solo coverage and the crossover to the B element (above) and either D (below) or finally the bottom edge of coverage.
Each element is tuning as a stand-alone system in its area of coverage. The levels for each are set to create the same level at near and far locations. Therefore provision should be made in the design for the A system to have sufficient power capability to go the distance.
After the individuals are tuned we can investigate the transitions A-B and the B-C etc, to see if they are maintaining a uniform response with their A and B soloists. If the overlap between the elements is too high the transition will be too loud and if too wide, they will be too low. Splay angles can be adjusted as needed to minimize the transition errors. Delay can also be added if needed to compensate for path length differences to the crossover point.
The process follows the alphabet. A combines with B first to become AB. Then AB is combined with C, ABC is combined with D and so on.
The equalization will need to be modified as the aggregation progresses. The combination of A+B should have minimal impact on the respective HF responses, since their ranges enjoy maximum isolation in their respective zones. The LF response by contrast, will be a shared resource between all subsystems so we can expect to have to taper the LF response as we add elements.
Practical Application of ABC to line arrays
It would be nuts to sub-divided our 12-element line array into ABCDEFGHIJKL subsystems, even if we had the budget and the time. Instead the practical, repeatable and manageable approach is to break the line into the same sorts of complexity levels as we have managed for 25 years: 3 or 4 levels. So our 12 elements might break down to 3 sets of 4 or 4 sets of 3 for example. Asymmetric quantity grouping could be used as well such as 6-3-3 or 5-4-3 for example. Our A, B and C subsystems are in turn comprised of A1, A2, A3 and A4 and B1-4 etc. This creates what I term a “composite” element, since we are grouping several speakers to together to tune with a common eq and level. The process of subdivision is fairly common practice in line arrays but there is another critical step to making it act like (and tune like) an ABC array: we make each composite element internally symmetric. Why?
In order for us to know where the center of coverage is for element A it will need to have one. Sounds elementary but its not so simple. If element A is a single speaker the center is on axis to the speaker. If A is comprised of two elements at some splay angle, the center is at the midpoint A1-A2. Still simple. If we add a 3rd element we have A1-A2-A3 and the center will be at A2. Right? Maybe……………why.
If the A1-A2 splay angle is the same as the A2-A3 splay angle, then symmetry rules apply and the center is found at A2. If A1-A2 is 2 degrees and A2-A3 is 4 degrees where is the center now? The answer is somewhere between A1 and A3 but now comes the real plot twist: It is different at every frequency. The isolating behavior of the highs will cause the center to move up towards the A1-A2 crossover since there is more overlap combination there than the A2-A3 crossover. The low end, by contrast, will laugh at difference between splay angles of 2 and 4 degrees and will maintain a center position over the 3 speaker spread.
If we have 12 elements with 11 different angles there are no independently locatable centers that do not shift position over frequency. To tune such a system is the ultimate challenge. Any given mic position may be the center at one frequency and is guaranteed NOT to be the center at others. If you see a peak at 2 kHz this may be the center of the 2kHz beam or maybe it will get louder 10 rows back You won’t know till you move the mic there. Then you will have to do that again for other freqs because they all have different centers.
The ABC approach built from composite symmetric elements ensure that you can place a mic in the A, B and C element areas and that the coverage will transition logically between them. If you can make A, B and C uniform in their respective areas and taper the combined LF response, the transitions will be smooth and predictable, giving you a continuous line of coverage…………. just like we did with the old systems. The result with the modern systems is we get all of the advantages of fine-slicing the coverage (and easy rigging) without losing our ability to tune the array from top to bottom.
This by no means a comprehensive treatise on this subject, but rather the shortest way I could figure out to say it that holds together.
As always, your comments, questions, musings, tried and true methods, better ideas, etc are welcome here.
I am adding some graphics excerpted from the 2nd edition that show the ABC method applied to a theater, opera house and arena. In each case you can see that the A, B and C composite elements are made up of several boxes. e.g. a 20 deg speaker might be 4 boxes at 5 degrees or 5 boxes at 4 degrees etc. There is an icon on the pics that shows where the on axis mic would be placed (the center of the A, B or C speaker) and another indicates where the tranisiton between composite elements is found.