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Articlefrom issue 03/2014

TW Audio – named after its founder and now co-managingdirector, Tobias Wüstner – is celebrating its tenth birthday in2014. Not only was the top part/subwoofer combination of T24and B30 one of the first speakers in the TW Audio range tenyears ago, it has been able to remain part of it, unchanged, to thepresent day. According to Tobias Wüstner, several thousandmodels of the T24 top part alone have been sold over the years,and the box continues to enjoy ongoing popularity. The T24 thusfortunately never shared the same fate as many other knownhorn systems, which today eke out a miserable existence behindthe line arrays (or have completely disappeared from the mar-ket). How could this be, even though TW Audio’s range naturally alsoincludes line arrays in the form of the VERA series? It may be dueto the good performance despite an extremely compact design.Weighing “just” 46 kg, the original T24 has always been a light-weight compared to the “100 kg plus” offered by some othermanufacturers. The T24 thus escaped this extinction phase andwithout any doubt: there are still many areas in which classichorn systems are equal or even superior to line arrays. When wethink about several top 40 bands, club concerts, fittings, smallerdance floors or monitoring systems on large stages, the horn topis always the first choice. A lot of work has gone into speaker chassis development inrecent years – the drivers have become lighter, more resilient,and better in general – and it is only logical to modernise andupgrade the tried-and-tested T24 design. The T24 is thereforecurrently being presented at the Prolight + Sound 2014, in itsnew, locally tested T24N version.

T24N for “new” or neodymiumExternally, an expert eye is needed to distinguish the T24N fromthe original T24, with only the slightly more rounded rear sidegiving it away. But lifting up the box reveals another significantdistinction. 33 instead of 46 kg is a crucial difference when itcomes to transporting and even setting up a top on one’s own.

TW Audio T24N and B30 The second generation of the TW Audio classic has been com-pletely reworked and equipped with state-of-the-art drivers

The weight was primarily reduced by usingneodymium drivers for the two 12" chassisand the compression driver. All drivers are made by Italian manufactu-rer B&C, and are not freely available cus-tom-made models. The tweeter is based onB&C’s 3" titanium models, and the bassreflex construction for the woofers is anot-her innovation. The large horn area nowhas four openings, which work as resona-tors, increasing the box’s frequency respon-se by over 6 dB to between 70 and 100 Hz,and even have an impact up to 200 Hz,which is particularly important for full-ran-ge operation. The two 12" chassis operate ona large horn, whose opening takes up theentire front of the box. Both drivers areequipped with a phase plug, designed forthe top chassis in the form of the tweeter. The entire tweeter unit is mounted ontothe box’s baffle on four metal slats, and canbe easily removed by undoing the fourscrews. This last step is relevant becausethe tweeter horn is rotatable and exchan-geable: there is a 60 ¥ 40 and a 90 ¥ 50-degree version. The T24N’s cabinet comprises two halves:The front section with internal front panel

contains the horn with the 12"drivers atta-ched from the rear. The back section is acase shell added for bulk. If the 12" driversneed to be serviced, the rear case section isthus simply removed to provide free accessto the drivers. The two woofers are 8-Ohm chassis, whichwork electrically parallel as a 4-Ohm unit.The box can be set for passive or active ope-ration using a solid switch at the back. Bothare connected via the NL4 bushing. From aperformance perspective, a T24N top goesperfectly with two B30 subwoofers (8-Ohmsystems), meaning a power amplifier canbe optimally utilised with two subs andone top. The active operation option has been intro-duced to TW Audio by international custo-mers, and is therefore also available.

AccessoriesThe T24N is identical to the T24 in size,meaning existing accessories can continueto be used. And there are plenty of them.Starting with the dolly – suitable for T24Nand B30 -, to protective covers, cases, smalland large flying frames, to a solid swivel

bracket with which the T24N can either bemounted on a stand or attached to a truss.Complex clusters can be built using therelevant accessories, and tops can be array-ed clustered vertically and also horizontallyat 45-degree angles. So nothing has beenspared here, and this comes as no surprise,given thousands of the boxes have proventheir worth in harsh conditions for the lastten years. In terms of power amplifiers, TWAudio recommends the Powersoft K DSPseries or the Lab.gruppen PLM series. TWAudio’s systematic concept means it sup-ports both platforms with compatible sys-tem presets. The test unit was suppliedwith a Powersoft rack with two K3-DSPamplifiers. Further details to come.

