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The CXM 15 from Dynacord, a manufacturerbased in Straubing (Germany), is a 15"/1.4" boxwith a wedge design. The cabinet has beenkept especially flat and compact so as to be asunobtrusive and easy to position as possibleon stage. Despite the advent of in-ear monito-ring, floor monitors are no relic of the past.On grounds of cost alone and the relativelytime-consuming handling, in-ear monitoring

is often not possible, and a great many per-formers and technicians in any case preferclassical monitoring on stage – and for a widevariety of reasons. The demands placed onthe loudspeakers used on stage are very high:"unobtrusiveness" is of particular importanceif the set is to remain uncluttered and thelines of sight of audience members (andcameras) unobstructed.

From an acoustic standpoint, a consistentdirectionality and a frequency responsefree from narrow-band ridges are what isrequired, the aim in each case being toreduce susceptibility to feedback, as – alongwith the polar pattern of the microphones– the performance of the monitors plays aleading role here. Depending upon theapplication, further factors, such as the ade-

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quacy of sound pressure levels, are of grea-ter or lesser importance. Since stage moni-tors are usually operated at close range –between one and three metres from theperformer – the sound should as far as pos-sible radiate from a single source.

CXM 15 For the aforementioned reasons, the CXM15 is conceived as a coaxial two-way sys-tem. A further advantage of this constru-ction is the reduced area of the front panel,which makes the box more compact. Thetwo sides of the enclosure are angled at 33°and 45°, the depth of the cabinet being 329mm. These compact dimensions for a 15"woofer indicate already that the box is notnecessarily optimized for deep bass repro-duction, but this is in any case not what isgenerally wanted in a stage monitor. Thesize of the enclosure is clearly here the pri-mary consideration. Genuine deep bassreproduction on stage is only required, if at

all, for the drum monitor, which can alwaysbe complemented, should the need arise, bya subwoofer or shaker.With a weight of only 19 kg, the CXM 15 isone of the lightweights in its class and iswonderfully easy to handle. Particularlypractical are the integrated gripping edgesrunning all the way round both sides of theenclosure. The recessed side panels accom-modate three Speakon sockets, the acti-ve/passive selector switch and an integra-ted stand adaptor. Two of the Speakonsockets are on the side with the stand adap-tor and the third on the opposite side. Thismeans that whichever way the loudspeakeris mounted, a short Speakon cable is all thatis needed to run the signal from box to box.The socket for the stand reveals that theCXM 15 can naturally also be used as a 'nor-mal' full-range box or as the top box in asmall PA or even as a fill system.Overall, the birch plywood enclosure withits textured finish has a very robust look toit and the same goes for the equally solid

front grille with its foam backing. The waythe front grille is attached here, with verysolid threaded screws, represents a greatimprovement over the screw or clampdesigns you usually find. You can removeand replace the grille umpteen times herewith no problems whatsoever. Dependingupon the type of show, monitors on stagecan be exposed to a considerable amount ofdirt, so it is important to be able to cleanthe front panel regularly and swiftly.When you unscrew the grille, the 15" chas-sis with its coaxial high frequency hornappears. The relatively large horn is positio-ned extremely close to the diaphragm ofthe 15" woofer. For the woofer, the hornfunctions as a type of small-volume band-pass chamber. Interference caused by thesound radiating from the woofer dia-phragm reflecting back off the horn is onlyof relevance at very short wavelengths – inother words, at high frequencies that are inany case above the crossover frequency of1,200 Hz. On the other hand, thanks to itssize, the directionality of the horn is alreadyso pronounced at the crossover frequencythat the woofer diaphragm lying behind itis scarcely of any importance. The object ofthis approach is to obtain greater control ofthe sensitive midrange, which is otherwisealways a weakness with coaxial drivers.And here, we can say at the outset, theapproach has proved successful.If you unscrew the chassis, it is easy toremove the driver with its two neodymiummagnets from the box. The saving ofweight that comes with the use of neody-mium magnets is especially noticeable inthe case of coaxial systems with two dri-vers in one chassis. Comparable chassiswith ferrite magnets usually weigh morethan the entire box does here.

