The VT4889 is the first loudspeakerfrom the new VT-series (VerticalTechnology) to be offered by JBL Pro.The Line-Array, as such, is nothing new;its method of functioning can be tracedback to 1930 i n r e l e v a n tl i t e r a t u r e . D e t a i l e d descriptionsof coverage patterns can be found inAcoustics (Beranek) from 1954 and in1957 from Harry Olson. At the beginningof the VerTec-Array-Guide, in anhistorical synopsis, the reader’sattention is proudly drawn to an old JBLproduct, the 4682, with four verticallyplaced 10-inch drivers and to the ClairBrothers legendary S4-System thatuses this positioning for the midrangefrequencies with four JBL-woofers. Allhistorical Line-Arrays of this type workas discrete arrays of single,

conven t i ona l , d i r ec t r ad ia to rs o ras vertical rows of horns. The problem ofside lobes and interference effects areunavoidable in this discrete arrangement,caused by overlaps ofindividual spherical wave-fronts. More on this topiccan be found in the article,’Calculation of Line-Arrays’ on page 118.

Histor ical Line-Arrays vsmodern l inesourcesModern line-arrayscircumvent this problemby the dense positioning

of several high-frequency drivers withspecial waveguides for the affected(and particularly critical) frequency

Line-Array

JBL VERTEC VT4889With VerTec JBL makes its move into the Line-Array market. The first systems bearingthis name were put at the disposal of the PRODUCTION PARTNER team for evaluation ofsound and measurements.

Left: Conventional System: 4 sources with spherical coverage pattern.Right: VTV4889, 6 sources that emit a coherent wavefront via JBL’sWaveguides.

range above 1 kHz, in an attempt tocreate an even wave-front. Only if this prerequisite is fulfilled, canthese elements be positioned aboveeach other to create a genuine linesource. A continual line-array like this,otherwise found only in ribbon tweeters,radiates a wave-front in the near field ina segment form according to thehorizontal opening angle of a cylindricalsurface.For deeper frequencies, with accordinglylarger wavelengths, this effect is easilyachieved with a simple line-up of directradiating systems, but the distancebetween loudspeakers must be smallerthan the wavelength. With frequencies upto 1 kHz (wavelength 34 cm) a row ofconventional 8“-drivers fulfill this purpose.The low-frequency ranges are lesscritical, so that the high frequency rangestands as the central point of a line-array.The moment the prerequisites for the

emission of cylindrical wave-fronts arefulfilled, the wave-front spreadshorizontally with 0-degree verticalaperture angle in the near field. Thecylindrical wave-front only transformsinto a spherical wave toward the end ofthe near field. The aperture angle in thefar field depends on the length of theline source in relation to wavelength andon the transition distance from near tofar field. More on the relatively simplecalculation possibilities can be found inthe article „Calculation of line-arrays.“on p.118 The following formula servesas a simple guideline to calculating theexpansion of the near field:

In an array measuring 4 metres in length,the transition for 10 kHz lies at 230metres, for 1kHz at around 23 metres, for100 Hz at 2.3 metres. Therefore highfrequencies can be transmittedespecially well over large distances.Energy-wasting in te r fe rence e f fec tso r uncontrolled coverage patterns areonly minor, thanks to the coherent wave-front of the line source. Because thewave-front expands only on one plane inthe near field the total sound poweremitted spreads less quickly than aspherical wave-front, meaning that thesound level reduces by 3dB per distancedoubled compared to 6 dB. This effect isdesirable because of the increase in airabsorption at higher frequencies.

CurvingOnly in rare cases would an array beused exclusively for long distances with anarrow vertical coverage angle. That’swhy all systems of this type are puttogether from separate elements thatallow a slight splay in relation to the nextloudspeaker. The curving of a line-arraymeans that even on the vertical plane adefined aperture angle is attained, that

can be adjusted to the limits of the area tobe covered.Because of the trapezoid casing and aslight vertical aperture in the VerTecswave guide, the curving is basicallyalready integrated into the loudspeaker.This means that the otherwiseunavoidable gaps in the line sourcecaused by the curving that occurs with apurely cylindrical wave-front statementare skilfully circumvented. The VerTecallows for a maximum angle of 10degrees from one loudspeaker to theother, that can be adjusted easily with thehelp of the cabinets rear hinge bars withdegree markings. The correspondingfigure shows an array consisting of 12VerTec units that are curved at a baffleangle of 60 degrees in the verticalplane. All in all an array like this willhave an aperture angle of 90 x 60degrees. This raises the question ofhow the array compares to a classicalconfiguration. A configuration of 2 x 3tops each with a coverage angle of 35 x35 degrees would theoretically cover acomparable area. For deep frequenciesa cluster like this would acousticallycouple, dividing the middle and highfrequencies up within the area.

Near-field with cylindrical waves front using four VerTec-elements with an overall opening baffle of 0degrees.

Near-field with cylindrical wave front using with four VerTec-Elements with an overall opening baffle of 30degrees.

l in m and f in Hz

Near-field with cylindrical waves frontusing 12 VerTec-Elements with an overallopening baffle of 60 degrees.

