Line Array Speakers/Systems

Isn't a line array that is not at least 4 times the wavelength of the frequency being measured, and the drivers have to be at least 1/2 wavelength apart, otherwise it's just a column of speakers?

For example, sound travels at approximately 1130 feet per second (in general)
1130 / 100Hz = 11.3 ft wavelength. To get a true line array at that frequency, the drivers need to be 5.6ft apart, no problem there. It also has to be 45 feet long, that's a problem usually!

Go to 500Hz
1130 / 500Hz = 2.26ft wavelength
Drivers need to be 1.16ft apart from center to center - no problem with a 6 inch driver.
line array needs to be 9.04 feet long - no problem there either.

1000 Hz
1130 / 1000Hz = 1.13ft wavelength
center to center driver distance needs to be .56 ft - not much of a problem with 4" drivers mounting lip on the frames is small, or no problem for a 3" driver
array needs to be 4.52 feet tall, not much of a problem there either.

Go up to 5000Hz now
1130 / 5000Hz = .226 ft wavelength
The drivers need to be .113 inches from center to center. Now that's a problem. However, it only needs to be .904feet long, that's not a problem

So, a true line array is only "real" in a narrow frequency range.

Somebody that knows more than me can go into more detail please, or correct me if I'm completely wrong and I'll bow out of this discussion.

My theory on Line array:

It just doesn't matter. I don't measure and I don't calculate. I listen. If it sounds good to me and to my clients, and it looks good to the clients and it's easy to me to be mobile with the stuff, it works for me. I leave my meter and calculators at home.

Wether its technically a line array or stack o' speakers really isn't going to affect what I do. The only question for me is do I need 1 or up to 6 to do the job.

I work on my making my job as simple as possible while giving my clients what they want.

At least that's my theory.

Scott Hanna said:

My theory on Line array:

It just doesn't matter. I don't measure and I don't calculate. I listen. If it sounds good to me and to my clients, and it looks good to the clients and it's easy to me to be mobile with the stuff, it works for me. I leave my meter and calculators at home.

Wether its technically a line array or stack o' speakers really isn't going to affect what I do. The only question for me is do I need 1 or up to 6 to do the job.

I work on my making my job as simple as possible while giving my clients what they want.

At least that's my theory.

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very good...see with your ears, not your eyes.

What a great read! Thanks for sharing!

cclark65 said:

Isn't a line array that is not at least 4 times the wavelength of the frequency being measured, and the drivers have to be at least 1/2 wavelength apart, otherwise it's just a column of speakers?

SNIP...

Somebody that knows more than me can go into more detail please, or correct me if I'm completely wrong and I'll bow out of this discussion.

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You are completely wrong. Go back and re-read the original link posted:

http://www.meyersound.com/support/papers/line_array_theory.htm

That article references an excellent book on all things acoustical, "Acoustical Engineering."

In terms of the discussion here, understanding a few of the "rules of thumb" for line arrays is sufficient.

- Line arrays should always be vertically arrayed.
- Line arrays narrow the vertical dispersion of sound.
- Horizontal dispersion does not differ from a point source to a line array.
- The near field effect is proportional to the square of the height of the array.
- The near field effect varies with the height of the array and the frequency of the sound.
- The near field effect is commonly cited as a "-3dB/doubling" rather than "-6dB/doubling" drop in SPL. This is overly simplistic, and is rarely achieved for more than a narrow frequency band in real world conditions with air currents, room humidity, and arrays less than 4m high.

In general, the benefit of a line array for a mobile DJ is the improved vertical pattern control, not the change in SPL drop with distance.

Scott Hanna said:

My theory on Line array:

It just doesn't matter. I don't measure and I don't calculate. I listen.

Click to expand...

Depending on how well you can listen, you might be OK. Could you set a 31 band EQ "by ear" to deliver a flat response (within 1-2dB per slider) accounting for room modes? I can't. I need an RTA to get me in the ballpark. If you can, then more power to you.

Very few people have "perfect pitch." Many people have trained to have good "relative pitch." Part of that training involves a lot of listening to reference tones or music or speech in an effort to learn to hear variations from that reference.

I still use an RTA to help "listen" for room modes and to do the first pass at setting the EQ for any given combination of equipment. Since you are using between 1 and 6 cabinets, it is likely that some kind of meter would be helpful during setup.