Measurement valuesWhen it comes to measurement values, weusually start with the impedance curves. Inaddition to the values for nominal impe-dance by standard, minimum and similar, italso provides information on the speakeritself, e.g. the tuning frequency of the bassreflex cabinet, driver resonances, and evenwhen the tweeter’s membrane starts to

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The T24N HF-horn can be rotated as an entireunit, or replaced by the alternative 90 ¥ 50horn; the openings on the sides of the largehorn are bass reflex ports

Frontal box links enable intermediate anglesto be set, and even larger horizontal/verticalclusters to be formed

The swivel bracket allows the T24N to bemounted on a stand or attached to a truss

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show partial vibration. Figure 1 shows threecurves relating to this for the T24N – the LFand HF branches in active mode, and theentire box in passive mode. The minimumfor the LF path and the passive box is at 3.3Ohm, right on the tuning frequency of 73Hz. It is thereby still safely within the 20-percent standard tolerance range. The tweeter is shown as an 8-Ohm system.In its sphere of operation over 250 Hz, theminimum here is a completely uncritical 8.1Ohm. The driver resonance is 473 Hz, andthe first partial vibrations only start appe-aring from13.4 kHz upward. The T24N’s frequency responses were initi-ally measured in the active version. Figure 2shows the individual measurements for theLF and HF paths. They each have two curves– for the 60 ¥ 40 and 90 ¥ 50 versions. Theangle of radiation is changed purely byreplacing the tweeter’s horn. But there isstill a small impact on the woofer, as thetwo horns have different external designs,meaning the phase plug also changesslightly for the upper woofer.The 90 ¥ 50 tweeter horn is approx. 1.5 dBlower than the 60 ¥ 40 in terms of sensitivi-ty. The correlation is also plausible, as thesame driver in the 90 ¥ 50 version distribu-tes its acoustic output over a much larger

spatial angle than the 60 ¥ 40. It must benoted that the two tweeter curves are mea-sured for 2 V / 1 m. The 1 W / 1 m sensitivityis thus 3 dB higher, sitting between 1 and 10kHz at a mean of 109 dB. This sort of figureis as expected for a compression driver witha 3" membrane. It has to be mentioned thatfor the curve for the higher frequencies allmeasurements were taken with the frontgrill. Without this grill, the curve wouldhave been noticeably smoother above 5kHz. Grills create reflections, which runback into the horn, where, coupled with thedirect sound, they cause interferenceexpressed as undulations in the frequencyresponse.The 2 ¥ 12" woofer unit is truly spectacular.Its sensitivity continually rises from an ini-tial 98 dB at 100 Hz to 110 dB at 1 kHz. Apartfrom the actual values, the main surprise isthe evenness of the curve. Filtering canthus be relatively modest, and we see clearevidence of largely low-resonance behavio-ur.The integrated passive crossover bringsboth paths together very simply, with thecrossover limited to high and low-passfunctions. All other filtering then takes pla-ce in the controller, where it can be perfor-med without loss. The advantage of passiveoperation compared to the fully active solu-

tion lies in the wiring and in the fact that itspares the need for another amplifier chan-nel. For most users, the passive separation ofthe T24N will be the first choice, which iswhy only this version is described in moredetail below. The crossover function for ful-ly active operation has largely approxima-ted the passive crossover to cause the leastpossible change to the radiation in thetransition zone.