The Advantages of the ArrayIn this configuration the line-array has 3distinct advantages:1. Because of the different splay anglesthe array’s upper range will provide arecognizably higher density in relationto room angle, which provides a longerrange to reach audience areas,situated further back. In the lowerranges the near field energy will bedistributed over a wider angle.Comparable performance could only beachieved using narrow radiating hornswith several planes in the upper part ofthe cluster with the same directionalposition, but this would lead to well-known effects on the aiming with sidel o b e s a n d i n t e r f e r e n c e .2. Loss due to interference effects isclearly minimised by using the coherentcoupling in all frequency ranges of theindividual units. These interferenceeffects cannot be fully obviated even invery cleanly functioning horn systemsin clusters.3. Compared on the basis of power andrange the line-array can be seen tohave a clear head start on aspects likearea coverage, weight and setupspeed. In general one can thereforesay that line-arrays have aconsiderable advantage in their flexiblearrangement of vertical coveragepatterns with adjustable range. In anycase the coherent wave-front and thusthe coverage is maintained from onesource. An age-old wisdom inloudspeaker technology is that thelarger the number of loudspeakers thelarger the loss in sound quality. Thiscan be cleverly circumvented becausea line-array, despite its form alwaysfunctions as a single unit. Prerequisiteis of course the maintenance of themaximal splay angle of the loudspeakerboxes to each other. On the other handthe line-array doesn’t offer the flexibilityof horn clusters on the horizontal plane,and in certain rare situations this couldbe seen to be a disadvantage. VerTecelements are suitable for small tomedium- sized halls, in which speakerscan be suspended or stacked on stagein units of four and will provide suitable90 x 40 degree coverage, and also for

massive open-air events where extremelylong ranges are possible with long line-arrays. Now to the details of theVT4889.

The DriverJBL as manufacturer of loudspeakersystems and woofers of course grabsthe chance to use drivers tailor-madefor the needs of the new VT4889. A 1.5”driver (type 2435) with neodymiumiummotor, 3” beryllium dome and theaddition of a new patented phasing plugis used for the high-frequency range.The driver’s motor is cooled with Ferrofluid in the air gap that conducts theexcess heat to the metal parts quicklyand with little thermal resistance. Thealuminum wave-guide serves well asheat conductor for the comparativelysmall driver. The 8” mid-frequencydrivers (Type 2250H) are also fitted withNeodymiumium magnets, with a motorwith dual voice coil fitted in its air gapand a 300-Watt power rating.At JBL one modestly asserts that this isthe most powerful direct radiator mid-range loudspeaker in existence. The 8”driver works on a minute amount ofvolume that is completed in theassembly with a cover plate on thereverse side of the driver. For improvedheat conduction the larger heat sinkstands out at the back of the internalcasing to avoid heat build up. With the2255H we have a new 15” low-f r e q u e n c y d r i v e r a l s o w i t hneodymiumium motor arriving on thewoofer-scene. Because of its very highperformance quality this should makesubwoofers obsolete for manyoccasions. Both 15”ers in one VerTecunit work on relatively small volumewith a port tuning of around 45 Hz thatshould allow for real full-range usageeven for music playback.JBL really demonstrates its strengthsas loudspeaker and drivermanufacturer with its three new drivermodels creating tailor-made high-techdrivers for each different application.Last but not least the very agreeable

weight of 72kg can be partiallyattributed to these three new drivermodels with their effective neodymiummotors. Nevertheless, the VT4889contains all of nine motors!

Cyl indrical Wave-front Forming with Wave Former and RBIAt JBL the VerTec project teamdeveloped the “High frequency WaveFormer” and the “radiation boundaryintegrator” (RBI) for the difficult task ofwave- guiding to a cylindrical wave-frontat high frequencies with shortwavelengths. RBI is being registered asa pa ten t and rep resen ts a newdeve lopmen t i n l oudspeake rconstruction. The Waveguide takesover the difficult task of creating acoherent wave-front for the high-frequency ranges. Each of the 1.5”drivers has its own wave former that,arranged one on top of the other createthe line source for the high-frequencyrange. Due to its lightly rounded shapethe wave-front emitting from the 1,5”driver is formed in such a way as toproduce an even wave emitting from thefront gap.The “ radiation boundary integrator”takes over the task of bringing togetherthe sound of all four 8” mid-rangewoofers and the sound from the waveformer into the desired waveform.Viewed from the front, the exit gap ofthe wave former sits right in the middleof the RBI. The 8” mid-range system’sexit gaps are positioned on the sides ofthe RBI. The mid-range systems coverplate improves the transition for thehigh-frequency range, which meansthere are no interruptions caused by thediaphragma. On the other handcoverage of the entire range from200Hz to 1kHz via a small gap withh igh compress ion cou ld beproblematic.What’s interesting are the differentproblem-solving attempts of thedifferent manufacturers: at L-Acousticsthe mid-range membranes are integrated

Fig. 1: Frequency response JBL VT4889 2x15” Low measured at a distanceof four metres. The sensitivity of around 95 dB between 50 and 100 Hz is theequivalent of a typical 2 x 15” system. The lower transition frequency is niceand low considering the enclosure size.

Fig. 2: Phase response JBL VT4889 2x15” Low.

Fig. 3: Impedance curve JBL VT4889 2x15” Low; Tuning frequency ataround 45 Hz.

Fig. 4: Waterfall plot JBL VT4889 2x15” Low. First resonances occur at 260Hz and 340 Hz.

Fig. 5: Frequency response JBL VT4889 4 x 8” Mid measured at a distanceof 4 metres. The sensitivity values of around 102 dB are relatively high for adirect radiator.

Fig. 6: Phase response JBL VT4889 4 x 8” Mid.

