So what's a line array ?
(Reminder that I am literally just finishing school therefore I still have a lot to learn and what I may be saying might be erroneous at times from a misunderstanding of some audio notions)
Well before we get in the nitty gritty of things, let's visualise what we're talking about. You see the big column (line) of speakers on the left of the stage ? That's a line array : (picture by me of Bons Sons Festival)
They are a solution to the need for louder systems (and more problems). As concerts got bigger, the concert halls became bigger and bigger as well, however there's a terrible thing in audio called attenuation and that's when the sound level diminishes because of the air resistance. The further a soundwave goes, the less powerful it becomes and the less you hear it. There's a very simple equation for that : number of dB (sound level) lost = 20log(distance). It means you would lose 32 dB in 40m, that's a lot !
There's a real need for louder system. And at first we had the Wall of Sound. A monster made of speakers, stacked on top of each other, up to 10m (32 feet) tall. This is the grandpa of line arrays.
There is a problem however. If the simple solution to "one speaker alone cannot produce enough sound for a whole crowd" seems to be "well just put more on top of it", this doesn't take into account the fact that the soundwaves produced by one speaker will interact with the soundwaves produced by another speaker and this will result in overlapping zones where it either adds the two (bigger sound), or substract one from the other (holes in the spectrum). This leads to using more speakers than necessary to cover those holes.
If these speakers could create a cohesive line front then you wouldn't need as much speakers. This means you want them to act as ONE speaker : as one SOURCE. What you need is for your line array to act like a line source.
In the early 90s Christian Hail (founder of L-Acoustics) determined the physical conditions needed for a line array to have a coherent wavefront. There are 5 criterias to the WST (Wavefront Structure Technology). If a line array conforms to all 5 then it will behave like a line source and be of great help for large concert halls and open air events. Nowadays a lot of softwares have been developped (usually one per manufacturer) to help with all the calculations : you only need to create a model of the venue in the software, select what speakers you are using and then give a maximum height (and other information that you have) and you can visualize how each speaker will behave, where will their energy be focalized, how they will interact with heat map and a lot of cool stuff.
(yes I know ArrayCalc is for d&b but it's the one I used the most at school)
Anyway the 5 criterias :
First we have the ARF, the Active Radiating Factor. This says that the wavefront created needs to be bigger or at least 80% equal to the total length of the line.
Then you have the STEP. The Step is the distance between two sources's acoustic centers. It should not be bigger than half of the wavelength of the highest frequency producessed. For example if the higher frequency produced is 100Hz (3.4m), then the distance between two speakers should not exceed 1.7m. However this becomes 0.17m for 1kHz and 0.01m for 16kHz. It becomes apparent that for higher frequencies it is almost physicaly impossible to have the correct step.
The third criteria treats of the wavefront's curve. It can be accepted, as it will natureally be when created by a speaker. The wavefront being curved there will be a slight difference in the distance needed to get to the audience between the top of the curve (the highest point) and the sides who are curved. However there is a limitation on how curved it can be. It can be calculated and it must be inferior to 1/4th of the highest wavelength. Waveguides were created to help "sculpt" that wavefront.
The last two criterias focus on keeping the line source's angle variable without losing the coherence of the wavefront.
The fourth criteria takes into account the sound level attenuation according to the distance. To keep the attenuation at 3dB per doubling of the distance, the angle between each speaker must be inversely proportional to the distance to the adience. This means the further away the audience member is, the more closed the angles between the speakers are.
And finally the last criteria determines the maximum angle between each element of the line array. This is to avoid the holes in the audio spectrum. It is an equation that I'm not going to try and write on tumblr.
Taken right from my thesis haha.
If the line array agrees to all these criterias it can be considered a line source. Line arrays usually take care of the audience further away on the ground and also in the balconies/stories/heights. There is a real need to understand how far your stage speakers will produce good sound (especially with all the fills you have on the front of the stage) and adapt the placement of your line arrays (or its general curvature).
Its line aspect also influences the shape of the hot spot. For a lone speaker the hot spot is going to be near it, at it's acoustic center, then attenuated in a spherical manner. For a line array, it will produce a hot spot in a line. This transforms the heat map. That's how you can have big concert halls with a general sound level that is very homogenous (the people in the front still have the most sound level/sound pressure). Understanding how they work allows you to create blind spots voluntarily (for example because of architecture).
Line arrays are almost present at every concerts now. But be careful with the fingers when setting them up !!
