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MillimeterA look at line arrays
Mar 1, 2000 12:00 PM, Glenn Leembruggen
The directional behavior of line arrays is due to three main factors. The first is the relative differences in the path length from a point in space to each drive unit. The second factor is the amplitude and phase shift of the signal applied to one drive unit compared to the others. The third is the directivity of the individual drive units.
In a line array, the sound on the main axis of radiation results from constructive interference between all the drive units, while the off-axis behavior is the result of partially constructive or destructive behavior. On the main axis of the array, SPL increases by 6 dB for each time the number of drive units in the array doubles. This is because the path length from each drive unit to the listener is essentially the same, causing equal phase shift to each driver and allowing constructive addition of the sound pressures of all drive units.
Off axis, the path length to each drive unit is different, causing phase differences between each drive unit's acoustic signal and resulting in partial cancellation or destructive interference. The longer the array, the more off-axis destructive interference occurs and the more directional it is at lower frequencies.
The main problem with line arrays is that they exhibit strong lobing effects at off-axis angles when the array becomes acoustically long. This is due to the path length to some or all drivers becoming close to multiples of 360 degree (one or more full cycles) and occurs at those frequencies at which the separation of the drive units is more than one quarter of a wavelength. Constructive addition then results at this off-axis location and the off-axis attenuation at lower frequencies suddenly turns into a boost at higher frequencies.
Lobing can be prevented using a combination of three techniques. The first is to make the array's acoustical length constant with frequency by acoustically shortening the array as the frequency rises. Acoustical shortening is achieved by using low-pass filters to roll off the signal fed to the outermost drivers, followed by filters progressively rolling off all but the innermost drivers. This type of array is called an electrically tapered array. The second is to make the spacing between drivers operating at these frequencies less than 0.25 wavelengths. The third is to adjust the relative broadband level of each drive unit.
Another advantage of electrically tapered arrays is their ability to generate much higher sound pressure levels (SPLs) at lower frequencies, which requires high piston area and high drive-unit excursion. In addition, sound systems are also usually required to deliver much more SPL at lower frequencies than at higher frequencies. In electrically tapered arrays, all units share the output at low frequencies, resulting in the effective piston area being increased. As the frequency rises, the drive units are progressively rolled off leaving the inner drive unit to do most of the work, but as less SPL is required at these frequencies, this is not a problem.