Active or passive?Once we have removed the driver, we get aclear view of the crossover: the circuitboard contains two passive second-orderfilters – a high-pass and a low-pass – aswell as a protective circuit for the tweeter.The compact circuitry on the board calcula-tes in the manner of a simple analoguecomputer the RMS power received by thehigh-frequency driver and opens a relaywhenever necessary (i.e. whenever there is

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a danger of overload). The tweeter is not,however, fully switched off when thisoccurs but driven via two light bulbs con-nected in series, which offer secure prote-ction against burnout. When the overloadcondition no longer exists, the bulbs areonce again bypassed by the relay. A switchon the panel allows you to choose betweenPassive and Active modes. In active mode, both the passive filter andthe high frequency protection remain inthe signal path, only the input to the passi-ve crossover is now split so that the HF andLF can be driven separately via pins 1 and 2of the NL4 connector. This obviates the riskof defects caused by faulty wiring. In activemode, the only thing to bear in mind is thatthe passive filter is already in the signalpath. The crossover frequency in the activeversion can no longer therefore be raisedsignificantly beyond the 1.2 kHz of the pas-sive filter, which is in any case somethingbest avoided. For active mode – naturallyfor passive mode as well – Dynacord recom-mends a power amplifier from its ownPower H series equipped with the control-ler module IRIS-Net RCM-26. The H5000power amplifier and the RCM-26 modulewere already the subject of a detailed testreport in PRODUCTION PARTNER (Issue10/2007) so we only need outline here tho-se functions most relevant to the CXM 15.

IRIS-Net software and the RCM-26 moduleThe Intelligent Remote Integrated Supervi-sion software platform (IRIS), which hasbeen available now for seven years, con-tains the manifold control and supervisionfunctions for Dynacord and EV poweramplifiers and loudspeaker controllersequipped with their own DSP systems.Independently of any signal processingfunctions, the RCM-26 module adds a two-channel digital input and output inAES/EBU format, a CAN bus interface andtwo GPIO control and supervision conne-ctors for external functions to the poweramplifier. The networking for the remotesupervision and remote control is handledby the CAN bus.To connect to the PC, a further USB/CANadapter is therefore needed. The UCC1supplied for the test offers in addition to itsadapter function the possibility of listeningin to the signals of the connected amplifiervia an audio output, which is especiallypractical when searching for errors.For remote supervision, IRIS-Net offers aseries of useful functions both for thepower amplifiers and the connected loud-speakers. In addition to pilot-tone supervi-sion, these allow the load impedance of thenetwork and the prevailing thermal condi-tions to be checked. Another very impor-tant function is measurement of the impe-

The RCM-26 IRIS-Net module

dance of the connected loudspeakers. Here,instead of a single reading, an entire impe-dance curve for the frequency range 20 Hz –20 kHz is measured. This can then be com-pared with a stored tolerance mask (see Fig.14) so that a reliable judgement can be for-med as to whether the connected loudspea-ker is still perfectly OK or in fact defective.With this type of measurement, you canmake a reliable and at times highly specificdiagnosis of most loudspeaker errors. Sucha function is extremely useful for thesupervision of fixed installations but evenmore so in the touring and rental sectors, asit makes it possible to conduct a swift andreliable check at the end of each show tomake sure all the loudspeakers connectedare still functioning correctly.The RCM-26 controller in the H5000 poweramplifier offers the choice of running theCXM 15 in either active or passive modes –in each case with either IIR or FIR filters.With 2 x 2.5 kW of sustainable power into 4ohms, the H5000 naturally offers morethan enough power for the operation of theCXM 15, so one can easily imagine a confi-guration with four boxes per channel and atotal load of 2 ohms. In this case, the H5000would deliver 3.3 kW per channel with 825watts going to each box. The CXM 15'spower handling is given as 900 watts pro-gram and 450 watts RMS, so this configura-tion should suit it nicely. One H5000 canpower eight CXM 15s in passive mode andfour in active mode.

