NHT Xd active loudspeaker system Measurements

Sidebar 3: Measurements

Their active, digitally crossed-over and equalized nature meant that I could examine the intrinsic behaviors of the NHT XdS satellite and XdW subwoofer in some detail. However, it is important when looking at the graphs that follow not to assume that any problems and idiosyncrasies that are revealed in the measurements of the raw drive-units are indicative of problems in the overall Xd system's behavior.

With that warning out of the way, fig.1 shows the impedance magnitude and phase for the XdS satellite's woofer and tweeter. With the speaker intended to be driven exclusively by the system's dedicated XdA amplifier, these curves are really only of academic interest. However, the peak at 73Hz, reaching 26.3 ohms, indicates that the woofer is tuned by the sealed box to a frequency below the satellite's intended passband, meaning that the equalized drive-unit will not be asked to deliver large cone excursions. There are some small discontinuities in the woofer's impedance traces in the treble that might indicate the presence of resonances of some kind; similarly, the glitch at 27kHz in the tweeter's traces is due to the metal dome's primary breakup mode. The XdS's small, rigid cabinet, however, seemed free from enclosure-wall resonances. Fig.2, a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of the sidewall, reveals only a couple of very-low-level modes. I found nothing of note in the XdW's enclosure, so I haven't shown a graph of its vibrational behavior.

Fig.1 NHT XdS, electrical impedance (solid) and phase (dashed) of woofer (top) and tweeter (bottom). (2 ohms/vertical div.)

Fig.2 NHT XdS, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Fig.3 is particularly interesting, in that it shows the acoustic responses of the XdS's unequalized drive-units. The tweeter (blue trace) has quite a flat response above 2kHz and is significantly more sensitive than the woofer (red). The latter rolls off at 12dB/octave below 100Hz due to its sealed-box alignment, but features both a rising response above 600Hz and some sharp spikes in its output at 8kHz and above. That these are due to resonances in the small-diameter magnesium cone is revealed by the raw woofer's cumulative spectral-decay plot (fig.4). The fact that these resonances are well above the driver's intended passband and should therefore have little or no effect on sound quality will become evident in a moment.

Fig.3 NHT XdS, responses of unequalized woofer (red) and tweeter (blue) on tweeter axis at 50", corrected for microphone response, with nearfield response of woofer plotted below 300Hz.

Fig.4 NHT XdS, cumulative spectral-decay plot of unequalized woofer at 50" (0.15ms risetime).

First, fig.5 looks at the other end of the woofer's frequency range. The black trace is the same unequalized nearfield response shown in fig.3; the colored traces to the right of this graph indicate the nearfield response of the XdS's woofer with the appropriate equalization applied by the XdA amplifier. (I didn't have any ground-loop problems with the XdA and XdW.) The red trace is with the front-panel mode set to "1," magenta is with it set to "2," green "3," and blue "4." Each mode applies a slightly different amount of lower-midrange boost to compensate for a different placement option. More important, the XdA flattens the XdS woofer's output in the midrange and rolls it off steeply below 150Hz or so, thus removing the need for the little cone to handle frequencies that require significant excursion.

Fig.5 NHT XdS, nearfield response of unequalized woofer (black), and of equalized woofer set to Mode 1 (red), Mode 2( magenta), Mode 3 (green), and Mode 4 (blue). Left-hand red trace is the nearfield response of the unequalized XdW.

The red trace to the left of fig.5 is the unequalized nearfield response of the XdW powered subwoofer. It can be seen to roll off below 60Hz and above 150Hz with intrinsic 12dB/octave slopes. When driven by the XdA (fig.6, left-hand trace), its output is increased for an octave below 60Hz to give excellent bass extension, but is rolled off rapidly below 28Hz to avoid overloading the twin 10" drive-units. At the other end of its passband it is rapidly rolled off to give a measured crossover point to the XdS satellite of 110Hz. The equalized satellite's woofer and tweeter can be seen from this graph (middle and right-hand traces) to have basically flat responses within their passbands when driven by the XdA with the crossover between them set to 2.5kHz, as specified. Note the very steep filter slopes achieved by the XdA's digital-domain crossover: greater than 40dB/octave. The woofer-cone breakup modes seen in figs.3 and 4 should not be excited to any significant extent when the XdS is driven by its partnering amplifier.

Fig.6 NHT XdS & XdW driven by XdA, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer and subwoofer plotted below 300Hz.

Fig.7 shows the overall response of the XdS and XdW, 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 subwoofer 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 energy, 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!

Fig.7 NHT XdS & XdW driven by XdA, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and subwoofer responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.

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

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.

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.

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 shown in fig.11. The first thing to note is the different time scales of these two graphs. Fig.10 shows that the wavefronts from the unequalized drivers took 3.75 milliseconds to reach the microphone, which was 50" away. By contrast, the outputs of the equalized drivers in fig.11 took 7.5ms longer to reach the microphone. This is the time taken by the XdA's digital circuitry to perform its filtering and equalization functions.

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

Fig.11 NHT XdS driven by XdA, step responses on tweeter axis at 50" of equalized tweeter (red) and woofer (blue). (5ms time window, 30kHz bandwidth.)

What can also be seen from fig.11 is that each drive-unit's step is preceded by some low-frequency ringing. But because the tweeter's and woofer's acoustic outputs appear to have opposite polarities, this pre-ringing should to a large extent cancel, at least on the tweeter axis. That this does in fact happen is shown by the XdS's overall step response (fig.12), the tweeter's positive-going step smoothly handing over to the woofer's negative-going step, this in turn correlating with the superb frequency-domain integration between the two drive-units seen in fig.7.

Fig.12 NHT XdS driven by XdA, equalized step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Finally, the XdS's waterfall plot on the tweeter axis (fig.13) features an impressively clean initial decay across the band. There are two ridges of ultrasonic delayed energy apparent, one at 26kHz and one at 30kHz, at the rightmost edge of this graph. These are high enough in frequency not to have any subjective consequences. However, there is some low-level hash evident in the mid-treble, perhaps resulting from the woofer's residual cone breakup modes.

Fig.13 NHT XdS driven by XdA, cumulative spectral-decay plot at 50" (0.15ms risetime).

The combination of the XdS, XdW, and XdA offers superb measured performance in both the frequency and time domains. Is the speaker perfect, therefore? Unfortunately, I have no means of assessing dynamic range, but it must be remembered that this superb performance is obtained by shaping the output of what is still a small drive-unit. Even though the crossover to the XdW will eliminate the low frequencies that would otherwise demand large excursions from the XdS's woofer cone, I felt in my own auditioning that the system would not play quite as loud as I wanted it to in my wilder moments. (I will report further on my experience with the Xd in a "Follow-Up" in the January 2006 issue.) But other than that minor limitation, this system offers extraordinarily good performance at what is really an affordable price. I agree with Kal Rubinson—the Xd system is the best thing to come down the pike in a long time, and, along with the pioneering designs from Meridian, a harbinger of speakers to come.—John Atkinson

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