B30 subwooferTW Audio recommends the B30 subwooferfor the T24N, and this product is offered inthe PA-SYS-ONE set at a ratio of 2:1. Rewor-ked two years ago, the B30 is equipped withtwo particularly strong 15" chassis byEighteen Sound. The two drivers with rub-ber surrounds are arranged on the V-sha-ped, inward-running sound panels in a spa-ce-saving manner. The bass reflex port issituated in the centre of the box itself. TheV-shape additionally creates a type of hornstump, which has a favourable effect on theradiation. The inverse assembly with themagnets facing outwards enables opti-mum driver cooling, as the waste heat canescape more easily compared to insidemounted magnets.

Fig. 1: Impedance measurements of the T24N in passive mode (green)and active mode with separate curves for woofers (red) and com-pression driver (blue). The tweeter is an 8-Ohm system. The wooferunit works with two parallel 8-Ohm drivers as a nominal 4-Ohm sys-tem. The bass reflex tuning is at 73 Hz.

Fig. 2: Frequency responses and sensitivity for the LF (red) and HFpath (blue) in the T24N with 60 ¥ 40 horn. The dotted curves repre-sent the 90 ¥ 50 version. The sensitivity for all curves is based on 2 V/ 1 m. The 1 W / 1 m value for the HF path as an 8-Ohm system is thus3 dB higher. All measurements are taken with the front grill.

In terms of dimensions, the B30 measures 706 ¥ 446 mm at thefront (like the T24), with a depth of 800 mm. It similarly weighs avery reasonable 39 kg. The side and back of the cabinet eachhave two grips, making it easy to handle. A dolly, also suitable forthe T24N, can be fastened to the front. The B30 has solid feet,enabling upright or flat assembly. Each of the opposite sides hasa suitably deep recess where the feet of another B30 or T24N canlock in to provide stable support during stacking.The impedance curve in Figure 4 shows an 8-Ohm system with aminimum impedance of 5.8 Ohm at a tuning frequency of 43 Hz.Two B30 subs are typically operated on one amplifier channel. The frequency response is where the B30 really shows what it’smade of: An average sensitivity of 97.2 dB is achieved between50 and 100 Hz, while the lower edge frequency of 38 Hz is prettydeep for a 15" subwoofer. Combined with a programme power of2 kW (1 kW limiting continuous thermal withstand power), twoB30s can, with a 4-kW amplifier, achieve a theoretical maximumof 136 dB. More information on this can be found in the sectionon maximum levels and distortion values.

Control ler and ampingThe test system was equipped with the relevant standard amprack with two Powersoft K3-DSPs. Each amplifier channel has upto 3.3 kW at 4 Ohm, depending on the signal’s crest factor. Theintegrated DSP system, coupled with the Armonia software, pro-vides extensive filter and limiter functions, as well as all kinds ofsystem monitoring options. In addition to the IIR and FIR filters,the DSP also offers the Raised Cosine Filter, which enables parti-cularly detailed adjustment of almost any given filter curve.Further details on this can be found in the K3-DSP test report inPRODUCTION PARTNER, 4/2011 edition.The K3-DSP power amplifiers contain setups for the active andpassive versions of the T24N. Both variants then, in turn, havetheir own separate variants with (cut) and without (full-range)additional subwoofers. Figure 6 shows the relevant filter curves,together with the curve of the B30 setup. There is also the optionof operating the T24N in full-range mode with subwoofer, rai-sing the level from 60 to 200 Hz, which may be beneficial in cer-tain cases or for certain setups.

Overal l systemFigure 7 shows the overall performance of the speaker and con-troller. In full-range mode (red curve), the T24N reaches down asfar as approx. 70 Hz, whereby the box also does well without asubwoofer when the output is not excessively bassy. If the T24Nis used in cut mode, coupled with the B30, the frequency respon-se is extended a crucial further octave down. This of courseresults in significant gains in maximum level (see also Figure 15).When it comes to the phase response (Figure 8), the T24N acts asexpected with and without B30. Each separation and each elect-rical high-pass filter causes a 360° phase rotation. It is also worthremembering that a transition is not just about each electricalhigh-pass and low-pass function, but also the speaker’s acoustic

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Fig. 3: Frequency responses and sensitivity for the T24N with passivecrossover in the 60 ¥ 40 and 90 ¥ 50 (dotted) version

Fig. 4: B30 impedance curve: Both 16-Ohm drivers are connectedparallel to an 8-Ohm system, the bass reflex system is tuned to 43 Hz

Fig. 5: B30 frequency response and sensitivity. Between 50 and 100Hz, the average sensitivity is 97.2 dB; in terms of this, the lower edgefrequency (–6 dB) is 38 Hz.

