Question About Line Array Systems Placement

[gfa]

Why is it that the various line array systems (Bose L1, JBL Eon One, etc.) allow you to place the stick behind the microphones? Doing that with a more traditional PA would cause feedback. What's different here?

[Cuki79]

From wikipedia article (line array)

by "Point source" understand "normal speaker"
by "line source" understand "line array".

In the free field, sound which has its origin at a point (a point source) will be propagated equally in all directions as a sphere. Since the surface area of a sphere = 4π r² where r is the radius, every doubling of the radius results in a four-fold increase in the sphere's surface area. The result of this is that the sound intensity quarters for every doubling of distance from the point source. Sound intensity is the acoustic power per unit area, and it decreases as the surface area increases since the acoustic power is spread over a greater area. The ratio between two acoustic pressures in deciBels is expressed by the equation dB = 20log(p1/p2), so for every doubling of distance from the point source p1 = 1 and p2 = 2, thus there is a sound pressure decrease of approximately 6 dB.

A line source is a hypothetical one-dimensional source of sound, as opposed to the dimensionless point source. As a line source propagates sound equally in all directions in the free field, the sound propagates in the shape of a cylinder rather than a sphere. Since the surface area of the curved surface of a cylinder = 2π r h, where r is the radius and h is the height, every doubling of the radius results in a doubling of the surface area, thus the sound pressure halves with each doubling of distance from the line source. Since p1 = 1 and p2 = 4 for every distance doubled, this results in a sound pressure decrease of approximately 3 dB

As line array approximates a line source, it experiences less attenuation and can reach further audience. The result is that you don't need as much power to reach the audience. As you can play quieter, you have more headroom before feedback (as far as the public is concerned).

Note: Bose L1 mI and mII are line arrays (...and mII can be questioned) but L1 compact and JBL eon one are not, they are indeed quite far from a line source. They have very few speakers. So there is a bit of marketing too. However they do work for what they are intended... So whether they behave physicaly speaking as a line source has no real importance.

Please read this post for real data:
http://www.acousticguitarforum.com/f...&postcount=119

Cuki

[zhunter]

Originally Posted by Cuki79

From wikipedia article (line array)

by "Point source" understand "normal speaker"
by "line source" understand "line array".



As line array approximates a line source, it experiences less attenuation and can reach further audience. The result is that you don't need as much power to reach the audience. As you can play quieter, you have more headroom before feedback (as far as the public is concerned).

Note: Bose L1 mI and mII are line arrays (...and mII can be questioned) but L1 compact and JBL eon one are not, they are indeed quite far from a line source. They have very few speakers. So there is a bit of marketing too. However they do work for what they are intended... So whether they behave physicaly speaking as a line source has no real importance.

Please read this post for real data:
http://www.acousticguitarforum.com/f...&postcount=119

Cuki

I would point out that this level of performance is highly idealized and will often be difficult to realize given the low likelihood of a stage mounted system being free of interference. In other words, almost never a free field. Given that, I think another factor is simply the number of speakers involved and the distribution of sound among them. The sound is distributed among several speakers indifferent planes. This has an effect of reducing the SPL any aligned speaker sends to a microphone.

hunter

[gfa]

Thank you both. What I'm getting from this discussion - and I don't pretend to understand the Wiki article - is that the SPL from a line array is lower at any given point due to the dispersion, including at the mic. Is that close enough?

[Wyllys]

Originally Posted by Cuki79

From wikipedia article (line array)

by "Point source" understand "normal speaker"
by "line source" understand "line array".



As line array approximates a line source, it experiences less attenuation and can reach further audience. The result is that you don't need as much power to reach the audience. As you can play quieter, you have more headroom before feedback (as far as the public is concerned).

Note: Bose L1 mI and mII are line arrays (...and mII can be questioned) but L1 compact and JBL eon one are not, they are indeed quite far from a line source. They have very few speakers. So there is a bit of marketing too. However they do work for what they are intended... So whether they behave physicaly speaking as a line source has no real importance.

Please read this post for real data:
http://www.acousticguitarforum.com/f...&postcount=119

Cuki

Actually, all of the above are varying forms of column array. True Line array is a different thing. But nomenclature aside, the main reason low-output column arrays are seemingly more tolerant of placing microphones in front of them is partly due to a wider, more diffuse sound field which spreads the total acoustic output...meaning that a point source cabinet of equal output of acoustic energy will concentrate it in a narrower pattern, the result being a higher SPL at the microphone and less headroom. Frequency variation within the two different polar patterns also figures in, but that's a bit beyond the basic explanation.

The bottom line is this:

LOUDEST SOUND AT THE MIC WINS.

If that's the direct sound from the prtformers, then you're good to go.

If it's the sound emanating from the speakers (no matter WHAT their positioning/orientatio), you lose.

It's just physics and there are no magic loudspeakers.

Edit:

If you want to see a high-volume PA working with the system BEHIND the band, look up the Grateful Dead "Wall of Sound" PA.
I heard it a couple of times in my younger days. It CAN be done...if you know what you're doing.

[jseth]

I am not a sound engineer, but I am a performer... working in night clubs, bars, restaurants and coffeehouses for the past 40+ years. In 2009, I bought a used Bose L1 Model I Classic, and have been gigging with one of their units ever since.

I do not care what the actual definitions or words for something are saying, I am speaking from a USER standpoint...

And all I can say is that, in all my years of performing with an acoustic guitar and my voice, I have NEVER, repeat NEVER played through anything that sounded as good or was as "manageable" as the Bose units are...

The fact that I can place the L1 Model II 6-10 feet behind me on a slight diagonal means I can hear everything "pretty much" like the audience is hearing it and darn close to the same volume they are getting, as well!!!

My vocal mic is a Shure Beta 58A; I can walk away from the stage with the volume still set to playing level, and absolutely NOTHING comes back through that mic... honestly, I have never had any feedback from the microphone in the past 8 years of using my Bose rigs... (Now, I HAVE had a few issues with my guitars, especially that peaky little low-mid around 200-400 cycles, but that was more "operator error" than a fault of the system.)

If you have not spent a decent amount of time playing live through one of these rigs, you are both missing something special as well as just blowing smoke...

Again, I don't care what someone "says", I am much more concerned with how something actually performs... and those Bose rigs kick some serious butt for a solo singer/guitarist.

[Cuki79]

Thanks jseth,

As a former Bose L1 mII owner (Only sold it to buy an expensive guitar), I totally agree with you. It works. Who cares about the rest?

Cuki