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NHT Xd
The basic Xd configuration tested
here was: the XdA, a box containing
four channels of power amplification
and crossover/equalization for up to
two satellites and two subwoofers; a
pair of two-way XdS satellite speakers
with stands; and the XdW, a powered
bipole subwoofer module. Other con-
figurations of electronics, speakers, and
woofer can accommodate anything
from 2.1 to 6.1 channels and beyond.
Brains in a box
The Xd system may be a significant
conceptual advance in home audio,
but it will be a tough pill for the tra-
ditional audiophile to swallow. Al-
though each component was de-
veloped by a different specialist
company—speakers by NHT, ampli-
fiers by PowerPhysics, EQ/cross-
over by DEQX—it is an integrated
active system that does not allow
easy substitutions of other gear.
www.Stereophile.com, November 2005
MEASUREMENTS
T
heir active, digitally crossed-over and equalized
nature meant that I could examine the intrinsic
behaviors of the NHT XdS satellite and XdW sub-
woofer 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 indica-
tive 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 dri-
ven exclusively by the system’s dedicated XdA amplifier,
these curves are really only of academic interest. Howev-
er, 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; simi-
larly, 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 enclo-
sure-wall resonances. Fig.2, a cumulative spectral-decay
plot calculated from the output of a plastic-tape acceler-
ometer 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.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
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.4 NHT XdS, cumulative spectral-decay plot of unequalized woofer at
50” (0.15ms risetime).
Fig.1 NHT XdS, electrical impedance (solid) and phase (dashed) of
woofer (top) and tweeter (bottom). (2 ohms/vertical div.)
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.