Fig. 7: Impedance curve JBL VT4889 4 x 8” Mid. The resonance frequencyat around 300 Hz is very high due to the small volume.

Fig. 8: Waterfall plot JBL VT4889 4 x 8” Mid with some resonances in theworking range.

Fig. 9: Frequency response JBL VT4889 3 x 1.5” High measured at adistance of 4 metres: Very high sensitivity with a later roll off to the higherfrequencies. Hardly any brake up modes at high frequencies.

Fig. 10:Phase response JBL VT4889 3 x 1.5” High.

Fig. 11:Impedance curve JBL VT4889 3 x 1.5” High. Fig. 12: Waterfall plot JBL VT4889 3 x 1.5” High.

without further ado into the high-frequency horn surface, in contrast,Electro-Voice inserts cone drivers withflat diaphragma in their X line.

Enclosure ConceptThe VerTec element’s trapezoid casingis made of a waterproof, lightweightcomposite material that contains allnine drivers. The woofers’ front exits areprotected from mechanical influencesand rain with strong grilles and backedwith foam rubber. The mid-ranger’snarrow gaps are covered by a thickerlayer of foam rubber and the wave-guide’s sound exits get enoughp r o t e c t i o n f r o m a f i n e m e t a lg r i l l e . A t o ta l o f 8 i n teg ra tedhand les and a t r anspo r t -do l l ythat serves s imul taneously as thef ron t -cover tu rn the handling of thismere 72 kg loudspeaker box into purepleasure. The lightness of thisloudspeaker is even more surprisingwhen one considers that the casing isfitted with a permanent rigging frame,which only has to be secured with hingebars and pins. Considering the highpower-density, dimensions measuring489 mm x 121 mm x 546 mm (H x W xD) and the weight of only 72 kg we haveto be talking about a real sensation. JBLagain sets the standard, when it comesto the construction of modern,lightweight loudspeakers, whilsteven now other manufacturers offerloudspeakers that plague their ownerswith well over double the weight of aVerTec unit.

Suspension SystemThe VerTec’s suspension system isbased on the side frames on each boxthat allow for the linking of as many as18 units to each other with a safetyfactor of 7:1. The entire weight iscarried by the steel frames, withoutstressing the speaker enclosures in anyway. As already mentioned, the splayangle can be easily adjusted via the

hinge bars inserted in the frames.Assembly of the uppermost box on therigging frame also takes place viathese hinge bars and enables you toset an angle at this early stage too.Rigging frames for the VerTec areoffered in two variants:One for compact arrays, and the bigversion for up to 18 units. Both framescan be reversed for use on the groundfor ground stacks with 4, or 6 units.Another assembly-choice is a largearray with smaller sub arrays. Here asmaller frame is connected to the lowerend of a large array. All in all therigging system appears flexible andeasy to use.The assembly of an array can becompleted on the ground and then step-by-step the locking pins can be pushedinto the back hinge-bars whilst hoistingproceeds, to attain the desired splayangle. In more confined conditions theboxes can of course be mounted one byone without having to assemble thearray in its entire length on the ground. In the US VerTec units have been field-tested in use for the past 6 months. Theunits have proved their excellence andthe construction principle behind therigging system remains confirmed.

AmpingThe system put to our disposal fortesting by Audio Pro Heilbronn wasfitted out with Crown-Amps MA5002VZand a completely new controller dbx480.The entire cabling was done withNeutrik-Speakon-Connections NL8.With altogether four MA5002 as manyas four VerTec units and two additional2 x 18” subwoofers can be driven. As analternative controller, JBL suggests theDP226 from XTA and the OmnidriveCompact 366 from BSS in its user-guide. The parameters for all threemodels are found with the respectivesystems.

Individual Measurements

Prior to our measurements of the entiresystem the three ways of a single VerTecunit were measured directly. Consideringits small casing size the 2 x 15” wooferunit (Fig.1-4) supplies a good sensitivityof 95 dB at 2V/1m in the 50 –100Hzrange, which increases to 99dB at200Hz. The first resonance effects occurat around 260 and 40 Hz.Fittingly the 4 x8” mid-range unit (fig.5-8) starts at 200Hz and has a sensitivity of 102 dB in itsrange to around 1 kHz. Here the highresonance frequency of 300 Hz isnoticeable, caused by the small volumebehind the driver. At 700 Hz and 1,1 kHzthere are two, somewhat smaller,resonances noticeable within thetransmittable range. When consideringthe sensitivity of the low and mid-rangesections in the VerTec one mustn’t forgetthat one is dealing with direct radiatorsnot with horn systems, where, with themid-range, one can at best talk about akind of wave-guide. The high-frequencyunit with its 1.5” drivers (fig.9-12) suppliessensitivity values from 110 dB at 1 kHz,and up to 116 dB at 2 kHz, that canalmost be maintained up to 8 kHz. Overand above this, the course is broken,although the first break-up modes occurat around 14 kHz. In the important high-frequency range between 4 and 8 kHzthe new JBL drivers succeed in beingalmost 10 dB louder than comparableolder models.Figures 14 to 17 show how a VT4889 unitperforms as a whole. The accompanyingcontroller-set up in fig.13 performs asmall system EQ and a strong increase inhighs in order to balance the frequencyresponse, in addition to its actual X-Over function.Noticeable is the parametric EQ at 300Hz in the mid-range, which brings thelevel up +4 dB at a point that is alreadyresonating. The resulting frequencyresponse is shown in fig.14 gives an all-encompass ing and ba lancedimpression, although these tests on oneindividual system are not yet reliabletestimony.The phase response is shown in fig.16as minimal phase response for a threeway system with passages at 4th order.

