[John Casler]

I was reading about the Dali Megalines.

While I know some of the virtues and properties of "line arrays" I am curious about one point, regarding these speakers.

They state on their website that:

Such a configuration is known as a line source and has a number of useful characteristics for sound reproduction in enclosed spaces. The dispersion pattern of a line source is not spherical like that of the point source, but cylindrical as it extends outward from the loudspeaker. Of special interest is the fact that there is virtually no sound at the upper and lower ends of the line. Consequently, with a line source extending from floor to ceiling, there will be no reflected sound from these surfaces, eliminating a major source of room colouration.

While I can certainly understand that their ribbon drivers might have "limited" vertical dispersion, how can the vertical dispersion of their 6" drivers be "limited" by this line configuration?

Or is it?  Doesn't the lowest cone driver have the same vertical dispersion downwards as a single 6" driver might have there?  and if not, why?  Wouldn't the same be true for the upper 6"?

I can see that since multiple drivers will each be playing "softer" that they might "measure" collectively, to have less dispersion than a single driver at the same SPL, but are the laws of Acoustics really changed here, or is it a relative reduction, based on their collective SPL?

[_scotty_]

John, you have asked a seemingly simple question that has a complicated answer.  I have put two links in this post that discuss this aspect of line arrays.  Read the section on interference  patterns in this link
 http://www.livesoundint.com/archives/2002/novdec/la.php
and read  E:Line Sources, in this link   http://www.linkwitzlab.com/frontiers.htm
I hope this information helps answer your questions.  
Scotty

[AndrewH]

It depends if your line source actually stretches from floor to ceiling.  If it's a perfect theoretical line source, it'll benefit from floor and ceiling reinforcement and propogate as a cylinder, suffering half the SPL decay over distance (I think 3db versus 6db for a point source).

If it's not a perfect theoretical line source, meaning if it doesn't stretch from floor to ceiling, it'll act somewhere between a point source and a line source.  It'll benefit from the floor bounce but not the ceiling bounce at least at closer listening distances.

Vertical dispersion on the top side will be like a point source.  The bottom, if the line source is resting on the floor, will be like a cylinder because of the floor bounce.

[_scotty_]

AndrewH, The question John asked dealt with real world radiation patterns of nonperfect linesources. Under these conditions the behavior you are describing is a frequency dependent phenomenon.
Scotty

[John Ashman]

Here's how I  understand it.  Think of air molecules as little grains, because that's what atoms basically are.  If you push forward with a cone, the molecules seek the path of least resistance.  Since resistance will be strongest directly in front of the cone, they will start to push outward until they all are experiencing approximately the same resistance, hence the spherical radiation pattern.  BUT, let's say you have a bunch of drivers above and below.  The ability to move to the left and right is mostly unchanged, but for any molecules trying to go up or down, there is increased resistance above or below because the molecules there are *also* trying to move.  They encounter resistance above and below and/or bounce off each other so they have to go either forward or sideways, not up or down.  So, with one driver, imagine an inflatable ball that is having air pumped in and out at a rapid rate, the sphere grows and shrinks.  But imagine a cylindrical "ball" capped by the floor and ceiling.  The cylinder moves in and out but only in two dimensions, not three.  A single driver is far enough away from the floor or ceiling that the radiation is spherical until it hits the limits of the floor.  But with a line array, the floor/ceiling are hard limits, the sound can't even head in their direction effectivly.  So the sound "slides" along the floor and ceiling in a parellel motion, it can't really reflect off of it.  Of course, things aren't perfect, so you might get *some* lower amount of turbulence and bouncing, even maybe a tad of comb filtering, but it should be extremely minimal and only because a series of cones isn't a *true* line source, but is just acting as one.  In theory, since the vibrations are only in two dimensions, there'd be zero energy heading up or down, only out into the room, hence the 3dB radiation, not 6dB (pi R loss instead of pi R squared).  That's why the volume would remain unchanged from floor to ceiling as well.  Did that make sense?

