Did I Hear Someone Say “Line Array?”
The problems and limitations of the conventional speaker array were very clearly understood by the mid 80s. The answers were a little slower in coming. One of the reasons could only be called ‘cultural mindset’. A culture of “this is how we do it and don’t argue” persisted for some time, (could also be called…”this is all I can understand“), along with the big ugly arrays and uglier ground stacks of both horn-loaded and frontal radiating boxes. ‘Loaded’ in this instance, refers to the air mass loading on the driver at the apex of the horn. A new approach lay buried in theoretical texts from the 40’s and 50’s. It was called the ‘Line Source’ effect
So what is a Line Array and how woes it work?
To understand this answer, we need to go back to the Inverse Square law and our Spherical Propagation of Sound Model.
You can see that by segmenting the Spherical Propagation model we are stuck with 6 dB loss with a conventional square cluster of boxes. When we start stacking the sound sources on top of each other in a vertical array, something happens to our spherical model. With a tall line of transducers, the source of the wave is emanating from a line source instead of a point source.
The line source propagates the wave so it emanates like a cylindrical shape, instead of a spherical. Harry F. Olson first published an extensive and very influential work on this subject in the early 50s, in the book Acoustical Engineering.
Line array loudspeakers date from the early days of acoustical research when it was observed that a simple vertical array of acoustic radiators produced increased directivity in the vertical plane. Old early line source boxes can still be seen in many old town halls and public venues. They were much smaller than horn-loaded boxes and were found to project voice well. Note: by directivity, we mean the degree, to which the wave is directed to a narrower or more directional field of coverage,
In a Line Source, the Inverse Square Law No Longer Applies, Or Does it?
A line source of wave propagation has the characteristic of having its SPL fall off at a rate of 3 dB per doubling of distance, (as oppose to 6dB). This is a well-known characteristic of infinite line sources. As an empirical example consider the observed behaviour of the noise pollution from a highway: People living near highways are the unfortunate victims of the highway noise only falling at only 3dB per doubling of distance from the highway. That's because the highway noise source precisely fits the model of an infinite line source, especially if it’s in a valley and you live on the hill above.
Infinite Line Source Theory
The SPL from a theoretically infinitely long line source falls off at a rate of 3 dB per doubling of distance. This is because the energy distribution is now over the surface of a cylinder, rather than a sphere as in the case of the point source. Because the surface area of the expanding cylinder is inversely proportional to distance, not distance squared, it follows that the energy density falls simply with distance from the source, rather than distance squared.
This works for an ‘infinite’ line source but for a finite full bandwidth line array, the line source effect breaks down after a distance and the wave again takes on the appearance of the point source and will again attenuate at 6dB. Remember, a real line array PA system is not a theoretical infinite line source.
What happens to a line array in the real world?
As soon as we start stacking speakers in a close proximity to each other, we are again (back) in the land of interference effect, phase shift and the associated geometries. For a practical example; a concert line array of 15 cabinets (each using 12-inch low-frequency cones), a slight ‘cylindrical wave-like’ effect can be considered in the low mid area, where there is only a 3dB drop between 2m and 4m from the array. As we start to move further from the array, the sound will begin to spread spherically, losing 6dB per distance doubling.
At frequencies below 100Hz, the drivers in a practical line array will be omni directional because the array length will be small compared with the sound wavelength, so the system will not conform to line array theory uniformly across all frequencies.
Above about 400Hz the low-frequency cones become directional, again violating the theory’s assumptions, and at high frequencies, many practical systems use directional waveguides whose behavior cannot be described using classical line array theory. In short, the geometry of real-world audio line arrays is too complicated to be modeled accurately by ‘pure’ line array theory.
A Remember, the original research and formulas are intended for line source boxes that were a long line of small speakers, all the same producing a relatively narrow band of frequencies.
As many technically knowledgeable authors have pointed out, the terms “line array” and “line source” are not synonymous. To propagate low frequencies via a line source, with the (presumably) desired –3 dB loss per doubling of distance, you need an extremely tall line. A reasonable approximation of a “cylindrical wave front” requires a line height of about 4x the wavelength. So for 50 Hz, we need a 26.6m tall line, or say, 60 cabinets. Many commercial “line array loudspeakers” are slightly less than .5m high.