Fig. 13: Controller settings Set-up No. 23 with an unusual boost of +4 dB inthe mid-range – exactly on a resonance point.

Fig. 14: Frequency response of single system JBL VT4889 at 4 metresdistance with Set-up No. 23.

Fig. 15: Waterfall plot of single system JBL VT4889, with a strong resonanceat 300 Hz.

Fig. 16: Phase response of single system JBL VT4889.

Fig. 17: Group delay of a single system JBL VT4889 with a strong increaseto the lower frequencies, caused unavoidably by the high pass filter 8. Order(acoustically and electrically 4 orders each).

Fig. 18: Horizontal isobars shown at +/-90 degrees, measured on a singlesystem. The transition is very smooth considering that this is a system thatwas primarily optimised for perfect coupling on the vertical plane.

Fig. 19: Vertical isobars shown at +/-90 degrees, single system. The cylinderwave coverage causes the impression of continual constriction whenmeasured on an orbit around the loudspeaker. This is technically correct, butthe actual wave dissipation isn’t rendered. (Please read the explanations tothis phenomenon included in the accompanying article.) The line sourceworks right up to 16 kHz.

Fig. 20: Maximum SPL (at 3-%- and 10-%-threshold for the THD), measuredon a single system. Measured at a distance of 4 metres, converted to 1metre.

Fig. 21: Maximal SPL, this time measured on a 4-unit line-array. Fig. 22: Rapid drop in level beyond the cylinder wave: 4-unit line-arraymeasured on a sound reflective floor at 4 metres distance. Measurementheight: Upper red curve 1,75 metres (within the range of the cylinder wave);other curves were each 25 cm higher, beyond the cylinder wave.

8” mid range driver (300W) withNeodymium magnet and heat sink15” woofer 2255H with Neodymium motor

Because of the quite drastic high passfiltering of altogether 8th orders a steepascent of up to 30 ms at 40 Hz for thegroup delay to low frequencies occurs.This compromise is unavoidable inorder to protect the woofer fromexcessive over-steering.

Array MeasurementsAfter individual measurements a line-array consisting of 4 elements was setup on the reflective floor of a soundproofed half room. With uprightassemb ly a r i gg ing f rame wasdispensed with, in order to attain a goodground coupling. Acoustically the array mirrors itself on the almost perfectlyreflecting granite flooring, so thatvirtually an array of 8 units with 4 metresin length is produced. The measurementdistance was also 4 metres so that forfrequencies down to 170 Hz it shouldhave been possible to prove acylindrical wave-front character. The upper redcurve in fig. 22 shows the averagedcurve of tests in 25 cm steps from thefloor to a height of 1,75 metres at adistance of 4 metres from the array.The curve is determined by a strongbass boost because of the acousticcoupling of the loudspeakers at lowfrequencies, so that the level rises thereby about 12 dB. Above 300 Hz the linearfrequency response remain constant,where all units appear to emit acoherent wave-front. The other curves

Fig. 23: Level drop of only 3 dB per distance doubled! 4-unit line-array onsound reflecting flooring measured at 1,2 and 4 metres, each averaged over9 points at a height from 0-2 metres.

Fig. 24: Line-array in a hall, averaged over 30 points from 5-50 metresdistance in the middle and outer areas of the audience area; gr =measurement without EQ; rt = measurement with EQ.

Fig. 25: 6-unit line-array in a hall, each averaged over 15 points from 5-25metres (rt) and from 25-50 metres (bl) distance. Example of a 5,4 metreslong array from a cylinder wave coverage in the near-field to a sphericalwave front in the far-field.

Open mid-range next to the WaveForm gap. Left shows thereverse of the 8” cover plate with affixed wave-guide and exitgaps for 8” speakers.

were also measured at 4 metres fromthe array. The curve is determined by a strongbass boost because of the acousticcoupling of the loudspeakers at lowfrequencies, so that tyhe level risesthere by about 12 db. Above 300 Hz thelinear frequency response remainconstant, where all units appear to emita coherent wave-front. The othercurves were measured at 4 metresdistance with increasing height over theexpansion of the array. For the entirefrequency range above 100 Hz the levelbegins to drop off severely, even a merecouple of centimetres outside thecylindrical wave-front. A second set isshown in fig. 23 where measurementsare taken at a distance of one, two andfour metres from the array at respectively9 points from the centre to the edge.As is easy to discern, the level drops amere 3 dB with the doubling of distance,as opposed to 6 dB, which can beconsidered further proof that the line-array can be seen as a cylinder wave-front radiator in its function.

