It is a series of loudspeakers that covers the same frequency range and is stacked above one another.The key to a line array is that the speakers face slightly different vertical angles, allowing them to consistently cover a greater depth of field than a single PA speaker can. The effect is that people experience similar sound whether they’re in the back rows, the middle, or the front of the venue, providing a better experience for everyone in the venue. If you’re working in larger spaces and need reasonably high SPLs (Sound Pressure Levels) and clarity, then this is one option you should definitely consider.
The Inverse Square Law
The inverse square law tells us that SPL drops by 6dB each time the distance doubles from a point source of sound in a free field of intensity (meaning no boundaries). This is the behavior we’re normally used to with speakers, though there are many nuances to it.
The inverse square law assumes the speaker is radiating omnidirectionally. Except at very low frequencies, this is rarely the case. However as distance increases, even a typically directional loudspeaker (e.g., 90° horizontal dispersion by 90° vertical dispersion) acts like a true point source (i.e. omnidirectional) with respect to how the inverse square law applies.
Phase cancellation is usually one of the things you try to avoid in a sound system, yet it plays a central role to the way line-array speakers work together to provide a system of speakers with narrow vertical dispersion characteristics. Even with advanced speaker cabinet designs to shape the vertical dispersion, there’s still plenty of natural overlap between speakers in a line array. However, each speaker is at a slightly different distance from the audience, which introduces a small degree of phase cancellation. By introducing a small amount of additional delay, you can fine-tune these phase differences to reduce each speaker’s vertical dispersion.
A Few Important Caveats
While applied phase cancellation can shape the vertical dispersion of the speakers in a line array, their horizontal dispersion is not affected. So in effect, an individual speaker in a line array may wind up with a 90° horizontal dispersion by only a 20° vertical (for example). Also, even though phase cancellation can achieve a line-source distribution and dramatically improve long-distance coverage, as distance increases, even line arrays begin to take on point-source characteristics and succumb to the -6dB per doubled distance of the inverse square law.
There are limitations and caveats to a line array’s ability to approximate a line-source function. First, the overall top to bottom length of the array determines the lowest frequency that will behave accordingly. This is simply because as the wavelengths get longer, the relevant time arrival distances at the listening position must be greater to achieve the effect. That necessitates a longer array. At the other end of the spectrum, the wavelengths become so short that the drivers are too big to be placed close enough together, so the relative phase differences become too great to achieve line-source function. In those cases, waveguides (horns) are used to achieve enough directionality to approximate something between a point-source and line-source function.
Because vertical dispersion per speaker is so tight, you can effectively think in terms of dividing the listening space into sections from front to back, with each front section covered by only one or two speakers, while more speakers may cover the rear sections. This idea gives rise to the popular J-shaped line array so commonly seen in concert halls and outdoor venues.
Even at their very best, line arrays are unlikely to deliver the purity of sound that high-quality single driver or single two- or three-way cabinets offer, which is one reason why they’re much less likely to be found in smaller spaces where a small number of speakers are more appropriate. Cost and space are considerations too, of course.
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