Dynacord CXM15 - Brochure - Pagina 4

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D y n a c o r d C X M 1 5

TEST

reflects a small portion of the sound back

into the horn, some interference between

the waves flowing in and out is inevitable.

In terms of sound, though, fine structures

like these are hardly of such critical impor-

tance as to justify dispensing with a grille –

least of all that of a floor monitor... Taking

the mean sensitivity of 99.5 dB between

100 Hz and 10 kHz, the lower and upper 6

dB corner frequencies are found at 110 Hz

and 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 compactness

of the enclosure and its use as a monitor. As

soon as the box is laid on a boundary surfa-

ce i.e. in its normal working position (see

Fig. 3), the lower corner frequency falls to

88 Hz. Thanks to the flat cabinet constru-

ction, the much dreaded 'monitor hole' in

the frequency response is scarcely discerni-

ble here. The phase response in Fig. 2 shows

the high-pass characteristic of the bass

reflex enclosure, with 360° of phase shift at

the lower end of the frequency band and a

relatively small phase shift in the area

around the crossover frequency. The slight

spatial displacement of the two paths

seems partly to compensate for the 180°

shift produced by the second order crosso-

ver. The electrical measurement of the

impedance in Fig. 4 reveals the minimum

impedance of 6.8 ohms as well as the 70 Hz

tuning frequency of the bass reflex resona-

tor.

All in all, what we are dealing with here is a

thoroughly 'good-natured' box, from which,

thanks to the secure foundation afforded

by its coaxial driver, the desired results can

be obtained with scant use of filters. The

small 'nose' in the frequency response at

800 Hz is revealed by the spectrogram in

Fig. 5 to be a resonance the causes of which

could be many and diverse. Here we might

be looking at an enclosure mode or perhaps

a horn resonance – we cannot be sure at

this stage.

Di r ect iv it y

In the data sheet, the nominal coverage

angle of the CXM 15 is given as 60°, which

due to the coaxial design and circular horn

is valid for all planes. Any discrepancies

could only be attributable to the enclosure

and edge effects produced by it. Figures 6

production partner 12/09_01/10 special reprint

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

and 7 show the isobar curves for the hori-

zontal (narrow side) and vertical (wide

side) planes. The nominal value of 60° as a

mean value above 1 kHz is arrived at by a

somewhat tortuous route. Above 8 kHz, the

horn then begins to focus the sound more

tightly and the radiation angle shrinks to

40° (-6 dB). Obviously you cannot expect a

coaxial horn to satisfy the same demands

as those imposed on comparable – and usu-

ally far larger – 15/2 boxes with individual

systems, since a coaxial design necessarily

involves compromises on one side or the

other, though here they are balanced out

well.

Maximum SPL

When measuring the maximum SPL, only

the passive version of the CXM 15 was con-

sidered, since no important differences

were to be expected between this and the

active version. The maximum power for the

series of measurements with an 185 ms

sinus burst was limited to 900 watts refe-

renced to 8 ohms. The green curve in Figure

8 is calculated from the sensitivity and the

900 watts (+29.5 dB referenced to 1 watt). In

red and blue, you can see the values actual-

ly achieved with a maximum of 10% and 3%

total harmonic distortion respectively. As

far as 3 kHz, the 10% curve achieves with

near perfect consistency the calculated

maximum level, which is just beneath the

130 dB line – this, it must be said, is an

outstandingly good result. At higher fre-

quencies, the curve inevitably begins to fall

away on account of the compression driver

but still remains around the 120 dB mark.

Also very impressive is the linearity of the

maximum level curves, which indicates

that the loudspeaker has no "weak frequen-

cies".

FI R or I I R fi lters

The RCM-26 controller in the H5000 power

amplifier offers the choice of running the

CXM 15 in either active or passive modes –

in each case with either IIR or FIR filters.

The concomitant filter functions for all four

variants are shown in Figure 9. The red and