Dist inct ive Character ist icsof Direct iv i tyBecause of the special line-array andcy l i nde r -wave cha rac te r i s t i cs adirectivity measurement has need of afew reservations and explanations. Ameasurement of polar or isobar curveson a turntable has as precondition thatthe wave form is spherical. With line-

arrays this, at least in the vertical plane,is not given. More information on thiscan be found in the article “Line-arrayCalculations” on page 120.However, the horizontal measurementsof a single unit, for the VerTec, asshown in fig.18, shows a completelyaccurate picture of all variants that areconstituents of this unit. The VerTec isspecified by JBL with a nominal 90degrees for the horizontal plane, which,according to fig.18 can generally befulfilled. Certain irregularities likeexpansion at 500 Hz and constriction at2 kHz have to be seen under the aspectthat this is no optimised horn but rathera wave guide that was developedprimarily for the creation of a line sourceon the vertical plane. All in allconsidering the difficult preconditionsacross the entire frequency range a verybalanced pattern is achieved that,thanks to the external arrangement ofboth woofers, stretches right up into the125 Hz region. Directivity is excellenteven at high frequencies where noconstrictions occur and the full 90degrees are held.In a second measurement set a singleVerTec unit was tested on the turntableon the vertical plane, with the abovenamed reservations taken intoconsideration. The measurement at acircle of four metres distance from theloudspeaker should have the effect thatthe frequency range with cylinder wavecharacteristics should reduce thedirectivity to an extremely narrow range.This effect can only be observed in asingle element, and that, as expected,only at high frequencies, as seen in fig.19. At lower frequencies the coveragepattern steadily transforms into aspherical form with reduced directivity fordeeper frequencies. Despite this themeasurement (fig. 19) serves to assessthe wave-front coherence at highfrequencies. If the wave-front broke up,disrupting the principle of line source, thiscould be seen as a deviation from theneedlepoint-shaped path in the isobar-diagram. With VerTec this effect firstkicks in at about 16 kHz; proof of the goodfunction of the wave-guide. Because ofthe size and weight of the VerTec itwasn’t possible to test several systemstogether on the turntable, as was donewith the d-VDOSC.

Determinationof maximal SPLThe customary procedure of a Maximal-SPL-Test is to choose the measurementdistance according to the size of theloudspeaker-with large loudspeakers aminimum of 4 metres, where the soundpressure levels are then converted intoa distance of one metre. The conversionfactor here is +12dB (factor 4). Thisformula has no relevance for a cylindricalwave emission; in this case +-3dB perdistance doubled or halved would apply.As yet this poses no great mathematicalproblem. The problem starts when inmeasurements for lower frequencies thewave emission is spherical (6 dB apply)and for the mid- and high frequencyranges cylindrical (3 dB).The transitionbetween these areas is floating. Themeasurements in fig.21 took place on aquadruple (4) array in free fieldconditions at a distance of four metreson sound-reflective flooring. (acousticalmirroring of the array). The transitionfrequencies for cylindrical waveemission in this configuration is ataround 170 Hz. For a single unit underthe same conditions, as in fig.20, thetransition frequency stands at around2.7 kHz.As can be seen from these twomeasurement sets, the systems couplewell in the low and mid-ranges, with themaximum level rising to 9 dB inaverage. For lower frequencies thiseffect can be attributed to the acousticcoupling of the woofer, whereas for themid-range section, the cylindrical waveemission is primarily responsible for therise in sound level. In the working rangeof the high-frequency driver above 1kHz the curves for one or four stackedunits are pretty similar, becausestacking only makes the cylinder wave-front higher, not louder. On the contrary,in some cases the sound level evendrops off slightly with four elements,compared to a single unit, which isprobably caused by unavoidableinterference effects. Overall only oneweak-spot, affecting the curves between300 and 600 Hz, is revealed. This couldbe the result of compression effects atthe narrow gaps at the front of themidrange systems.

Listening Evaluat ion

Each of the three 1.5” drivers 2435 has it’s own Waveguide.

which are strongly dependent ondistance. This has to be accepted as acondition of acoustic principles. Onemustn’t forget that the VerTec is in itsearly stages when it comes to collectingdata and experience, and in the nearfuture new methods will be developed tooptimise coverage patterns. The usercan rest assured that with a company ofrenown like JBL, one isn’t left alone inthis area.When the system is driven to its limits,the limits are first noticeable in thelow-mids, which is also confirmed bythe measurements . Overa l l theperformance was a great success,which was reflected by the sale of 32systems each to the Crystal SoundCompany and Sirius. Additionally the hall was used toconduct a series of measurements thatare summarised in fig. 24 and 25. Figure24 shows a green averaged curveconsisting of 30 test points spread overthe entire audience area. 6-secondsweep signals were used formeasurements, to make sure the entireroom was included. In addition to thedirect sound the curve also shows thecomplete reverbant and/or diffuse fieldin the area, which is significantlyeffected by the directivity of theloudspeakers. Equalisation of a curvelike this promises to be much harderthan a direct wave-equalisation, whereone is “only” concerned with ensuring a

The hypothetical blue curve wasconsidered only as guideline for the EQ-Tuning of the VerTec in the EuropaHalle. The resulting EQ curve is shownin red. Naturally the obtained EQ-settings used here have no generalvalidity due to the fact that the room isincluded. Two further interesting curvesresulting from the same measuringmethod are shown in fig.25 where thered curve was averaged on the first halfof the test points and the blue curve onthe rear half. The sound levels aresimilar in both areas, only the low endreduces less with increased distanceand the air absorption is still slightlypresent. Without cylindrical waveemission for high frequencies the soundlevel drop would have been stronger forthe high frequencies. All in all thefrequency response is pretty balancedeven without EQ, so that a dream-curvecan be attained with minor adjustments.At this point a special thanks goes outto Carsten Peter from Audio Heilbronnand Hanns Hommen from the CrystalSound Company for the smoothexecution of tests and listeningevaluations.