Measured valuesThe connectivity of the CXM 15 with itsindividual LF and HF paths when set to bi-amp mode makes it easy to measure thetwo paths separately. What this revealsimmediately is how well the two interact inthe 1.2 kHz crossover zone. Fig. 1 illustratesthis nicely. Despite the simple design of thepassive crossover, the response when thetwo paths are summed is linear. Between200 Hz and 3 kHz, the sensitivity is justunder 100 dB; above 3 kHz it is around 103dB. The fine interference structure in thehigh frequency range is not the fault of thedriver but largely an unavoidable side-effect of the grille, as the black line sho-wing the response when the grille is remo-ved makes plain. Since the grille inevitably

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reflects a small portion of the sound backinto the horn, some interference betweenthe waves flowing in and out is inevitable.In terms of sound, though, fine structureslike these are hardly of such critical impor-tance as to justify dispensing with a grille –least of all that of a floor monitor… Takingthe mean sensitivity of 99.5 dB between100 Hz and 10 kHz, the lower and upper 6dB corner frequencies are found at 110 Hzand 17.4 kHz respectively. For a 15" tweeter,110 Hz is very high for a lower boundary fre-quency, but this is due to the compactnessof the enclosure and its use as a monitor. Assoon as the box is laid on a boundary surfa-ce i.e. in its normal working position (seeFig. 3), the lower corner frequency falls to88 Hz. Thanks to the flat cabinet constru-ction, the much dreaded 'monitor hole' inthe frequency response is scarcely discerni-ble here. The phase response in Fig. 2 showsthe high-pass characteristic of the bassreflex enclosure, with 360° of phase shift atthe lower end of the frequency band and arelatively small phase shift in the areaaround the crossover frequency. The slightspatial displacement of the two pathsseems partly to compensate for the 180°shift produced by the second order crosso-ver. The electrical measurement of theimpedance in Fig. 4 reveals the minimumimpedance of 6.8 ohms as well as the 70 Hztuning frequency of the bass reflex resona-tor.All in all, what we are dealing with here is athoroughly 'good-natured' box, from which,thanks to the secure foundation affordedby its coaxial driver, the desired results canbe obtained with scant use of filters. Thesmall 'nose' in the frequency response at800 Hz is revealed by the spectrogram inFig. 5 to be a resonance the causes of whichcould be many and diverse. Here we mightbe looking at an enclosure mode or perhapsa horn resonance – we cannot be sure atthis stage.

DirectivityIn the data sheet, the nominal coverageangle of the CXM 15 is given as 60°, whichdue to the coaxial design and circular hornis valid for all planes. Any discrepanciescould only be attributable to the enclosureand edge effects produced by it. Figures 6

and 7 show the isobar curves for the hori-zontal (narrow side) and vertical (wideside) planes. The nominal value of 60° as amean value above 1 kHz is arrived at by asomewhat tortuous route. Above 8 kHz, thehorn then begins to focus the sound moretightly and the radiation angle shrinks to40° (-6 dB). Obviously you cannot expect acoaxial horn to satisfy the same demandsas those imposed on comparable – and usu-ally far larger – 15/2 boxes with individualsystems, since a coaxial design necessarilyinvolves compromises on one side or theother, though here they are balanced outwell.

Maximum SPLWhen measuring the maximum SPL, onlythe passive version of the CXM 15 was con-sidered, since no important differenceswere to be expected between this and theactive version. The maximum power for theseries of measurements with an 185 mssinus burst was limited to 900 watts refe-renced to 8 ohms. The green curve in Figure

8 is calculated from the sensitivity and the900 watts (+29.5 dB referenced to 1 watt). Inred and blue, you can see the values actual-ly achieved with a maximum of 10% and 3%total harmonic distortion respectively. Asfar as 3 kHz, the 10% curve achieves withnear perfect consistency the calculatedmaximum level, which is just beneath the130 dB line – this, it must be said, is anoutstandingly good result. At higher fre-quencies, the curve inevitably begins to fallaway on account of the compression driverbut still remains around the 120 dB mark.Also very impressive is the linearity of themaximum level curves, which indicatesthat the loudspeaker has no "weak frequen-cies".

