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-

N H T X d

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. Although each component was developed by a different specialist company—speakers by NHT, amplifiers by PowerPhysics, EQ/cross- over by DEQX—it is an integrated active system that does not allow easy substitutions of other gear.

M E A S U R E M E N TS

Their 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 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. 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 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.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.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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

Fig.4 NHT XdS, cumulative spectral-decay plot of unequalized woofer at

 

 

bandwidth, 2kHz).

 

50” (0.15ms risetime).

 

 

 

 

 

 

 

 

 

 

 

 

 

www.Stereophile.com, November 2005