The NHT Xd’s are “digital” speakers; they do crossover functions using digital signal processing (DSP). The Xd’s are far from being the first speakers that are digital in this sense—models from Philips, Quadrature, and, on a continuing basis, Meridian have been around for many years. But digital speakers are still something of a novelty; moreover, the Xd’s are set apart from their digital predecessors by a combination of factors: ease of setup (no computer science degree required), elegant appearance, relatively moderate price (considering that amplification is included), and fine sonic performance. The Xd’s look like a “lifestyle” system, but they sound very high end, indeed. (The internal functions of the Xd’s crossover/amplifier unit are digital, but the only input is analog. The first thing that happens is an A-to- D conversion to 96/24 digital. This is audibly transparent in my opinion.)
Digital crossovers offer an option that is not available in the analog domain. In particular, really steep crossover slopes can be implemented without phase nonlinearity problems. The Xd’s use 48 dB/octave slopes, but their impulse response is excellent. And the minimal driver overlap gives a surprisingly small variation of frequency response with change of listener position.
Moreover, what I think of as grunge level is very low. Drivers are best behaved when not asked to operate outside their normal bandwidth, and the Xd’s are extraordinarily free of the sonic junk which can arise from the residual out-of-band activity of drivers without steep slope crossovers. (Mid-drivers that are still getting a lot of signal at 8kHz seldom put out nice sounds there!) In this sense, the Xd’s are like electrostatics: grunge-free and very transparent. Have a listen to something involving a large chorus, such as Reference Recordings’ Rutter Requiem. The clarity of enunciation is striking. “Requiem eternam” and the like—on some speakers the final “m” and other such endings are all but lost, and one almost hears them only because one knows they are there (at least if you have sung in church choirs, you do). Here they are nicely nicely enunciated, without being exaggerated. This feeling of hearing into the textures of things to the details is uniform on all materials— and addictive.
The steep-slope crossovers also enable the Xd’s to play far more loudly than one would expect of such small speakers. The satellites have only 4.5" drivers, but they will blast away without strain when they are asked to.
Incidentally, driver integration from the mid to the tweeter is very good but not quite perfect to my ears. I do not hear the drivers separately as such, but there seems a slight shift in the character of the sound at the transition point. However, this is very subtle. It is a lot less observable than, say, the dead spot in the power response of the all-toonearly- ubiquitous 24dB-per-octave Linkwitz-Riley crossover.
DSP allows complete adjustment of frequency response, and, naturally enough, the Xd’s have been made very smooth on-axis (a little toppy, however, for some reason, a deliberate choice perhaps). And they are neutral in a broad sense—specific detailed character momentarily. (There is an adjustment for close-to-the-wall placement and for woofer level, but otherwise the speaker is fixed in behavior, no DSP adjustments.) In my two listening rooms, I much preferred the two-woofer, highercrossover setup. The Xd’s are available as a system with two satellites/one woofer or, at a slightly higher price, with two woofers. The two-woofer system can use a 150Hz crossover without stereo problems (strictly speaking, one could use this crossover with one woofer, but a lower crossover offered makes surer there is no interference with stereo effects). In my rooms, the two-woofer, higher crossover was much more naturally balanced and convincing-sounding.
With the two-woofer setup installed and after some careful work on placement, I got good integration of satellites and woofers, and a quite even room response. But—and here we get to the “character” referred to above—the system sounded a bit on the lean side with a slightly projected mid, relatively speaking. People used to electrostatics will feel right at home, since they are often similarly lean in the 150–200Hz octave. I want to emphasize that this is not a big effect, but it is there. Music sounds very transparent with the Xd’s, but it is not very warm and full. (I know, I know, I am obsessed with Wagnerian warmth— my dogs are not named Freja, Brunnhilde, and Sieglinde for nothing!) I think what goes on here is that the mid-driver in the satellites runs out of steam a bit before the hand-over to the woofer. This seems a “room sound” effect: measured anechoically, the satellites are flat to the bottom of their operating range. The crossover from woofer to satellite mid-driver is quite steep, too, so turning up the woofer will give you potentially too much bass without filling in the 150–200 region all that much. Lean sound is pretty much just what you get. The deep bass further down is good though, and has lots of dynamic punch. (Even the legendary “Sanctus” on the Rutter did not ruffle the woofers.)
The Xd’s have extremely wide and uniform radiation pattern. Only at the extreme top end is there any real narrowing of the pattern. So there is a lot of room sound and it has a quite uniform energy with respect to frequency. Even in my very damped room, the steadystate “RTA” response is flat to around 10kHz. In fact, there is more response at, say, 6k than in the (slightly lean) 150–200Hz region.
This contrasts with what has been historically many people’s ideal of room response, one that rolls gradually from bottom to top. Ironically, this whole matter of rolled-off room response has been brought emphatically to the fore by DSP itself, by room-correction systems and the perceived need for sloping “target curves” when RTA measurements are used. The whole logic of declining diffuse-field response to off-set the ear’s (relatively) rising diffuse-field response and so on has thus been dragged out of the theoretical closet and into the marketplace. (This theory is discussed in my “Records and Reality” on www.regonaudio.com.) Like it or not, the Xd’s sound very much like a widedispersion speaker TacT-corrected to a flat target curve—with no TacT correction in sight. It is impressive how flat they are in this sense, but is it really ideal? Your call….
The wide dispersion of the Xd’s also has an effect on their imaging. The contrast with a pair of highly directional electrostatics at hand was considerable. On Let No Man Write My Epitaph [Classic Records gold CD reissue], the panels gave Ella Fitzgerald singing into a microphone—what is really recorded. On the Xd’s, one heard her singing as if in one’s own room—musically nice but not what is actually recorded. Similarly, on the Waterlily Mahler Fifth, the “soundstage” was larger with the Xd’s, but there was less sense of hearing into the original recording venue. This is just a choice: Beamy speakers present stereo differently from speakers that fill your own room more with sound and bounce more off the walls (this controversy goes back forever in TAS!).
Speaker reviews usually amount to seeing how well a design meets its goals. But this review turned inevitably into a discussion of the goals themselves precisely because the Xd’s seem so nearly perfectly to be what they were intended to be. They sound almost exactly the way a point-source speaker with flat response, uniformly wide radiation pattern, and very low distortion would be expected to sound. Besides the slight leaning out of the 150–200Hz region and the feeling of a bit of extra energy in the 4–8kHz octave, there was little to criticize in the context of what was apparently intended. But whether this particular engineering ideal is the ideal that you are pursuing, whether this somewhat light-weight, top-oriented sound (relative to other speakers) is what sounds musically natural to you or even whether the ideal makes theoretical sense to you are questions for you to answer for yourself.
This is a speaker everyone who is serious about audio needs to hear for the near-perfection with which it realizes one ideal of speaker behavior. If clarity, transparency, and large soundstage are dominating goals for you in audio, I think you may fall in love. If warmth, fullness, and hearing into the recording venue are your goals, perhaps not. I would really like to hear what NHT would do with a digital system where elegant appearance was not a consideration, and ease of operation was supplemented by some DSP room adjustments. I call for an NHT monster—sound above everything! But the sound of the Xd’s as they are is not to be sneezed at.