Shortly after the arrival of the firstsystems in Heilbronn a test set-up withthe VerTec Systems was madepossible on the initiative of the CrystalSound Company. This took place in theKarlruhe Europa-Halle. An array of sixsuspended on the ceiling served ascentral cluster and could be operatedwith or without additional HLA-Subwoofers, as desired. As is usual atsuch happenings, the event turned intoan impromptu meet for the Germanaudio-scene and there was plenty todiscuss. The general opinion was thatthe rather petite-looking unit of sixVerTec systems had enormous levelreserves to offer, which effortlesslyfilled the entire depth of the hall ofaround 60 metres. Special praise wasgiven to the almost unbelievable bass-level, which caused one to involuntarilywonder whether or not the subwoofershad been turned on. With the factorytuning the very restrained highs werenoticeable; they had to be boosted withan additionally looped-in EQ in order toachieve a balanced sound from theFOH point at a distance of around 50metres. Getting closer to the array onthe middle axis from the FOH point, onecouldn’t avoid noticing light tonalvariances. However, these had alreadybeen predicted by the line-arraycalculator. The variances probablyresulted in the transitional frequenciesfrom the array near field into far field,

Input panel

Rigging-frame.

ConclusionJBL steps into the suddenly fiercelycompetitive market of line-arrays. Justas with the last large scale soundsystem from JBL, the HLA, all high-techguns have been drawn: In order tocreate the VT4889 Element the mostadvanced drivers have been used, aninnovative enclosure has beendeveloped that is light and solid,together with a special wave former andthe RBI Technology. Despite its modestsize the box is unreservedly suitable asfull-range system and delivers a cleanhorizontal directivity performance.Clustered vertically the principle of acoherent wave-front to create cylinderwaves works very well - right up to 16kHz. Overall it is a technically perfectproduct as one has come to expect frommanufacturers like JBL. It is not just theaudio quality of the VerTec, but ratherthe combination of this with theextremely light and easy to handleenclosures and the ingenioussuspension system, which should bedecisive on the world market for thisproduct. In summary, the VerTec line-array has, in relation to its size andweight, a very high power density, isflexible and easy to use and despite therather high costs makes good economicsense for the user.

¹ Text and MeasurementsAnselm Goertz

Photos:A.Goertz, JBL, Archive

© Reprinted with permission from PRODUCTION PARTNER magazine.

www.production-partner.de

Calculat ion of l ine-arraysIn its classical form a line-array is madeup of a series of individual radiators.Each of these sources radiates a wavewith a spherical front that then overlapsin the sound-field. It should bedifferentiated between the near-fieldand the far-field of such anarrangement, as its transition at adistance depends on the relationshipbetween the wavelength and the lengthof the radiation arrangement. An easyformula calculates this distance as:

Length 1 stands here for the overalllength of n single elements that have acertain distance d from each other. Forthe far-field the following formula resultsfor the directivity of such a line-array:

The two illustrations show the directivitybehaviour of the radiation arrangement.For the individual radiators a simplifiedspherical characteristic is assumed. Thefour curves represent the averageddirectivity behaviour for the 1 kHz and 2kHz octave and the three accompanyingthirds. A two metre long row consistingof ten sources at a distance of 20cmfrom each other was used for thecalculation. For the 2 kHz octave the directivityincreases as expected and the mainmaximum narrows compared to the 1kHz octave. Interesting about theseillustrations is the appearance of strongside lobes in the 2 kHz octave, where

there was nothing to be seen in the 1kHz illustration. This is due to thetransgression of the critical frequency,whose wave length equals the distancebetween two of the line-arrays sources.For the chosen distance of 20cm thecritical frequency lies at 1700 Hz. Abovethis frequency a line-array starts tocreate distinct side lobes from discretesources and is therefore unsuitable forcontrolled coverage. With furtherincreasing frequency the number of sidelobes increase and they move in towardthe main maximum. This effect occursas a matter of course with all line-arrayswith discrete sources even when theseconsist of high Q horns. The directivitycalculated based on spherical radiatorsoverlaps the directivity of a singlesystem.If a line-array is assembled out ofnormal horn systems, as it is presentlyoften done, then the narrow coveragebehaviour in the main maximum on anaxis is obtained, but the distinct sidelobes are only repressed within thecapabilities of a single horn, whichusually leads to compromising results.Modern line-arrays like the V-DOSC,VerTec or X-line differentiatethemselves from the previouslydiscussed discrete line-array throughtheir “genuine” cylinder wavecharacteristic, that can be producedover the entire frequency range withonly one line source, which is notpossible with discrete radiators.A line source means that a continualinfinite length and evenly radiatingsource is produced. These line sourcesradiate a cylinder shaped wave-front,that, due to the infinite expansion of theline irrespective of the distance,maintain their expansion shape.In reality there are some restrictions andreservations to be made. Because of thefinite length of a real line source thewave expansion area has to be dividedinto near- and far-field. The formula forthe transition equals that of the discretearray. However there are differences in

the formula for the directivity function ofthe line-arrays far-field, that acts as realline source of the length 1. The followingpolar diagrams show the resultingpolars, again for the 1 kHz and the 2kHz octave. The shape of the main maximum on themiddle axis of 0 degrees is identical to adiscrete array. The difference here isthat side lobes don’t occur.

