Monday, December 12, 2005

How to Evaluate a Line Array System in the Field

from Alcons Audio web site

With the current hype around “line-array”, it has almost become a magic word. A lot, if not most currently available line-arrays (even from the “big names”!) claim to be a “line-array” but in fact aren't.

The relative short experience (”learning curve”) that most users have with this type of sound system, gives plenty of room for badly engineered line-arrays, before they realize there are very significant quality differences ”. But the effects are very audible; Lack of HF throw (a result of lobing), spikes/lobes, frequency / angle dependent dispersion, excessive high-mid response in front and low gain before feedback, are some of the effects that usually occur with a poorly designed array system.

Below we've given some indicators of the engineering quality of a line-array.

Important features of a good engineered line-array are;

- even, consistent coverage of all frequencies from front to rear of the room. Changing response in MF/HF represents bad coupling in the vertical plane (so not according to line-array criteria).

» Check by walking away from the array (near-field) to the end of the area (far-field).

- “throw”, The ability of a system to actually bring critical info rmation to the end of the listening area; The main design reason for line-arrays! Most line-arrays do not have this at all!!

» Check by standing at the farthest point in the room and listen if the sound is “in the face”. The throw can also be heard when walking away from the array; At some point you loose the intelligibility of the sound, that's the limit of the throw of the system.
Note that “intelligibility” is something completely different than “SPL”..

- even, consistent coverage in the horizontal plane; The audible coverage of HF determines the real actual dispersion of a system.

» Check by walking left to right in front of the array. Sound should be “in the face” and intelligible from the farthest dispersion angle of the system. After that, the sound should “roll-off” with similar frequency response. Most systems in practice do not meet their figures on paper!

- absolutely no HF sound directly under the array; Any MF/HF sound under the array represents bad coupling in the vertical plane. What you hear is actually a huge “side-lobe” (with a vertical array, side-lobes are under and above the array; Whereas with a horizontal array, these lobes are really on the side of the array). The same amount of energy that is being fired downwards (under the array, problems with feedback!) is also being fired upwards, against the ceiling (problems with reflection!).

» Check by standing under the array and listen. The MF/HF sound should come to the listener ONLY though reflection from the room (and not from the array).

These pragmatic criteria are audible and excellent references for a quick evaluation of a line-array system.

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