User manual ELECTRO-VOICE DH1012A

[. . . ] this design is roughly six inches, relating to a wavelength of 2. 26kHz. The line array behaved very well up to that 2 kHz range. It should also be noted in the Figure 3 that a high frequency horn was employed above that frequency limit in order to achieve appropriate extended bandwidth and fidelity up to and beyond 10 kHz. This is a classic embodiment of a limited bandwidth line array and as we shall see in this presentation, only recently have solutions been brought to the state of the art to enable line array technology to truly be full bandwidth and extend beyond the 10-15 kHz region. [. . . ] The product theorem simply says that a simple source array has a multiplying factor that is described by the directional nature or "Q" of each horn loaded element. Or put another way, the result of a nonsimple array equals the simple array directionality plus the individual device directionality. Figures 25 and 26 illustrate this very graphically. It, again, is the a long vertical arrangement of simple point sources each spaced 12 inches apart. The frequency is 630 Hz and, again, is relatively long compared to the device spacing (in this case, the wavelength is 2 times the device spacing). Comparison of this polar with the same array where the simple sources have been replaced with horns, each bringing their own directionality, shows the change in vertical radiation. Substantially higher Q and associated higher directivity index are the result of the combination of the directionality of the array with the simple sources and a multiplier of that directivity that is the directionality of each horn device that has replaced the simple radiating source. A simple example is shown in Figure 23, Equation 3 where we arbitrarily set vd to 4 inches per second and the area of the diaphragm is arbitrarily set to 4 square inches (these are thoroughly arbitrarily quantities simply selected to make the arithmetic very simple). We now arbitrarily set the area of the throat to 1. This is where the term compression driver comes from, as the area of the radiating diaphragm is many times greater than the area of the throat. The air displaced by the diaphragm then encounters a substantially reduced area in the throat. The air is compressed and the diaphragm is able to "do more work" against the air in the throat. The full drawing in Figure 27 is 3 Hydras vertically stacked, thereby generating 21 "point source" radiating surfaces coupled to a horizontal wave guide with an included angle varying between 90 and 120 (model dependent). Figure 28 shows a Hydra without the driver or wave-guide coupled. Each hydra has 7 output "slots". The driver is coupled to the input side of the hydra and the 7 outputs are then interfaced with a horizontal wave-guide to produce the required horizontal included angle. The space b for a hydra is . 826 inches, which equates to a wavelength of 16, 434 Hz. Again, it is always best for wavelengths to be longer than that spacing, so in this implementation, the Hydra presents excellent high frequency control in the 15kHz to 16 kHz range. The Array Show plot Figure 29 shows 21-point sources in a vertical orientation with the exact spacing provided by a hydra. = 16, 434 Hz Figure 28 Figure 27 8 Figure 29 Realized Line Arrays/Horizontal Geometry Figure 30 represents two possible methods of orienting a full bandwith line array. The two methods are axis symmetric and axis asymmetric. The most common realization is that of an axis symmetric. It is the left hand drawing on Figure 30. The high frequency section is in the horizontal center of the enclosure and is flanked by two mid drivers of 6 to 8 inch diameter and two low frequency drivers of 12 inch to 15-inch diameter (depending on individual realization). [. . . ] Because of that improvement in front to back uniformity, flying subs are highly recommended where improved full frequency coverage is required. Figure 40 Line Arrays and Very Low Frequencies Traditional practice with low frequency radiators, or subwoofers, has been to groundstack the subs. Groundstacking produces the familiar 3 db doubling of pressure, because of the conversion in the acoustic load from a 4 steradian to 2 steradian load. Figure 41, Equation 6 and Figure 42, Equation 7 show the change from full space to half space loading and the subsequent pressure doubling. The physical height requirements of a full band with line array, however, bring an important performance advantage to flying subs. [. . . ]