In summary it can be said that a discreteline-array consisting of individualsources below the critical frequenciesbehave similarly to a genuine linesource.Significant differences occur only abovethe critical frequency, where theundesirable side lobes occur. The art ofcreating a line-array suitable for theentire audio-range lies in creating acoherent wave-front not only for low andmid-ranges, but for the high frequencyranges too. For the low frequencies thisis no problem with the alignment of thedrivers in a straight line. As our exampleshowed, for frequencies up to around1500 Hz, normal 8” drivers in a closearrangement are perfectly adequate.This is exactly how it is done in all well-known line-array systems: each boxcontains one or two 8” drivers as closeto one another as possible. With across-over frequency between 1000 and1500 Hz to the high-frequency driverthis is on the safe side. For the highfrequencies it isn’t enough to merelystack the drivers above each other,because the sources would separateand cause side lobes. In order toaccomplish this coherent wave-frontwith “ normal drivers” wave guides areused, that, depending on the

l in m and f in Hz

manufacturer are variously calledWaveguide (JBL), Hydra (EV) orWave-Front Sculpture Technology (L-acoustics). When evaluating the coverage patternsof the far-field the wave-front spreads ina spherical shape, even for a lineradiator. For a 5.4 metre long lineradiator this means r>43 metres at 1kHz, and accordingly at 100 Hz r>4.3metres. In the near field however, thewave-front takes a similar form to acylindrical wave with the useful attributethat the sound pressure level reducesby 3 dB when distance is doubledrather than 6 dB. For the calculation ofthe opening angle of the mainmaximum in the far field an additionalsmall formula comes into play that the 6dB opening angle of the mainmaximums BW-6dB can be calculatedas:

For an example with 5.4 metres inlength that would be about 4.4 degreesat 1 kHz.So much as to the theory of linesources, that in part were alreadyexplained in 1930 in a publication of theJASA. The large ribbon-tweeters likethe Z-line from Sonus (see the 3/2000issue of our sister publication for mediatechnology PROFESSIONAL SYSTEM)fulfil the requirements of a line sourcewith coherent wave-front over the entireaudio-frequency range in near-toperfection.In order to make a light curving of theline-array possible, to cover a largerarea the line-arrays consisting of smallunits have to be capable of graduationwithout the wave-front breaking open.Trapezoid casing and a small verticalopening gap in the individual unitsupports this application.A.G.

Directivity behaviour of a line-array from discrete spherical radiators below (first figure) and above (second figure) thecritical frequency, the wavelength which corresponds to the distance between two sources.

For comparison: Directivity behaviour in a continual line-array with coherent wave-front, for the two frequency rangesshown above, for discrete line-arrays.

Line-array CalculatorThe manufacturers of line-arrays arepresently in a difficult situation in thatthe simulation programmes like EASEor Ulysses that are on the market aren’tsuitable for their new loudspeakers.Despite the abundance of loudspeakerlibraries and the programmes wide usethese components cannot be found.This is connected, not with theunwillingness of the manufacturers, butwith the characteristics of the line-array,which is incompatible with the functionsof the simulation programme. Basicallythe simulation programmes assume aspherical wave-front from theiracoustic sources, which ideallyshows a directivity where distance isirrelevant. In normal loudspeakers thisis largely true, so that a directivityballoon measured at 4 metres will alsobe valid at 10 and 20 metres. Thisprerequisite is fulfilled if one ispositioned in the far field of a source.With “normal” loudspeakers 4 metres isadequate for all frequencies in the audiorange.A simple formula calculates thedistance for the end of the far field asforementioned as:

Length 1 is the largest expansion of theradiator in one direction, which equalsthe diameter of the direct radiatorsdiaphragm. The transition from near tofar field shouldn’t be visualised as a gapor point, but as a continual transitionfrom a near field with local minima andmaxima and a far field with a continuous1/r level decrease. For the calculation ofa balloon one should therefore be wellinto the far field and choose a testdistance greater than:

For a 4 metre long line-array this wouldmean a distance of 47 metres at 1 kHz,for 5 kHz as much as 235 metres. Onlybeyond this distance would the classicalsimulation programmes produceuseable results for a line-array. Incontrast, the near field and thetransition area are much morecomplicated.

Problematic Bal loon DataStaying with the demonstration methodof balloon-data for a line-array onewould have to test a whole series ofballoons for different distances and thesimulation program would have tocalculate the point of impact and thencalculate the distance from the source,only to then apply this data to theballoon suitable for that distance. All inall a very complicated process, madeeven more so by the fact that theballoon-data would have to bedetermined for all the line-arraysvarious configurations, so that this kindof calculation for freely configurableline-arrays like V-DOSC, VerTec or X-Line doesn’t make much sense.An alternative method is the discretionof the array into such small elementsthat in a complex calculation like this,they replicate the line-array functionsexactly.The elements mustn’t be added asclusters, but instead, have to be addedtogether minutely, source by source. Toattain correct results for the upper 8-kHz-octave, the discretion has to takeplace in intervals smaller than thewavelength of the highest frequency tobe tested. For the 8 kHz octave thiswould mean around 3 cm. Accordingly a4 metres line-array would consist of at

least 133 single elements. Calculationwork increases considerably, but thismethod allows any configuration orcurve angle to be calculated on thebasis of a single set of data. All that beneeded would be a convenient way ofentering the line-arrays data into thesimulation program. The testing of thiskind of calculation of a line-array in asimulation program has yet to be done;the newest test results will be publishedat a later date.