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function. A 2nd-order electric low pass cou-pled with the relevant speaker can thusalso result in a 4th-order low pass. For theT24N, this is the transition between twee-ter and woofer at 360°, the box’s acoustichigh-pass behaviour at 360°, and the rele-vant electronic 4th-order high-pass filter.Subwoofers add a further 360°C for thetransition from top to sub.The delay curves in Figure 9 also differaccordingly.The spectrogram in Figure 10 shows a goodresult overall with the T24N and B30 com-bination. There are very few resonances inthe 1-to-2 kHz range and at the far top ofthe transition zone, where the partial vibra-tions of the tweeter membrane inevitablystart. The post-oscillation below 100 Hz isdue to the rise in group delay.

DirectivityIf all the T24N variants were to be coveredby directivity measurements, there wouldbe 16 test series in total. There are the activeand passive variants, each with a 60 ¥ 40and 90 ¥ 50 horn, and both in the rotatedhorn variant as 40 ¥ 60 and 50 ¥ 90. Thehigh flexibility in radiation should be parti-cularly important to the installation mar-ket, where the speakers need to adapt as

accurately as possible to local conditions.According to Tobias Wüstner, a weather-proof T24N model for stadium use is alrea-dy under construction. The differences bet-ween the actively and passively separatedversions are rather marginal in terms ofdirectivity, as the X-Over functions of theactive crossover have been adapted to thoseof the passive crossover wherever possible.We thus limited ourselves to the passiveversion in the most important variants 60 ¥40 and 90 ¥ 50 for the test.

In all cases, the tweeter horn fulfils its taskcorrectly, without any problem. With therelatively high transition frequency in the1.6 kHz range, the 2 ¥ 12" unit displaysgreater bunching than the tweeter unit,which is particularly noticeable for the 90 ¥50 horn and vertical level, due to the largerspread of the two 12" driver. A huge pluspoint is the complete omission of interfe-rence effects. The “almost” coaxial arrange-ment means no level has angle-dependentphase differences, which otherwise typical-ly cause interference at the vertical level.

M a x i m u m l e ve l a n d d i s t o r t i -on valuesA maximum level test with sine bursts for 3% and 10 % distortion limits was initiallyconducted to measure distortion. It ratesthe harmonic components k2 to k9. Themeasurement algorithm increases the leveluntil the relevant distortion limit value of 3% or 10 % is reached. A performance limitfor the speaker or, as in this case, thedetection of a limiter in the system, can beset as a third abort criterion. Figure 15 showsthe result with the blue curve for maximum3 % distortion and the red curve for the 10 %limit. At the point where both curves meet,the distortion limit values were not reachedbefore the internal limiter in the K3-DSPamplifiers stopped the measurement. Thedotted curves were measured with a combi-nation of T24N and two B30 subwoofers.Except for 1–2 dB, the red curve for the topparts approximates the theoretical maxi-mum value, which is calculated based onsensitivity and the maximum output per-mitted by the limiters. When it comes to theT24N, the limiters in the K3 are set for a sine

signal in such a way so as to ensure 63.6Veff as a maximum, corresponding to anoutput of 1 kW at 4 Ohm, which results in a30 dB level increase compared to the 1 W/1m sensitivity. The figures for the two B30s are as follows:The sensitivity of an individual B30 is 97.2dB plus the 3 dB gained by the acousticcoupling of two subwoofers. The K3 ampli-fier can provide a maximum output of 2.8kW for the duration of the sine bursts, cor-responding to a theoretical level increase of34.5 dB. The average value of the 10 % curveis only 1.5 dB lower, between 50 und 100 Hzwith an average 133.2 dB, allowing ampli-fiers, limiters and speakers to perfectly com-plement one another. The second test for non-linear distortionuses a multi-sine signal to analyse the dis-tortion. The signal consists of 60 sine sig-nals with a random phase, and is weightedas per EIA-426B based on average musicalspectrum. The signal has a crest factor of 12dB, and is therefore also very similar to anormal music signal.The green curve in Figure 16 shows the fre-quency weighting of the measurement sig-nal. The blue lines or 1/6 octave band cumu-lative curve show the signal transmitted bythe speaker. The red lines and cumulativecurve only contain the distortion percenta-

