Today, with few exceptions, there is hardly a manufacturer, domestic or foreign, that doesn't have a line array among its products. Soon we won't see a concert that doesn't use one of these systems. It's a story of more than half a century.
A Little History
Although it seems like the latest in sound reinforcement technology, its operating principles are more than half a century old. First came Auguste Jean Fresnel, who in 1814 demonstrated a multitude of phenomena exhibited by polarized light. He observed that two polarized rays in the same plane interfere with each other, but do not when they are polarized perpendicular to each other. This discovery led him to think that in a polarized ray something must be happening perpendicular to the direction of propagation, and he established that this something could be nothing other than the light vibration itself.
Its analogy in the world of sound shows that the reflected and refracted waves are formed by the envelope of the elementary waves produced at the same time at different points on the surface. The reflected ray is perpendicular to the reflected wave, just as the incident ray is to the incident wave. From this we can deduce that, to avoid significant lobes in the vertical polar response and to make the sum of the individual sound sources coherent, the maximum spacing between cabinets must be less than half the wavelength of the highest frequency they have to reproduce.
Harry F. Olson, in his book "Acoustical Engineering," published in 1947, had already put forward several theories applied to acoustics, such as subwoofer lines, arcs, directivity of low frequencies, line arrays, and so on.
Heil Acoustic's "WST" Technology
But it wasn't until Dr. Christian Heil presented the study "Sound Sources Radiated by Multiple Sound Sources" at the AES (Audio Engineering Society) in 1992 that the first line arrays began to be manufactured. This French doctor of acoustics reasoned that if you throw a stone into water, it produces a progressive circular wave emitted from the point where the stone fell.
If we throw a handful of stones, a network of interference is created. Since the water's surface doesn't let us see a progressive wave shape, it's as if we were in a chaotic sound field. But if you take all those stones, put them in the same bag and throw it in the water, we see a progressive circular wave again. In other words, the idea was to create a point sound source whose aperture could be controlled, in order to concentrate the energy on the part of the audience we care about.
This idea led him to develop WST, Wavefront Sculpture Technology, whose goal was to find the physical conditions under which a system with several loudspeakers is equivalent to a single large sound source capable of reproducing a continuous, manageable wave. In developing his theory he also observed that the ARF, Active Radiating Factor, must be greater than 80% of the total area of the complete system, including the spacing between cabinets.
Radiation behavior depends on the ratio between the length of the array and the wavelength of the frequency being reproduced. For a fixed frequency, if we increase the length of the line, the main lobe narrows and secondary lobes appear. For a fixed array size, as frequency rises the main lobe narrows and secondary lobes appear.
Sound Propagation
Spherical Waves
As we already know, according to the inverse square law, sound pressure level drops by 6 dB every time we double the distance. This is due to sound propagating as a spherical wavefront. Each time the distance from the listener to the source doubles, the radiated energy spreads over an area 4 times larger, so the energy density drops to a quarter, which accounts for that 6 dB drop.
Cylindrical Waves
In a line array, the wavefront generated by each element is cylindrical, remaining constant in the vertical plane. This wavefront is almost flat, so there is no interference between the individual sources, giving us a coherent sum that behaves as a single sound source.
Difference Between Near-Field and Far-Field Propagation
Because the array's length is not infinite, there will be a point, depending on frequency, at which the resulting wavefront changes from cylindrical to spherical. This point separates the near field from the far field, so the more cabinets there are, the farther the near field extends. With the following formula we get the relationship between array length and the near-field limit:
D = H2f / 2C
H = array height - f = frequency - c = speed of sound. If the array is 5 m long, then at f = 100 Hz D = 3.7 m, and at f = 1 kHz D = 37 m
In reality, the near-field behavior of line arrays is more complex. Any given point in the near field is on-axis with only one of the highly directional high-frequency horns, but it receives low-frequency energy from most of the components in the array. For this reason, adding more components to the array will increase the low-frequency energy in the near field, but the highs will stay the same.
That is why line arrays need EQ to boost the high frequencies in the far field; the EQ effectively compensates for propagation loss. In the near field, it compensates for the constructive summation of the lows and the proximity to the high-frequency waveguide.
Array Coverage
A system's coverage is the angle defined by a 6 dB drop in sound pressure level. For a 2 m flat line array, the vertical coverage angle would be:
If f = 100 Hz, lambda = 3.4 m, so length / lambda = 0.59, and in the graph we read > 150º
Whereas if f = 1 kHz, lambda = 0.34 m, length / lambda = 5.9, and in the graph we read < 15º
The Importance of Phase
John Meyer demonstrated the other theory of line arrays, in which the operating principle is considerably more complex than what we've described above and is a consequence of the phase relationship between the cabinets.