I have been in and around PA systems for 15 years. In the last two years I have worked through this book three times. What I hear in music today is night and day different from what I heard before I took the time to learn to critically listen. If you are going to rely only on your ears, you owe it to yourself to take the time to work through this material.


http://www.amazon.com/Critical-Listening-Skills-Audio-Professionals/dp/1598630237

Jeff Romard said:

I beg to differ. Most people we deal with in this business wouldn't know good sound if they fell over it. Alot of times I find they are so used to screachy midrange that they can't hear quality sound properly and find it's not loud enough

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I'm not sure how that differs.

But, I do agree with you. Most people (and most DJs) do not know how to critically listen to music. Most musicians do learn to listen critically (at least while learning to play an instrument). Even so, those critical listening skills do not automatically translate into an ability to understand how changes in the PA system setup will alter the sound field.

soundinmotiondj said:

You are completely wrong.

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I don't think that I am completely wrong. There are many papers about the physics of line arrays that correspond to my posting. Here are a few links:

http://www.rh.com/WhitePapers/LineArrays-WhitePaper.pdf
http://www.livesoundint.com/archives/2002/novdec/la.php
http://www.tasesound.com/news.asp?id=277

In fact, the paper you linked to states the same rules that I have mentioned.

So, I don't think that my statements are completely wrong, they just might not be what you are looking for in the answer.

cclark65 said:

I don't think that I am completely wrong. There are many papers about the physics of line arrays that correspond to my posting.

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Perhaps "completely wrong" was not fair - but that was your term. A better description was that your comments are "confusing with the use of very specific terms" and "focusing on the leaves instead of the trees or the forest."

You said this in your original post:

cclark65 said:

Isn't a line array that is not at least 4 times the wavelength of the frequency being measured, and the drivers have to be at least 1/2 wavelength apart, otherwise it's just a column of speakers?

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The reason that you were completely wrong is that the guidelines you were using were for a "line source." Those guidelines are not applied to "line arrays" which function over a wider range of frequencies. If you had said "line source" instead of "line array" you would have been in the ballpark. The devil is in the details.

As you math points out, at 5kHz a "line source" effect can be achieved using a column of sources that is less than 1ft tall. Even a single CD horn can reach that size. So, a single conventional front loaded direct radiating speaker turned on it's side will function as a "line source" over at least some portion of the frequency band. But, I am assuming that any "array" has at least two cabinets.

From the pdf link you posted:

http://www.rh.com/WhitePapers/LineArrays-WhitePaper.pdf

There are two conditions for line source behavior. The first is that the line has to be at least four times as tall as the wavelength it is radiating for a vertical pattern that is a reasonable approximation of a plane wave. A second requirement is that the sources on the line have to be less than 1/2 wavelength apart. This is the inverse of the first requirement. Olsoni calculated in the 1940’s that two adjacent sources radiate a spherical polar pattern (i.e. sum coherently) when they are less than 1/4 wavelength apart. Between 1/4 and 1/2-wavelength spacing the pattern narrows, but side lobes (interference patterns, caused by destructive interference) do not appear until the spacing is greater than 1/2 wavelength.

What does this mean in practice? Only very long line arrays can function as a line source at low frequencies, while only very short modules can couple at high frequencies. In the real world, the actual range of line source behavior is typically less than one octave. The answer to the question of whether a line array is a line source is “almost never.”

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The article is quite clear: "line source" and "line array" are not interchangeable terms.

In practice, line array sources should be less than 1/4 wavelength apart to constructively couple together. Having the sources 1/2 wavelength apart is the maximum recommended separation distance to minimize the onset of side lobes in the waveform pattern...but is certainly not a recommendation for the layout of the array.

The article that I pointed you to (that was posted at the beginning of this thread) clearly states:

http://www.meyersound.com/support/papers/line_array_theory.htm

As we have seen, practical "line array" systems as used in high-power applications are actually a combination of "classical" line arrays for the low frequencies and highly directive wave guides for the high frequencies. Because of this hybrid nature, it is difficult to apply predictions from classical line array theory across the whole audio spectrum. Nonetheless, line array systems can be made to work reasonably well in both the far field and moderately close to the array.

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Even two speakers stacked are likely to function as a "line array" at some frequency. Line arrays are hybrid systems that need to be understood as both line arrays (at low frequencies) and as wave guides (at higher frequencies). At this depth of discussion, I believe that it is better to stick to the general rules of thumb that I have given and to focus on the vertical pattern control rather than the near field effects of the array.

got it.