B30 subwoofer with two Eighteen Sound 15"drivers. The bass reflect port is in the middle,between the two drivers inside the box. Thehigh-performance drivers are fitted with neo-dymium magnets and flow-optimised bas-kets.

System amp rack with two Powersoft K3-DSPamplifiers. Two stacks, each with 2 x B30 and 1x T24N, are operated with one rack as stan-dard, though the design enables the use of upto four stacks (4 x T24 + 8 x B30, all channels 2Ohm)

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Fig. 6: Controller functions of the K3-DSP power amplifiers. Pale anddark blue for T24N active operation with low-cut. Red for the passivevariant without low-cut. Green for the B30 subwoofer filter.

Fig. 7: Frequency responses for the system with controller. Red forthe T24N in full-range mode and pale blue for low-cut when combi-ned with subwoofer (green). Without a subwoofer, the T24N can beused down to 70 Hz. The B30 takes the transition zone one octavelower. All curves are based on the T24N in passive mode.

Fig. 8: Phase responses for the T24N full-range (red) and with B30subwoofer (blue). There is a 360° phase rotation for each X-Overpoint, as well as for the electrical/acoustic high passes.

Fig. 10: Spectrogram of the T24N and B30 combination. There aresome small resonances between 1 and 2 kHz. Above 13 kHz, we cansee tweeter membrane resonances and reflection from the hornmouth / front grill.

Fig. 9: Group delay for the T24N full-range (red) and with B30 sub-woofer (blue). With a subwoofer, the phase rotations increase in thebass range, and the delay curve rises accordingly.

Fig. 11: Horizontal isobars for the passive version of the T24N with 60¥ 40 horn. The 60° is achieved upwards of 800 Hz. The isobars nar-row down slightly between 1 and 2 kHz.

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Fig. 14: Vertical isobars for the passive version of the T24N with 90 ¥50 horn

Fig. 15: Maximum level for 3 % (blue) and 10 % (red) distortion limitsfor a T24N in full-range mode and with two B30 subwoofers. Thesubs perfectly complement the top. There are no weak areas in thecurve.

Abb. 16: Intermodulation distortion for a Leq of 99 dBA at a distanceof 8 m. A multi-sine signal with EIA-426B spectrum (green) and 12 dBcrest factor was used as a stimulation signal. The red represents thedistortion components which make up 4 %. The peak level measuredfor this, based on a distance of 1 m, was 132 dB.

Fig. 17: Measurement of total distortion for:Green: T24N at 99 dBA / 8 m 132 dBpk / 1 mRed: T24N at 106 dBA / 8m 139 dBpk / 1 mBlue: T24N with 2 ¥ B30 at 106 dBA / 8 m

Fig. 12: Vertical isobars for the passive version of the T24N with 60 ¥40 horn. The 40° is largely upheld by the tweeter horn. The 2 ¥ 12"unit causes more bunching in the middle.

Fig. 13: Horizontal isobars for the passive version of the T24N with 90¥ 50 horn. The constriction in the affects optically stronger here compared to the 90° tweeter.

