Cylindrical Radiator / Extending the Height

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quote;

If it were possible to add another section to the top of the L1...
Even though the direct sound would be over the listeners head, what effect would this have on the behaviour of the line array?

Chris-at-Bose replied.

Ooooohhh! My favorite question! <squirms in seat, waves hand vigorously in the air> (Didn't you hate that kid in school?) Even though Hilmar already gave the right answer, I have to talk about this because it is so COOL!

1) You probably know that the L1 sends out a layer of sound that is as tall as the array. If you added another section to the top, then, as you might expect, the layer would get 50% taller. But the more interesting thing is that the sound inside the original layer is almost totally unchanged. Neither the musician nor anybody in the original layer would notice the addition of another section on top. Imagine that--it won't even be louder! The extra "speaker power" goes into a taller layer, not a louder sound. Now, if everyone were within, say, roughly 50 feet (or 15m) of the L1, that would be the whole story.

2) Beyond roughly 50 feet, the story is different and even more interesting. You'd have to be using an L1 outdoors to notice this, though, because reflections would obscure the effect. The layer can't go on forever, of course. Eventually as you get far away, the layer starts to spread out vertically and the sound starts to fall off with distance just like a regular speaker does. The coolest thing about line arrays is that, when we increase their height by only 50%, the distance that the layer projects before it starts to spread out doubles. (Actually it's 2.25x, but who notices?) We get a big return in distance projection of the layer for a small increase in height.

3) And, furthermore, everyone who was farther away than the original layer's extent, would hear the sound get louder by 3 dB when the third section was added, just like a regular speaker would do.

Isn't this amazing? Add another section on top and no one inside the original layer hears any change, but people 100 feet away would hear it get about 3 dB louder! It almost seems to defy physics. But remember, the extra energy that will make it sound louder far away is carried along in the new upper 1/3 of the layer, the part that goes well above the heads of the original nearby audience, which is why they hear no change.

This radical increase in the projection of a line array with more height is probably why nobody discovered the L1 back in the fities and sixties, when line arrays were first sold. Those line arrays were short, so their layers rarely projected far enough to reach anyone. The layer itself was an obscure footnote in textbooks and was thought to be always confined to very close to the speaker. But double the height and the layer goes 4 times as far. Double again and it goes 16 times as far as the original. And soon the whole audience is within the layer, where sound falls off very slowly with distance.

So Robert L's question gets at the heart of why line arrays are different from other speakers and it allows me to describe what I think of as their most fascinating behavior.

Chris <-- hoping this explanation doesn't go over anyone's head!

Original Post: Extending the Height

Another interesting discussion has sprung up in which we get more insight into the L1 here: Does elevating the L1™ change the line array performance?