Loudspeaker World

Everything You Wanted To Know About Line Arrays (And Then Some)

What's really at the heart of how they work?

By John Murray June 17, 2013

The critical distance for a given line array length varies inversely with wavelength (frequency). Shorter wavelengths (higher frequencies) have much farther critical distances than longer wavelengths (lower frequencies).

In theory,  this means that at greater distances, a line array will maintain more high-frequency content than low. However, air attenuation of the highs will counteract this characteristic.

Articulated Arrays
Articulated is the ten dollar term for curved. This describes the popular J-Array shape that most manufacturers currently offer.

On the other hand, the Duran Audio Intellivox system is a line array that covers from extreme near-field to far-field seating with a straight-line dead-hang approach. (Talking about articulated arrays with your clients is what gets your day rate increased and your job title changed from “sound tech” to “audio engineer.”)

Spiral Arrays
This is also a term for curved arrays of a particular type. Spiral arrays describe a curve that is increasing in the rotational angle from one end to the other, just as the common J-Array does from top to bottom.

Arithmetic Spiral Arrays
Mark Ureda, when working as a consultant to JBL, mathematically determined that spiral arrays that increase their angle of curvature in even increments perform better.

For example, at the top of a line array, the splay between cabinets is 0 degrees. Going down the array, the element boxes are successively splayed at 1 degree, 2 degrees, 3 degrees, etc. Or it could go in increments of 2 degrees (i.e.: 2 degrees, 4 degrees, 6 degrees, etc.).

These are arithmetically increasing spiral arrays.

Lobes
Lobes describe all the acoustical energy that emanates from a loudspeaker or group of loudspeakers. The specified coverage angle of a horn is its main lobe.

Spurious lobes are those that emanate out in a non-useful direction from the source.

Steering Lobes
Much has been made about lobe steering. Visions come to mind of front of house guys moving loudspeaker coverage around with a joystick.

Lobe steering is generally done by incrementally delaying drivers in a line array. This can only be done when the sources, (the drivers), are about 1/2 wavelength apart for a given frequency, and only in the direction of the line array’s axis. For typical live sound HF drivers with a 9-inch diameter, this means that they cannot be positioned close enough together to steer anything above 750 Hz.

However, using adaptive apertures to mimic a long line of smaller sources enables some steering at shorter wavelengths.

Side Lobes
Side lobes are artifacts of line arrays, and actually, they emanate from the ends (not the sides) of the array – at the top and bottom, as a typical line array is viewed in use. They’re caused by the individual elements being in-phase at a particular angle and wavelength at some off-axis position from the array’s main lobe.

It’s possible to eliminate side lobes, but there are limits and consequences to side-lobe elimination in line arrays.

Gradient Side Lobes
This is a synonymous term for side lobes. Gradient describes how these lobes occur at particular angles or grades with respect to the line array’s orientation.

Professional progress terminology tip: use gradient side lobes rather than side lobes in your technospeak. Sounds more complicated, making you seem even smarter than you already are…

Driver Spacing
Another of the fundamental parameters of line arrays is the spacing between individual elements.

The accepted limit is that for good line array behavior, the sources should be no more than 1/2 wavelength apart for a given frequency. This means that loudspeakers reproducing longer wavelengths can be spaced farther apart without any deterioration in performance.

But since 1/2 wavelength at 15 kHz is just under one-half of an inch, HF devices can never be close enough. One manufacturer maintains that because of this, line arrays do not really work at very high frequencies.

However, I disagree, because even at very short wavelengths, the 3 dB loss per doubling of distance still holds true, and this is what defines the line array effect (in my humble opinion). What does result from driver spacing of more than 1/2 wavelength is more pronounced gradient side lobing.

Logarithmic Driver Spacing
Duran’s Intellivox Series line array loudspeakers, for example, employ the logarithmic driver spacing technique. This provides denser driver spacing at short wavelengths and economizes on the number of drivers needed for longer wavelengths by spacing them in larger and larger logarithmic increments.

Isophasic Apertures
Isophasic aperture is one of my favorite high-tech terms. It describes the phase characteristic of the slot that loads the horn bell of some line array box HF sections.


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About John

John Murray
John Murray

Principal, Optimum System Solutions
 
John Murray is a 35-year industry veteran who has worked for several leading manufacturers, and has also presented two published AES papers as well as chaired numerous SynAudCon workshops. He is currently the principal of Optimum System Solutions, a consulting firm based in Colorado.

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Charles St Lucas says

John;
I read your post on Everything You Wanted To Know About Line Arrays (And Then Some)
Nicely done!
Chuck St Lucas PhD

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