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ges consisting of harmonic distortion andall intermodulation distortion. The measurement is typically performed fora standard useful signal level, approxima-ting a real operational situation. A level of99 dBA at a distance of 8 m under open-airtest-site conditions was set for the T24N.The level here is an LAeq (energy-equivalentcontinuous sound level), and the linearweighted level is approx. 3 dB higher. 12 dBmust again be added for the maximum

value. This means a 114 dB peak level at adistance of 8 m for the T24N, correspondingto 132 dB at a distance of 1 m. At –28.3 dB (4%), the total distortion measured was verylow. But the box has certainly not reached itslimit yet. The level was raised further for thecurves in Figure 17, starting with the 99-dBAmeasurement at a distance of 8 m. Thegreen curves once again show the trend ofthe overall level (solid curves) and distortionpercentages (dotted curve) from Figure 15.Controlling the amplifier until the limitersare set (red curves) raises the level by 7 dB to106 dBA Leq at a distance of 8 m, and 139 dBPeak at a distance of 1 m. In this case, thedistortion percentage then remains at anacceptable 10 %.The blue curves in Figure 17 were also mea-sured for 106 dBA, but with the addition oftwo B30s. Below 150 Hz, the level rises asexpected, without increased distortion. Qui-te the opposite, in fact – the values actuallydrop. But another phenomenon is observedby the by: Adding the subwoofers onlyincreases the distortion values by 2–3 dB inthe tweeter’s sphere of operation, thoughthere is unfortunately still no explanationfor this. It could be electrical or acousticphenomena which caused the distortion incurrently very high low-frequency soundpressure to rise slightly in our type of con-struction.

Listening testThe listening test involved building a stereoset consisting of a B30 subwoofer and aT24N top on each side in the measuringroom. Despite the rather unrealistic boun-dary conditions, the tonal behaviour can beevaluated very well here due to the lack ofspatial influences. Another advantage –also generated by the room’s lack of diffusefield – is the possibility of being able to lis-ten at a very loud volume, without thislevel becoming unpleasant. The listeningdistances in the low-reflection room aretypically 6–8 m. The T24N was initially heard full-range wit-hout subwoofers. The bass reproductionwas pleasingly strong, meaning that,depending on the music, nothing was miss-ed unless there was a direct switchover.With a subwoofer, the system was really

amped up, producing a high-end soundwith the dynamism of a live performance.At this point, it was pure enjoyment, andeven critical ears were unable to find anypoints of criticism. Many different sampleswere listened to, and every person presentcould clearly envisage the purposes forwhich the systems were best suited: Clubconcerts, marquee events, DJ gigs, voiceamplification for speeches over long dis-tances, and all kinds of installations ...Particularly striking were the system’ssound, which remained pleasant even atvery high levels, and tonal balance.

SummaryWith the T24N as the successor to the T24,which has proven its dominance over thelast ten years, TW Audio has achieved morethan merely a model upgrade. The extreme-ly compact, significantly lighter (thanks toneodymium drivers), full-range-compati-ble, completely horn-loaded top is the epi-tome of speaker design in every respect.There is now also the alternative 90 ¥ 50-degree horn and the serial switchover opti-on between passive and 2-way active ope-ration. Everything about it, even on top ofthe tonal and metrological performance, istop class. The workmanship is excellent, theaccessories are plentiful, and the speakertop and subwoofer are well designed andgenuinely easy to use. So it’s easy to seethat the new T24N, coupled with the B30subwoofer (reworked two years ago) andthe Powersoft or Lab.gruppen system ampracks, will be successful for many moreyears to come.

Phase rotation and groupdelay

Contrary to what is often misreported,intense phase rotations and a rise ingroup delay do not mean that low fre-quencies are reproduced too late. Theyinstead cause a delay in a frequency

group’s energy focal point. Our simulati-on explains this using two sine bursts:At the top are two original signals, andat the bottom are both signals afterbeing transmitted through a high-passfilter. The blue sine burst with the lowerfrequency peters out, and its energyfocal point shifts clearly backwards. Thered burst, on the other hand, remainsvirtually unchanged. This time diffe-rence in energy focal points, resultingfrom the transmission through the sys-tem, corresponds to the group delay dif-ference between these two frequencies.

Two sine bursts in the original (top),which are transmitted down to the lowfrequencies (bottom) through a systemwith increased group delay. The energyfocal point of the lower-frequency blueburst shifts by an amount equivalent tothe system’s group delay difference(green lines)

◊ Text and measurements: Anselm Goertz

Pictures: manufacturer