Radiation patterns and reflections
Here we have a point source speaker in a room. Let’s see what happens when it plays sound.
Sound radiates outward in all directions, “thinning out” quickly as it goes since it is expanding both vertically and horizontally. But what happens when it runs into the ceiling?
It reflects back down and continues to spread out.
The same thing also happens off the floor. Carpeting can help attenuate the reflection a bit at the highest frequencies, above about 10 kHz or so. But it helps less than most people think.
And of course, the same thing happens off the wall behind the speakers. Once again, this reflection has less energy in the highest frequencies, but it is a strong reflection. (If you were to stand behind a speaker playing out in the middle of an open field, you would still hear lots of sound, without any reflections.)
And of course, all this is happening at the same time. It presents your ears with a lot of confused, delayed versions of the original signal, along with the direct sound you’d like to hear.
On the other hand, when you have a line source (especially if it is mounted in or on the wall, eliminating the reflection from behind the speaker), things get much simpler.
Because the driver is extremely tall and narrow, the sound radiates outward in an expanding cylinder, as though from a line in space. Thus the sound is spread widely through the room from side to side, but not vertically. In fact, because the sound is concentrated where your ears are, it does not “thin out” nearly as fast, and you have surprisingly uniform SPL throughout the listening space. People sitting near the speakers and far away hear pretty much the same volume. This fact alone opens up the “sweet spot” significantly, since you no longer have to be on the center line between the speakers to hear them at the same volume.
The sweet spot gets even bigger because there are many fewer reflections confusing your ears. All those ceiling and floor reflections essentially disappear from about 530 Hz up. This range of frequencies include almost all the information our brains use to determine spatial cues (imaging, depth, a sense of space, etc.)
Since true line source arrays effectively eliminate floor and ceiling reflections, they are especially valuable for large rooms or those with architectural challenges including cathedral or coffered ceilings, hard floors, etc. But their singular benefit is heard in any room: you hear more of the speaker and less of the room. Everything is significantly clearer.
Go ahead, walk around the room…
It is rather uncanny, but line sources do not seem to get much louder as you get closer to them. This bears some explanation.
Most speakers in this world are point sources, whose sound expands away from the speaker as an ever-enlarging sphere. The reason for this is that the vibrating diaphragm is small compared to the sound waves it is producing. The sound spreads out like ripples from a pebble thrown into a pond… except in three dimensions.
When the vibrating diaphragm approaches the size of the sound wave it is producing, the sound becomes more directional, moving away from the speaker like a spotlight instead of a floodlight.
When you have an extremely tall and narrow driver like those in our tallest speakers, the sound radiates outward in a cylindrical fashion. This is because the width of the diaphragm is small compared to the sound waves (wide dispersion), while the height of the diaphragm is large (resulting in controlled directivity in the vertical plane).
Because more of the sound is being focused where your ears are (somewhere in the horizontal plane of the speaker, not up on the ceiling or down on the floor), the difference in volume as you move away from the speaker is significantly less. In technical terms, it falls off in a linear way rather than as the square of the distance.