Getting into line-array speakers is a nice additional tool to have when deciding what gigs get what kinds of speakers. In fact, line-array speaker cabinets have been popping up in unconventional applications, such as being stacked two high for medium- to long-throw spot reinforcement in nightclubs. In this piece, I want to run through the typical line array application, determine the amplifier/speaker processor settings and take another look at when more compact arrays are used.
Example One
In this first example, let’s look at two line array splays, one on each side of the stage with 12 cabinets per side. Using a full-size line array, like the popular JBL VerTec VT4889 cabinets, a 12-cabinet array is about 20” per cabinet or 240” (20’) tall for the “h” parameter. To calculate the maximum “near field effect,” which is the cylindrical coverage advantage that line arrays have over stacked long-throw cabinets, the low-frequency wavelength (ï¬) must be ascertained for the VT4889. Based on the VT4889, the vertical beamwidth remains good down to 200 Hz or 5.735’ based on a 1147ft/sec speed of sound for a humid day.
So, our maximum near field throw equation boils down to d = h2 / 2ï¬ for line arrays. With h at 13.3’ and ï¬ at 5.735’, then d equals 4.7 meters or 15.42 feet at 200Hz. As the frequency doubles, the wavelength of interest halves; and by the equation doubles the near field (cylindrical) coverage. But to get a reasonable low-frequency corner of the flown cabinets, the ol’ four wavelength rule is a nice practice to follow. With the array height at 240”, dividing by four gives us a 60” wavelength or about a 229 Hz of low frequency cutoff point. So, even with cabinets capable of 45 Hz low end, as an array the subwoofers need an extended upper low frequency if low-frequency pattern control is desired. In reality, operation below the cutoff point is very common and typically done right down to about 80Hz with the VT4889s 15” low-frequency drivers.
The JBL VerTec 4889 cabinets are three-way designs with a pair of 15” low-frequency transducers, four 8” mid-frequency drivers, and three 1.5 throat high-frequency drivers. These drivers are all Neo-magnet designs with nominal low/mid and mid/high crossover points of 200 Hz and 1 kHz. Using JBL’s recommended DSP settings (ex BSS FDS-366 OmniDrive), the sensitivities of each driver pass-band are set to +2 dB at LF, 0 dB at MF, and -11 dB at HF. Other than a couple minor parametric EQ bump removals at the MF and HF bands, the VT4889 operates pretty smooth. Of course, there is the necessary boost filter way up at 16 kHz with 0.7 octave width for getting the most sizzle.
From the above information, it becomes reasonably obvious that big line arrays like the 12 boxes of VT4889 can provide nice long-throw capability and reasonable near-field effects in the prime vocal ranges. But what about compact line arrays? Are we giving up a lot in that direction?
Example Two
If we look at a pair of eight-box compact line arrays, like the EV XLC-127; each cabinet is 14.25”. Rounding up to 15” and going eight cabinets high gives us 120” or 10 feet or 3.05 meters. Dividing by four gives us 2.5 foot wavelength or about 459 Hz limit in pattern control. If we use this wavelength to find it near-field cutoff distance, the d = h2 / 2ï¬ formula yields 6.1 meters or 20 feet. Obviously, doubling to 918Hz provides a 40 foot near-field limit, but you can see smaller height arrays mean shorter throw cylindrical field benefits (3 dB loss per distance double from array front).
The XLC-127 is not that much more compact than the full-size VT4889, but going from 12 cabinets to eight cabinets for fitment into smaller height venues has a price to pay in physics. The EV XLC-127 has a single 12” low frequency driver, a pair of 6.5” mid-frequency drivers, and two ND6 high-frequency drivers. Using standard tunings, the crossover points are 500 Hz at low/mid and 1.6kHz for mid/high. Pass-band gains are set at +1 dB at LF, -5 dB at MF and -6 dB at HF.
When analyzing the compact line array vs. the full-size line array, the extra sensitivity and power handling at the lower frequencies is the biggest difference in capability. The XLC-127 can handle low frequencies well below 80 Hz, but placing them in the right spots into the audience is not there, plus the increased distance losses at the lower frequencies means that more subwoofer support is needed beyond 80 Hz and preferably past 200 Hz.
Wiring Line Arrays
With all un-powered arrays of speakers, getting a rack or three of power amplification requires some thought. Thankfully, the Neutrik NL8 Speakon connector and 13-8 cabling helps bring the problem to manageable levels. Figure 1 shows the process.
With the eight contacts on the NL8 connectors, four pairs of wiring are brought into each line array cabinet. In the case of the JBL VerTec VT4889 cabinets, each low-frequency driver is individually wired to either the +/-1 or +/-2 circuits. This way, the 8-ohm drivers can be daisy chained into 2-cabinets (4-ohms) or 4-cabinets (2-ohms), and bought down to the stage level amp racks. With a typical amp rack having four stereo amplifiers powering subwoofers, and LF/MF/HF of the line arrays, four to eight cabinets can be powered by one amp rack and two cable assemblies.
Of course, if you choose 2-ohm loading, the eight wires of 13-gauge copper conductors will not efficiently move very much audio power at longer distances the line array to amp runs will result. So, very high SPL shows will likely run two cabinets per NL8 cabling instead of four. Running the numbers for each cabinet of VT4889, each LF driver wants at least 1,000 watts at 8-ohms, each quad of MF drivers desires 1,400 watts minimum at 8-ohms, and the three HF drivers wired in series for 16-ohms needs 225 watts minimum.
So, if you double the cabinets by daisy chaining a short jumper of NL8/13-8 patch cabling, the +/-1 and +/-2 pairs from the amp rack are running at a minimum of 2,000 watts to 4,000 watts at 4-ohms for the LF drivers. The +/-3 pair is cooking at 2,800 watts minimum to 5,600 watts at 4-ohms, and the easy +/-4 pair is taking at least 450 watts to 900 watts into 8-ohms. Talk about a little warmth on the copper.
Contact mark at marka@fohonline.com.