[JoshK]

John, the last statement you said is basically how it works.  Lets just say for simplicity that for a given SPL at a distance D in front of the two speakers the line source drivers are having to work much less, playing on 1/3 as loud, because of the coupling of the drivers forward wave propagation.  But still each of the drivers is playing only 1/3 as loud.  Now move up to an angle 30º above the top of the line source.  We are now in an area not benefitting from the coupling and is served only by the 30º off axis response of the nearest driver playing only 1/3 as loud as a point source would be playing for this SPL (in reality you are gaining more than just one drivers off axis response, but combing and other things complicate the matter), so the result is the off axis SPL here is quite low relative to in front of the line.  Therefore the off axis acoustic disturbance, should we say is going to be much less than it would be for a point source for a given SPL in the listening spot.  What SL says as the direct to reverbant ratio.  

One draw back is that the ear needs a certain amount of reverbant info for a sound to sound real, however stereo reproduction cannot do this reverbant info correctly for 99% of music to begin with, so we simply gain a lesser of two evils quandery IMO.

[Danny Richie]

Actually John,

Their posted statement can be viewed as true and false. It depends on the wavelength being played.

In the higher ranges what they say is true. The reason being is that the comb filtering effects cancel output above and below the length of the line.

As frequency decreases this becomes less true.

In frequency ranges below 200Hz or so then the woofers begin to play more like a single driver and the radiation pattern extends to and reflects off of the floor and ceiling making an infinite line.

but are the laws of Acoustics really changed here, or is it a relative reduction, based on their collective SPL?

They are the same and it is not based on SPL.

[John Casler]

John, you have asked a seemingly simple question that has a complicated answer.  I have put two links in this post that discuss this aspect of line arrays.  Read the section on interference  patterns in this link
 http://www.livesoundint.com/archives/2002/novdec/la.php
and read  E:Line Sources, in this link   http://www.linkwitzlab.com/frontiers.htm
I hope this information helps answer your questions.  
Scotty

Thanks Scotty,

I read through both, but still cannot undestand how the "radiation" (dispersion) of the top and bottom cone driver will fail to radiate/disperse up or down as it would it it were simply a single driver in that position.

What I seem to see so far, it that there is a "phasic" phenomenon that somehow causes this.

I'll read some of the other responses.

Thanks for your post.

[John Casler]

Did that make sense?.

Hmmmm, not really. since I'm talking about the upper and lower cone drivers.

Your example makes sense for all the drivers in the array except the top and bottom.  It is certainly easy to see that when all working together that they form a cylinder like pattern, but why do the top and bottom lose their normal vertical dispersion?

[John Casler]

John, the last statement you said is basically how it works.   ...

Ah Hah!!! so it is the relative measurment of dispersion of the collective, rather than some "phasic" anomalie.

Now that makes it simple, but it may lead to more question about LA's in the future.

I need to GROK this and absorb it.

Thanks Josh

[John Casler]

Actually John,

Their posted statement can be viewed as true and false. It depends on the wavelength being played.

In the higher ranges what they say is true. The reason being is that the comb filtering effects cancel output above and below the length of the line.

As frequency decreases this becomes less true.

In frequency ranges below 200Hz or so then the woofers began to play more like a single driver and the radiation pattern extends to and reflects off of the floor and ceiling making an infinite line.

They are the same and it is not based on SPL.

Thanks Danny,

That is how I felt it should work, and my main area of "question" was the lower frequncies handled by the "cone" drivers.

Thanks for your explanation.

[jholtz]

Hi John,

Here's what I consider to be the ultimate "bible" on line arrays. Jim Griffins white paper will answer all of your questions. Go here to download it. http://www.audiodiycentral.com/awpapers.shtml

HTH

Jim

Thanks Danny,

That is how I felt it should work, and my main area of "question" was the lower frequncies handled by the "cone" drivers.

Thanks for your explanation.