FI R or I I R f i l tersThe RCM-26 controller in the H5000 poweramplifier offers the choice of running theCXM 15 in either active or passive modes –in each case with either IIR or FIR filters.The concomitant filter functions for all fourvariants are shown in Figure 9. The red and

The coaxial chassis with its 15" woofer and 1.4" tweeter and large horn

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Fig. 1: The frequency response and sensitivity of the CXM 15 (red) aswell as the LF (green) and HF paths (blue) with passive crossover.Moved 10 dB upwards and shown in black: the frequency responsewithout the grille

Fig. 2: Phase response of the CXM 15 in passive mode

Fig. 3: Frequency response and sensitivity of the CXM 15 in floor-monitor position

Fig. 4: Impedance curve of the CXM 15 in passive mode showing theminimum impedance of 6.8 ohms at 217 Hz and the tuning frequen-cy of the bass-reflect resonator at 70 Hz

Fig. 5: The spectrogram of the CXM 15 in passive mode reveals a fairlypronounced resonance just below 1 kHz

Fig. 6: Horizontal isobar curves of the 60° coaxial system

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orange curves representing the FIR variantsare characterized in general by more detailsand in the active version considerably grea-ter steepness.If you look more closely at the FIR setup inthe IRIS software, you will notice at oncethat in the lower frequency range there arean additional two IIR filters, a bell filter anda high-pass filter. These minor reinforce-ments are necessary because the FIR filtersno longer afford sufficient resolution or fre-quency selectivity in this range. Depending upon the selected number ofcoefficients of the FIR filters, there is anincrease in the total latency of the system,which otherwise, independent of the num-ber of signal processing functions, is 1.03ms at a sample rate of 96 kHz and 2.3 ms at

48 kHz. With a sample rate of 48 kHz and512 coefficients, for example, the increase inlatency attributable to the presence of a FIRfilter in the signal path comes to 5.3 mswhen the filter is designed to be linear pha-se. This value can be calculated by dividingthe number of coefficients by twice thesample rate – in this case, then, by dividing512 by 2 x 48.000 1/s. At this point, it mustbe emphasized once again that the latencyis not conditioned inexorably by the princi-ple of the FIR filter but by the filter functionitself. If the FIR filter is designed with aminimal phase function, its latency can bejust as long or short as that of an IIR filter.The latency is only a by-product of the line-ar phase approach. What is needed, therefo-re, is to find a good compromise with a suf-

ficiently short linear phase FIR filter combi-ned with an IIR filter. The IIR filter is thenused where the short FIR filter for want offrequency selectivity can no longer interve-ne – i.e. at the bottom end of the frequencyrange. This is the thinking here behind thecombination of a 512-tap FIR filter and twoadditional IIR filters. These configurationscan be nicely appraised using the IRIS-Netsoftware (Fig. 15), in which the amplitudeand phase of all the filter elements can beviewed singly and together – if necessaryin conjunction with the frequency respon-se of the loudspeakers. At the present time,as far as we know, the IRIS-Net software isthe only controller software that offerssuch possibilities together with the measu-red values of the loudspeakers (see Fig. 16).

Fig. 7: Vertical isobar curves of the 60° coaxial system. Discrepanciesbetween the horizontal and vertical curves are attributable to theshape of the enclosure

Fig. 8: Maximum level at 3 % (blue) and 10 % (red) THD along withthe calculated curve (green) for the maximum supplied power of900 W referenced to 4 ohms

Fig. 9: Controller functions for the passive version (above) with IIR(blue) and FIR filters (red) as well as for the active 2-way version(below) with IIR (light blue) and FIR filters (orange)

Fig. 10: Frequency response of the CXM 15 in passive mode withoutthe controller (green) as well as with the controller and IIR (blue) orFIR filters (red)

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Fig. 11: Phase response of the CXM 15 in passive mode without thecontroller (green) as well as with the controller and IIR (blue) or FIRfilters (red). With the FIR filters, a linear phase response is achievedfor the medium and high frequencies

Fig. 12: Frequency response of the CXM 15 in active mode with FIR fil-ters, with which a steep slope can be achieved

Fig. 14: The window for impedance supervision with the measuredcurve shown against the background of the reference range

Fig. 15: Overview window in the IRIS-Net software showing theH5000 power amplifier (left) and a flow diagram of the routing andsignal processing performed by the dual channel RCM-26 module