Special Array CalculatorTo supply the user with an immediatelyapplicable calculation tool theloudspeaker manufacturers offer theirown programs to calculate the puredirect coverage patterns of a line-array.For the VerTec there’s a line-arraycalculator that works on excel. Itrequires Excel 95 or later with activatedVBA Add-in.Up to three audience areas and a line-array with up to 18 VerTec units can beentered in an intuitive fashion on anuser-friendly screen. The calculationoccurs solely on a vertical plane in aroom and shows the polar diagram ofthe complete array and the soundpressure level in one third octaveintervals. Even on slower PC’s thecurve appears almost without any delayas soon as the frequency is chosen, sothat an overview is immediatelypossible. Calculations are only takenfrom the far field perspective; thereforenothing can be said on the topic ofshort distances.A lso, the sound leve l resu l ts areon l y re la t i ve t o each i nd i v i dua lr e p r e s e n t a t i o n a n d s o n ocompar i son can be made o fabso lu te sound l eve l s i n t heaudience area.Comparison of the results of line-arraycalculators with their measurements

l in m and f in Hz

suggests that the Excel routine works with amodel of n-elements where each element has thevertical directivity of a single VerTec element andis included in the calculation with an angle-offsetthe size of the set-up angle. This method servesas a good compromise for a quick overview,without great emphasis on accuracy.Parallel to the acoustical data the calculator givesother useful information about weight anddimensions of the array and its set up angle.Overall the LAC (Line-Array-Calculator) is a usefultool on location and in the planning stages of aninstallation. Near field effects and the actualcylinder wave expansion are not taken intoaccount here. Fo r exac t ca l cu la t i ons , w i t hcons ide ra t i on o f t he acous t i cenv i ronment , implementat ion in o f the well-known simulation programs should be done assoon as possible. The trend is growing towardline-arrays and now almost all loudspeakermanufacturers are jumping on this F renchband -wagon . So f twa re manufacturers arewell aware of this development and everywherework is getting underway and speedy updates arepromised. Nothing, however, has been said aboutthe way in which the calculation of line-arrays willbe implemented. A.G.

Horizontal Coverage of Line-ArraysLine-arrays from the big manufacturers on themarket are at the moment all specified withhorizontal opening-angles between 70 and120 degrees. This scale is practical for manyapplications, especially with the combinationof different systems, for example 90 degreemodules at the top and 120 degree units at thebottom of the array, in order to attain a certainadaptability. Despite this, there will always besituations where larger angles are needed.For a central cluster above the stage of a wideaudience area 180 degrees would besomet imes des i rab le . Fo r t he oppos i t eext reme, coverage of a long, narrow area,a subs tan t ia l l y nar rower hor izon ta l ang lew i th i nc reased sens i t i v i t y on ax i s i sdes i rab le .

The LAC screen in Excel with settings for a 6-unit array for listening evaluation in theKarlsruhe Europa Halle. Near-field effects and the actual cylinder-wave expansion are nottaken into account by LAC.

Three of these drivers with Waveguides on top of each other createa coherent wave-front for the line-source.

Gene ra l l y ques t i ons a r i se asto va r i a t i on -poss ib i l i t i es andthe poss ib i l i t y o f assemb l i ngthe l ine-a r rays hor izon ta l l y inc lusters .Under this aspect we find aclassical construction in the line-arrays of V-DOSC or VerTec,with external positioned direct-radiating woofers and coaxial midto high range arrangements. Thehorn function in a horizontalplane is realized with simplelinear planes, that also serve assound wall for the mid-rangedriver. With Electro-Voice we find

a different arrangement in the X-Line. The high-frequency driveris built in separately and thewoofer and mid range driver arearranged coaxial. The woofersare designed here as flat-membrane loudspeakers to keepthe horn area as linear aspossible. In the VerTec the mid-range drivers are covered withslit covers for the same reason.These s imp le ho rn f unc t i onsor wave-guides are necessarydue to the necessary vertically closedarrangement of the paths in a l ine-ar ray.

A clean and narrow coverage patternof, for example, 40 degrees over a widefrequency range is difficult to attainwith this kind of horn. Especially in thelow- mid range strong expansions ofthe coverage angle would be caused,comparable to the behavior of anarrowly radiating horn with a directradiator. By adding the loudspeakers toa tight cluster, parts of the area wouldbe covered by single horns for the highfrequencies. For the lower frequenciesa smooth transition to an alloverradiator would be attained by theacoustic coupling of the close proximityof the drivers or horn openings. Fortypical line-array systems this is hardlypossible because the arrangement ofthe individual paths doesn’t allow for aclean acoustic coupling. For twoVerTec lines rigged next to each otherthe centrally arranged mid-high unitsthat cover the frequency area fromaround 2000 Hz up, would lie so farapart that interference would occur,even if the line-array would be perfectlystraight aligned. An alternative to thesymmetrical set-up of a VerTec or V-DOSC could be the solution offered byElectro-Voice with the X-Line, wherethe high frequency horn positioned onthe side next to the coaxial low midarrangement allows for a symmetricalmirror-like arrangement of two linesrigged next to each other. More to thisprobably in our 6/2001 issue afterdetailed measurements of the at thispoint still slightly modified X-Line.In the JBL and L-Acoustics handbooks itis advised not to cluster several lines,but rather to rig them spaced at a largerdistance around the stage area.A.G.

JBL Professional8500 Balboa Blvd., P.O. Box 2200Northridge, CA 91329 U.S.A.

A Harman International Company

Part # PPVERTWEC 5/20/01