User manual ELECTRO-VOICE LINE ARRAY BROCHURE
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Manual abstract: user guide ELECTRO-VOICE LINE ARRAY BROCHURE
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[. . . ] Line Arrays
Line Arrays -- History and Theory
Mention is made of the vertical orientation of sound sources as far back as 1896. Line arrays were also popular in the 1950s and 60s because of the ability to provide excellent vocal range intelligibility in reverberant spaces. Figure 1, Figure 2 and Figure 3 are excellent representations of high performance "vocal range" line arrays. These line arrays, like all vertically oriented sources in the past were, what could best be termed, limited bandwidth line arrays. [. . . ] 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). 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. In the example here using the arbitrary parameters, the equation becomes as shown. Solving for vt generates 16 inches per second, a substantial gain over the physical velocity of the diaphragm itself. In this case we have the velocity in the throat substantially greater then the velocity of the diaphragm, and we generate an additional conversion efficiency as a result. We have now illustrated two methods of achieving directional radiation, that of orientation of simple sources or of coupling a horn to a radiating source. An important concept at this point is to introduce the product theorem. Figure 24, Equation 4
(r, ~ , ø) =
AX (r) | He ( ~ , ø) H ( ~ , ø) | He ( ~ , ø) is the expression that describes the directional characteristics of each source.
Realizing a Full Bandwith Line Array
Full bandwidth line arrays are typically three way systems. The practice of dividing the band into 3 separate passes is done to enable the cross-over points to always be substantially low enough that the radiation from each pass exhibits wavelengths that are always longer than the physical device, or driver spacing. This is relatively easy to achieve for the low frequency section of any line array and is also easy to achieve for the mid-band section. In mid-band sections the mid range devices are 6 inches in diameter to 8 inches in diameter. The crossover points are selected so that the device spacing is always small compared to the wavelength radiated. The problem for a full bandwidth line array systems is the high frequency radiation. As mentioned earlier, historical line arrays were excellent in terms of low frequency and mid-band control of the pattern, but always suffered from polar lobing errors associated with the device space "B" being greater than the wavelengths being radiated. A 16 kHz wavelength is on the order of 3/4 of an inch and as a consequence device spacing must be comparable to those wavelengths or shorter, if possible. This was always a problem in the past because engineering techniques could not realize spacing closer than the driver diameters themselves. Even with modern neodymium iron boron based magnetics, the diameters were always at least 4 inches or greater (for large format diaphragm devices). That spacing limited good performance to below approximately 3 kHz, obviously not a full bandwidth device. As a practical example, fmax, the maximum high frequency control based on the relationship between the spacing of the devices b and the wavelengths is as follows. For base line arrays where we are interested in control up to 250 hz, the spacing needs to be at least 4. 5 feet. [. . . ] 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. While it is completely true that the pressure doubling is lost when the subs are removed from the floor, there is a substantial gain associated with a large vertical array of low frequency sources. [. . . ]
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