Another was that the basic DEQX unit cost about $3000, and the price rose with the addition of A/D, D/A, and preamp-type controls. Yet another was the need to have a PC or laptop to do the number crunching for the mea- surements, calculations, and setup. Still, the DEQX enjoys deserved popularity in recording and mastering studios and with DIY speaker builders.

The DEQX processing in the XdA does all the hard work for the user. Crossover and EQ are customized for

N H T X d

the two-way XdS satellite and XdW woofer to endow the XdS with a flat response on-axis and optimal dispersion in the horizontal plane. Perfection in the vertical plane is thwarted by the physics of stacked drivers, but due to the very high (108dB/octave!) crossover slope, the suckouts above and below the listening axis are narrowly restricted. The steepness of the slope makes possible a relatively low crossover frequency for the tweeter (just above 2kHz) without compromising

its power handling. This, and a smoothly shaped cabinet devoid of edges on its front surface, contribute to the smoothness of the XdS’s off-axis response. In addition, some low-end boost is provided at around 26Hz, which makes the XdW’s in-room response fairly flat into the 20Hz range before it begins to roll off.

The XdA also includes four Power- Physics class-D (switching) amplifier modules (www.powerphysics.com) to biamplify the two XdS speakers. While

m e a s u r e m e n t s , c o n t i n u e d

averaged across a 30° horizontal window centered on the satellite’s tweeter axis. Below 300Hz, this graph shows the combination’s estimated farfield response, calculated by adding the nearfield responses of the sub- woofer and woofer and taking into account acoustic phase. The region covered by the subwoofer, below 110Hz, can be arbitrarily raised or lowered using the XdW’s rear-panel volume control. However, it does look as though the integration between the two units is not quite as perfect as that between the woofer and tweeter. Other than that very slight lack of lower-midrange ener- gy, and the slight shelving-down of the speaker’s output above 12kHz due to the tweeter becoming more directional in this region, the NHT speaker system offers extraordinarily flat response!

It is not only a flat on-axis response that contributes to

Fig.8 NHT XdS driven by XdA, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

a speaker having a neutral balance; its radiation pattern also has an effect in all but anechoic rooms. Here the NHT XdS also offered superb performance. Fig.8 shows its lateral dispersion referred to the response of the tweeter axis, which has been subtracted from all the traces so that only the changes are apparent. The contour lines are evenly spaced, and there is only a trace of the usual off-axis flare at the bottom of the tweeter’s passband. Other than its restricted dispersion above 12kHz or so, this speaker’s radiation to its sides is not significantly different from what it puts out in front. In the vertical plane (fig.9), the use of noncoaxial drive- units means that suckouts in the crossover region between the XdS’s tweeter and woofer appear at extreme off-axis angles. But the use of very steep crossover filter slopes means that the XdS is otherwise very tolerant of listener ear height.

Performing a loudspeaker’s crossover and equalization functions in the digital domain means that its acoustic performance, in theory, can be optimized in both the frequency and time domains. So, when examining the XdS’s time-domain performance, I first looked at the step responses of the raw drive-units (fig.10). Without the XdA equalization, the tweeter (red trace) leads the woofer (blue) very slightly, with the tail of the latter’s step overlaid by the high-frequency ringing of the cone breakup modes noted earlier. Repeating these measurements with the XdS driven by the XdA gave the steps

Fig.9 NHT XdS driven by XdA, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Fig.10 NHT XdS, step responses on tweeter axis at 50" of unequalized tweeter (red) and woofer (blue). (5ms time window, 30kHz bandwidth.)

www.Stereophile.com, November 2005