A line array is a group of radiating elements arranged in a straight line, closely spaced and operating at equal amplitude and in phase. As Harry Olson describes in "Acoustical Engineering," line arrays are useful in applications where sound must be projected over long distances. This is because line arrays achieve very directional vertical coverage.
Line arrays achieve their directivity through constructive and destructive interference. A loudspeaker's directivity varies with frequency: at low frequencies it is omnidirectional, and as the wavelength shrinks with rising frequency, its directivity narrows. Stacking two of these loudspeakers, one on top of the other, and driving both with the same signal gives a different radiation pattern. At on-axis points between the two there will be constructive interference, and the sound pressure will increase by 6 dB relative to the sound pressure of a single unit. At other, off-axis points, the differences in path length will produce cancellations, resulting in a lower sound pressure level. This destructive interference is called "combing."
A common misconception about line arrays is the belief that they allow sound waves to combine into a single cylindrical wave with special propagation characteristics. Under linear acoustics theory, this could not be, so that claim is not science but a marketing technique.
At the sound pressures used in sound reinforcement, sound waves cannot merge; they pass through one another linearly. Even at the high pressure levels present in the throat of compression drivers, sound waves obey linear wave theory and pass over each other transparently. Even at pressure levels above 130 dB, nonlinear distortion is less than 1%.
To test what we've just said, we place two cabinets in a "crossfire" array and observe on the sound pressure map that neither affects the other on its own axis, as far as coverage and pressure are concerned.
* The graphics published in the article in the print edition of ISP Música magazine are pressure maps of what happens with 8 line array cabinets spaced 0.56 m apart, center to center. In the first three cases, the spacing between cabinets is less than 2/3 of the wavelength of the frequency being reproduced.
Principles Line Arrays Must Meet
Line array theory works best for low frequencies. As wavelength decreases, more and more loudspeakers, smaller in size and spaced more closely, are needed to maintain directivity. The most practical method for sound reinforcement systems is to use waveguides, horns coupled to compression drivers.
Ribbon Emulators and Horns
One principle the horns must meet is to have the smallest possible spacing, and the ideal way to do that would be to emulate a ribbon.
Each manufacturer has chosen a different technique for creating its waveguide. Christian Heil opted for the DOSC (Diffuser of Spherical and Cylindrical Waves). This horn design lets every sound wave take the same path, creating a ribbon-shaped, in-phase wavefront from a classic compression driver.
Many other brands, such as Adamson and Nexo, have followed this path with very similar designs. John Meyer opted for a REM ribbon emulator (Ruban Emulator Manifold). The two drivers are mounted at the back of the REM, and you can see that each driver has 4 output ports spaced at less than 2/3 of the wavelength of the highest frequency reproduced. And as hard as it is to believe, many cabinets sold as line arrays don't meet this last principle, although some are already correcting that.
Line Length
Another fundamental principle for a line array to work properly is that its length must be greater than the wavelength of the lowest frequency it can reproduce.
Frequency Response by Number of Cabinets
Another peculiarity is that stacking cabinets changes the overall frequency response of the system, as we see in the following graph, where an increase in low and mid frequencies is visible. The highs remain unchanged. This characteristic depends on the number of cabinets but also on their size.
Frequency Response and Environmental Parameters
We know that the speed of sound, and therefore its propagation, varies with temperature: the higher the temperature, the faster it goes. And there is also attenuation due to distance and air absorption. But this is not the only environmental parameter that affects sound. One of the most important is relative humidity, measured as a percentage. The interaction of these two factors changes the system's frequency response, but only in the high-frequency range.
Different Zones of a Line Array
With a line array it's always a good idea to set up several zones, that is, to group a certain number of cabinets to cover different areas. You need complete control over the level, EQ and phase of each zone relative to the others, which calls for digital or analog processors such as the BSS Omnidrive, XTA, DBX Driverack or Meyer LD3.
Acoustic Prediction and Line Array Configuration Software
Finally, to properly configure a line array you need a prediction program to help you choose the splay angles between cabinets. Almost every brand has its own program; without one, prediction is practically impossible. You can read the chapter we devoted to this by clicking here.
We have to keep in mind that, since line arrays have very narrow vertical coverage, an error of just a few degrees can have serious consequences and leave a section of the audience uncovered. Before designing a sound system for a venue with a line array, we will need essential tools such as laser distance meters, digital inclinometers, and so on.
The best-known general-purpose program, and one of the most complex for acoustic prediction, is EASE, which lets you make complete designs taking materials and surfaces into account. In real life, where we need to make predictions every day because of touring and we're in a different place every day, each manufacturer has its own program, most of them built in Excel environments, such as QSC's Ease Focus, HEIL ACOUSTIC's Soundvision, NEXO's GeoSoft, Adamson's Y-axis Shooter, and others like MEYER SOUND's Mapp On Line, which lets you view sound pressure and phase maps, and constructive and destructive interference.