Fig. 16: FIR filter functions (thin lines) and the frequency response ofthe loudspeakers together with the filters. The phase response andsumming functions can also be displayed

Fig. 13: Phase response of the active version with FIR filters. Despitethe steepness in places, a linear phase response is even possible inthe region of the crossover frequency

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universally applicable: this is not at all thesame thing as a "multi-function box" thatyou can – at a stretch – also use as a moni-tor. The CXM 15 is for this reason very lowslung, compact and light in its construction.The 15" coaxial chassis with its unusuallylarge high frequency horn offers the idealprerequisites for this task. The measuredresults are correspondingly impressive,with the very good maximum SPL valuesparticularly praiseworthy. The fact that youcan switch very quickly between active andpassive modes means that the hirer canswiftly adjust the monitor system to thewishes and budget of the customer. In com-bination with the H5000 power amplifierand the RCM-26 controller module, there isthe added option of IIR or FIR filteringIn the hearing tests, the box sounds fullerthan the frequency response would at firstlead one to expect. The directionality is pre-cise with the coverage relatively stronglyfocussed, though the runoff towards theedges is even. Voices in particular are deli-vered by the CXM 15 with great clarity,which is precisely what is required from amonitor. A high degree of presence in thereproduction is often equated with "aggres-sion" or "obtrusiveness", but this is not at allthe case here. The idea of using the CXM 15in combination with a subwoofer as a smallclub PA, side-fill or drum-fill is also one thatappeals. Bearing in mind the quality of theperformance and features offered, ataround 1,900 euros the CXM 15 has to beconsidered good value. The 4,350 euros forthe power amplifier and 795 euros for thecontroller module, on the other hand, mayat first sight seem less so. This is only relati-ve in the overall reckoning, however, becau-se you can drive up to eight monitors witha single power amplifier. Since even in pas-sive mode without a controller the CXM 15is still a very good box, the choice existsbetween a very wide variety of viable con-figurations to match the needs and meansof the user.

The software is supplied with a large libra-ry containing the measured values of manyDynacord and Electro-Voice loudspeakers.The difficult question that arises next iswhich of the four controller modes to choo-se. Whether the monitor is driven in two-way active mode or passive is likely to bedetermined by economic considerations orthe guidelines of the customer, so the choi-ce boils down to that between FIR and IIRfilters. Figures 10 and 11 show the amplitu-de and phase response of the CXM 15 inpassive mode without the controller ingreen as well as with the controller and IIR(blue) or FIR (red) filters. In terms of fre-quency response, both versions with thecontroller thanks to the equalization achie-ve markedly lower corner frequencies atthe bottom end. Furthermore, the frequen-cy responses achieved with the controllerare generally more linear and where thereare spikes, they are less sharp. The FIR ver-sion goes into greater detail than the IIR fil-ter. Far more marked are the differences intheir phase responses, where the FIR filterin the midrange and higher frequenciesachieves a linear phase whilst the IIR filterleaves the original curve of the box virtual-ly unchanged. Summing up, one can saythat the FIR filter achieves what are all in

all a somewhat smoother frequency res-ponse and a linear phase response overwide frequency ranges. The advantages ofthe FIR filter are still more pronounced inthe active version. If you look at Figures 12and 13, an extremely steep separation bet-ween the LF and HF paths can be discerned.With conventional analogue or digital IIRfilters, this would lead to pronounced pha-se shifts. Not so with the linear phase FIRfilters, which make it possible to achieveeven here a linear phase response in themedium and high frequency ranges. TheFIR filters for the active mode just like thepassive version introduce 5.3 ms of latency,which at the speed of sound translates to adistance of 1.8 m. This, even for a stagemonitor, is perfectly acceptable. With a ful-ly digital signal chain in which A/D andD/A conversion is performed in each caseonce only, a further 3 ms at most will beadded, which still leaves us well insidewhat is generally regarded as the limit i.e.10 ms.

SummaryThe CXM 15 from Dynacord is a coaxialloudspeaker optimized from the outset foruse as a stage monitor but one that is also

The passive crossover of the CXM 15 with (on the right) the circuitry protecting the tweeter

◊ Text and measurements: Anselm Goertz

Photos: Dieter Stork, Anselm Goertz (2)

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