Filter
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ed on the XdA amplifier’s front
panel. Turning off the amplifier and
turning it on again fixed the prob-
lem.)
Repeating this torture test with
the 150Hz crossover filters didn’t
result in any shutdowns with the
Clarke solo, and the chord that
begins “In the Flesh,” from Pink
Floyd’s Is There Anybody Out There?
(The Wall Live) (CD, Columbia C2K
62058), sounded positively cata-
clysmic. The low-frequency crash of
the wall being demolished on this
album gave the XdW subwoofers
one heck of a workout. However, the
drum-and-unvoiced-guitar intro of
Eric Johnson’s “East Wes,” from his
live Austin City Limits CD, still man-
aged to shut down the left XdS
woofer amplifier if I wasn’t careful
with the volume control. (Setting the
NHT PVC’s control to –6dB, equiv-
alent to an in-room SPL of around
108dB, could trigger an amplifier
shutdown with this track.)
I listened again to the half-
step–spaced tonebursts on Editor’s
Choice. Despite the revised filters, I
could still hear the slight modula-
tion noise accompanying the
tonebursts. Probably not too much
should be made of this phenome-
non; I mention it only because I
had never heard it before with con-
ventional loudspeakers.
What about the high frequen-
cies? The veiling that had bothered
me was much reduced, the top
octaves having more air apparent.
Perhaps more significant was the
minimizing of a slight mid-treble
“shoutiness” that had limited maxi-
mum volume with the original fil-
ters. The presentation was first-rate
in its lack of coloration, superbly
stable and detailed imaging, excel-
lent retrieval of subtle reverbera-
tion tails, and overall top-to-bot-
tom integration. Wow!
Measurements: Fig.1 shows the
response of the Xd satellite (black
trace) and the XdW subwoofer
(green), as well as the individual
responses of the Xd tweeter (red)
and woofer (blue), with the XdA
crossover running the original digi-
tal filters. The crossover points lie
at 110Hz and 2.3kHz, and the
Linkwitz-Riley filter slopes are a
very steep 48dB/octave. A touch of
excess energy can be seen in the
mid-treble, and the top octave is a
little shelved off both on axis (red)
and in the 30° averaged response
(black). It also looks as if the tweet-
er’s ultrasonic dome resonance has
been notched out.
By contrast, fig.2 shows a similar
set of response curves taken with the
revised filters (NHT’s “150Hzcross-
over.flt” file). (Because of continued
wind and torrential rain during
October, the responses above 300Hz
were taken indoors at 40" instead of
outdoors at 50"; this affects the accu-
racy of the measurements in the
midrange but is otherwise inconse-
quential.) The filter slopes are still
48dB/octave, and the crossover
between the XdW subwoofer and
the XdS satellite can be seen to have
been shifted up to 150Hz, as speci-
fied. While the upper crossover
point looks very similar to that in
fig.1, the XdS drivers look better
integrated overall, with a flatter mid-
treble. There is also now slightly
more energy apparent between
7kHz and 15kHz, and the tweeter’s
ultrasonic resonance is unfettered by
a notch. The response on-axis (red
trace) now extends flat to the tweet-
er’s resonance at 27kHz, though the
HF unit’s limited dispersion above
15kHz results in less energy in this
region in the 30°-averaged curve
(black).
But it is the response in the room
that matters most. To investigate
how the Xd system behaved in my
listening room, I ran my usual test of
averaging 120
1
⁄
3
-octave power spec-
tra taken individually for the left and
right speakers in a window centered
on the position of my ears. The blue
trace in fig.3 shows the in-room
response of the Xd system with dual
subwoofers and the XdA’s original
digital filters. The lack of energy in
the 50Hz and 63Hz bands is endem-
ic to my room and
seating position.
But note the exten-
sion to 20Hz and
the very flat mid-
range, meeting
±1dB limits from
250Hz to 16kHz—
superb perfor-
mance for an in-
room, listening-
position measure-
ment. There is a
slight lack of
upper-bass energy,
though not nearly
as much as I was
expecting from my
auditioning.
The red trace in
fig.3 is the in-room
response taken in
an identical man-
ner with the 150Hz
crossover/EQ.
Overall it looks
very similar, but
there are detail dif-
ferences that corre-
late with the listen-
ing impressions.
There is more
energy in the
80Hz, 100Hz, and
125Hz bands, indi-
cating better inte-
gration between
the subwoofers and
satellites. That
www.Stereophile.com, January 2006
FOLLOW-UP
Fig.1 NHT Xd system, original XdA crossover filters, anechoic response on
axis at 50", averaged across 30° horizontal window and corrected for
microphone response (black), with the nearfield XdS woofer (black)
and XdW subwoofer (green) responses plotted below 300Hz, and the
individual XdS woofer (blue) and tweeter (red) responses.
Fig.2 NHT Xd system, revised XdA crossover filters, anechoic response on
axis at 50", averaged across 30° horizontal window and corrected for
microphone response (black), with the nearfield XdS woofer (black)
and XdW subwoofer (green) responses plotted below 300Hz, and the
individual XdS woofer (blue) and tweeter (red) responses.