Floor Loudspeaker Reviews

I measured the Estelon XB Diamond Mk II in Jim Austin's listening room. I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the farfield frequency response and an Earthworks QTC40 microphone for the nearfield and in-room responses. As the Estelon speaker weighs 150lb, I wasn't able to lift it off the floor to raise the measurement axis sufficiently high to move the early reflections back in time. (The tweeter is 36" from the floor.) Instead, I used a small jack that Wilson Audio Specialties had loaned JCA for his December 2021 review of the Alexx V loudspeaker; still, the presence of reflections (see later) meant that the FFT-calculated response had limited resolution in the midrange.

Estelon specifies the XB Diamond's voltage sensitivity as 87dB/2.83V, presumably at 1m. My B-weighted estimate was lower, at 84.4dB(B)/2.83V/m. I used Dayton Audio's DATS V2 system to measure the loudspeaker's impedance. The XB Diamond's impedance is specified as 6 ohms, with a minimum value of 3.5 ohms at 50Hz. The impedance magnitude (fig.1, solid trace) is higher than 6 ohms for almost the entire audioband, with a minimum value of 3.1 ohms at 53Hz. The electrical phase angle (dotted trace) is occasionally high, causing the effective resistance (EPDR, footnote 1), which is calculated from the combination of magnitude and phase angle, to drop below 3 ohms between 18Hz and 36Hz, between 49Hz and 82Hz, and between 2.23kHz and 4.33kHz. The lowest values of the EPDR are 1.24 ohms at 59Hz and 1.93 ohms at 3.07kHz. Owners of the Estelon XB Diamond should match the speakers with amplifiers that won't be fazed by low impedances.

The impedance traces are free from the small wrinkles that would suggest cabinet resonances. Using a plastic-tape accelerometer, I found some low-level modes between 500Hz and 750Hz on all the enclosure's surfaces (fig.2). However, the low levels and relatively high frequency and Q (Quality Factor) of these modes make it unlikely that they'll have significant sonic consequences.

The saddle centered at 24Hz in the impedance magnitude trace suggests that the flared port on the rear panel is tuned to that frequency, which implies deep low-frequency extension. The port's output, measured in the nearfield (fig.2, red trace), peaked in that region but had a second peak at the same frequency where the woofer's nearfield output (fig.3, blue trace) was at its maximum. The port's output rolled off rapidly above 50Hz.

The woofer's nearfield response had the expected minimum-motion notch at the port tuning frequency and featured a very narrow passband, crossing over acoustically to the midrange unit (fig.3, green trace) at 65Hz. The crossover frequencies are not specified on Estelon's website, but the impedance peak at 95Hz suggests that the electrical woofer/midrange crossover is close to this frequency.

The black trace in fig.3 below 300Hz shows the complex sum of the XB Diamond's nearfield port, woofer, and midrange responses, taking into account their radiating areas, acoustic phase angles, and the fact that the port is on the rear panel. The midbass peak is due in part to the nearfield measurement technique, but the Estelon's low frequencies don't quite extend to the port-tuning frequency at full level. Higher in frequency in fig.3, the black trace shows the Estelon's farfield response averaged across a 30° horizontal window centered on the tweeter axis at 50". The trend is even up to 10kHz, though with a small lack of energy in the midrange and a slight excess in the octave above 4kHz. The response gently slopes down in the top audio octave. As with the Estelon Forza, which MF reviewed in October 2021, the XB Diamond's use of a pistonic tweeter with a high-Q ultrasonic dome resonance results in a lack of energy at frequencies just below that resonance.

To examine the loudspeaker's horizontal dispersion, I moved the microphone by 5° intervals up to 45° to the side of the tweeter axis. The Estelon XB Diamond maintains its spectral balance below 10kHz to the sides of the tweeter axis (fig.4), with the smooth, evenly spaced contour lines in this graph implying accurate, stable stereo imaging. (Reflections from the room's sidewalls will have the same spectral balance as the direct sound.) The dispersion plot in the vertical plane (fig.5) indicates that the tweeter-axis response is maintained over a wide ±10° window.

To examine the Estelon XB Diamond's spatially averaged in-room response (fig.6, red trace), I averaged 20 1/10-octave–smoothed spectra, taken with MLSSA for the left and right speakers individually, in a rectangular grid 36" wide × 18" high and centered on the approximate positions of JCA's ears. For reference, the blue trace in fig.6 shows the in-room response taken under identical conditions with the Wilson Alexx V. The two speakers excite low-frequency room resonances to a similar degree and behave similarly between 200Hz and 3kHz. Above that frequency, the Estelons have significantly more mid-treble energy than the Wilsons, which I could hear with the pseudo-random MLSSA noise signal as a touch of brightness. The responses of both speakers gently slope down above 5kHz due to their tweeters becoming more directional as the frequency increases and to the increased absorption by the room furnishings in the treble. (What you don't want to see with a spatially averaged in-room response is a flat treble output, which will sound excessively bright/shrill.)

In the time domain, the step response on the tweeter axis at 50" (fig.7) reveals that the tweeter and midrange unit are connected in positive acoustic polarity, the woofer in negative polarity. (I confirmed this by looking at the individual step responses of each unit.) The decay of the tweeter's step smoothly blends with the positive-going start of the midrange unit's step; the decay of the midrange unit's step blends smoothly with the negative-going start of the woofer's step. The early reflections of the upper-frequency drivers' outputs that I mentioned earlier can be seen just after the 7ms mark in this graph.

Finally, the cumulative spectral-decay or waterfall plot at 50" on the tweeter axis (fig.8) shows a clean initial decay, but there is then some low-level hash present in the mid-treble region. Again, the XB Diamond behaves similarly in this respect to the Estelon Forza.

The Estelon XB Diamond Mk II's measured performance is very good overall, though that excess of mid-treble energy may complicate setup and system optimization.—John Atkinson