Wednesday, September 14, 2005

Understanding Line Array Systems: Part 1




By John Murray

...this is a great article by John Murray/ProSoundWeb that provides a great overview of the different types of line arrays...

Behind the buzz, there are a lot of factors at work in line arrays. An explanation and comparison of current models.



At last count, I found at least 19 companies offering line array loudspeaker systems that are more than simple column designs. Rather than discussing over a dozen different product types, I thought we might approach the subject by defining the technological terms of line arrays. This way, we get a better grasp of the issues involved with line array systems and will be able to discern both the similarities of, and unique differences between, the products being supplied by manufacturers today.

This discussion can’t be contained in just a few paragraphs, so we must start with the more basic issues of line arrays and then follow with more esoteric topics that build on these basics.

A LITTLE HISTORY

Line arrays have been around for over a half of a century as column speakers, and other than those made by Rudy Bozak here in the US, most were voice-range only. Their application was generally for highly reverberant spaces, where a narrow vertical dispersion avoided exciting the reverberant field, provided a higher Q (narrower dispersion pattern) and, as a result, improved intelligibility of the spoken word.

Never losing popularity in Europe as they did in America, it’s no wonder that L-Acoustics V-DOSC loudspeakers from France were the first to show the concert sound world that more level and smoother frequency response can come from fewer drivers in a line array. After everyone realized that for a given listening area, the drivers have no destructive interference in the horizontal plane and combine mostly inphase in the vertical plane, the race was on.

CYLINDRICAL WAVEFORM

Basically, a line of sources will create a wavefront of sound pressure that is loosely cylindrical in nature at a particular range of wavelengths (frequencies). Its idealized shape is actually more like a section of a cake, and the wavefront surface area, as it expands only in the horizontal plane, doubles in area for every doubling of distance. This equates to a 3dB SPL loss of level for every doubling of distance.

SPHERICAL WAVEFORM

An idealized point source, imperfectly represented by a loudspeaker or nonlinear cluster of loudspeakers, radiates in a spherical waveform rather than cylindrical. This wavefront expands to four times the area with each doubling of distance, which equates to a 6dB SPL loss for every doubling of distance. This is commonly known as the inverse-square law, and it applies to all point-source radiant energy. Hence the big advantage for a line array is that for a given number of transducers, the resulting long throw level can be much greater than for a non-line array, or point-source, loudspeaker system.

INTERFERENCE PATTERN

This is the term applied to the dispersion pattern, or response balloon of a line array. It simply means that when you stack a bunch of loudspeakers, the vertical dispersion angle decreases because the individual drivers are outof- phase with each other at positions off-axis in the vertical plane. The taller the stack is, the narrower the vertical dispersion will be and the higher the sensitivity will be on-axis. In the horizontal plane, an array of like drivers will have the same polar pattern as a single driver. Some believe that the horizontal pattern is wider than for a single driver, but they are mistaken, likely fooled by the fact that the level is louder off to the side due to the higher sensitivity of multiple drivers. However, the actual polar pattern remains the same as for a single driver.

ARRAY LENGTH

In addition to the narrowing vertical coverage angles, the array length also determines what wavelengths will be affected by this narrowing of dispersion. The longer the array, the lower in frequency (longer in wavelength) the pattern control will occur.

CRITICAL DISTANCE

There is a limit to the 3dB per doubling loss, and it’s at this point where the array is far enough away to appear to be more of a point source and its level begins to attenuate according to the inverse-square law at 6dB per doubling of distance. The transition between these two regions is known as the critical distance for the line array. The region closer than critical distance, and the region beyond it, is termed as the Fresnel and Fraunhofer regions, respectively, so named by Christian Heil of L-Acoustics. Unless you’re a true math dweeb, near-field region and far-field region roll off the tongue a bit easier.

The critical distance for a given line array length varies inversely with wavelength (frequency). This was also discussed in depth in the last issue. Shorter wavelengths (higher frequencies) have much farther critical distances than longer wavelengths (lower frequencies). In theory this means, 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 very-popular J-Array shape that most manufacturers currently offer, save one. To date, the Duran Audio Intellivox system is the only 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, 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 ado has been made about lobe steering. Visions come to mind of FOH 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. They are called side lobes but actually emanate from the ends of the array, at the top and bottom, as a typical line array is viewed in use. They are 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 is 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. Chicks dig it.

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 3dB 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 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.



Live Sound Technical Editor John Murray is a 26-year industry veteran working for EV, Midas, MediaMatrix and TOA. John has presented two AES papers, chaired three Syn-Aud-Con workshops and is a member of the TEF Advisory Committee and ICIA adjunct faculty. If you have a question you’d like to ask John, e-mail him at jmurray@livesoundint.com .

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