On the other hand, a ribbon roughly 13 cm high would be an 8x wavelength tall line source for 20 kHz, and would have an extremely narrow front lobe. But if this 13 cm high line is composed of separate sources they need to be at most 0.8 cm apart: otherwise the “line” breaks apart into separate sources that interfere destructively with each other. We would need 3200 tweeters to maintain 1/2 wavelength spacing at 20 kHz over a 26.5m high line.
We can see now that not only is it extremely difficult to construct a full-range line source, it’s not actually very useful. In the near field, the vertical coverage of line source coverage does not exceed its height. The number of audience areas that require this tightly restricted vertical coverage is extremely small. So if they don’t work that well at a theoretical level, why are they now fashionable?
Another History Lesson
Back in the early 90s, an unknown (outside of France), French company called L-Acoustics burst on to the scene with the first (really) practical system marketed as a line array. It went against the conventions of the time (a normal French cultural engineering approach), and was a brave move that paid off with stunning results that moved the technical centre of gravity back to Europe.
A very clever design developed by Dr Christian Heil took everybody back to square one as far as arena and big gig shows went. The system was not so flexible for the small to mid sized companies but for large-scale shows and big auditoria; it was a real problem solver for a number of reasons.
Dr Heil came up with a number of practical solutions that didn’t necessarily solve every obstacle with building a workable full bandwidth line array, but he did end up with a functional system that delivered uniformly very good results in the intended market. Part of the technology included the L-Acoustics array and modeling software so that the system could be custom-rigged to suit each venue. He also only supplied the system to companies with the technical competence to implement the technology correctly. This previously untested marketing and technical support approach guaranteed good results.
Apart from the fact that the V-DOSC system sounded good, the introduction of the relatively compact line array system coincided with a major change in the visual fashion of pop music PAs. The shift from massive semi-circular left and right arrays, to much smaller J-shaped hangs drastically reduced the number of sound sources brought into the venue, and therefore the potential for multiple arrivals, interference and incoherent sound.
For the same venue, a “line” array will use as few as one quarter the number of drivers deployed within a big horizontal array. As lights, video and special effects become more important in the pop music presentation, stage and lighting designers were only too happy to embrace the new smaller, lighter line arrays. Road crews were happy as well. As we have already seen, much of the old-style horizontal array’s output was wasted due to interference and poor aiming. “Overkill” sound systems with five-high columns aimed at venue sidewalls were still common in 90s.
Audiences benefited from the new “line” array because they were no longer being pulverized by too many big lumpy horn loaded boxes turned up too loud, bouncing off every available hard surface. Coverage was more predictable with this type of system. It’s easier for a manufacturer to model in virtual space as it comes with fixed rigging in the vertical plane, there is only one way to ‘fly’ it and less ways to get it wrong.
An un-shaded straight vertical line array is a ‘tunnel’ of focused high SPL. It is still prone to interferences but certainly delivers where it’s pointed. Let’s introduce the ‘J’ curve.
A straight array. It’s still not perfect, but it is a big improvement on the big ground stack. The graduations indicate dB SPL, not frequency response.
The J-shaped array can produce reasonably even front to rear SPL (volume) using “Angular Shading” or “Amplitude Shading.”
Creating Even Coverage: Shading SPL
Amplitude (output), shading is where one cabinet is powered at full output and the next one is turned down a little. This produces a pressure difference and resultant side lobes: perhaps not as severe as those at the end of a line of equal pressure sources, but clearly audible nonetheless. Remember with changes in pressure, waves start bending and changing their behavior.
Rather than turn down boxes, what about gradually spreading the energy over a larger area?
The concept of angular volume shading is a line of equal pressure where the pressure is distributed over a wider angle, as the listener gets closer. Here’s what that looks like in an elevation view:
The goal is a zone that maintains equal pressure while its curvature is varied over a fairly wide range.
So What Does it All Mean?
It means that like all fashion, once the idea catches on, it is copied and done to death by everybody. The line array is not a panacea to all audio problems, it is a specialist tool, but one thing is for certain, light weight, hi-fidelity powerful sound systems are the new market demand whether we are talking about a semi full of gear or a pair of plastic boxes.
Have fun! enjoy life.
jojo
