Sonus Faber Electa Amator loudspeaker E le misure?

Sidebar 2: E le misure?

The Amator's plot of impedance magnitude and phase vs frequency (fig.1) reveals an easy-to-drive reflex design. The port tuning, indicated by the saddle between the two bass peaks, lies at 40Hz, supporting JE's positive feelings about the speaker's low-frequency extension. Though a slight wrinkle in the impedance amplitude trace at 480Hz might be thought to correlate with an enclosure resonance problem, the Amator's cabinet was relatively inert. The main side- and top-panel modes lay at 360Hz but were well down in level.

Fig.1 Sonus Faber Amator, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Moving to the time domain, looking at the impulse responses of the individual drive-units revealed them to be connected in phase, as might be expected for a design with a first-order crossover. The Amator's overall impulse response on the tweeter axis (fig.2) and the step response (fig.3), however, suggests higher-order filters, as does the acoustic crossover (fig.4), which also implies a crossover frequency nearer 2kHz than 2.8kHz. While the tweeter's response appears smooth, if a little uptilted in the high treble, that of the woofer near the top of its passband is somewhat lumpy, which might well tie in with the nasality noted by JE.

Fig.2 Sonus Faber Amator, impulse response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

Fig.3 Sonus Faber Amator, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

Fig.4 Sonus Faber Amator, acoustic crossover on tweeter axis at 45", corrected for microphone response, with the nearfield responses of the woofer and port plotted below 300Hz and 1kHz, respectively.

This lumpiness can also be seen to the right of fig.5, which shows the speaker's response on the tweeter axis averaged across a 30 degrees horizontal window. (Both this response and those in fig.3 have been corrected for the measuring microphone's own departure from a flat response.) Despite this, however, the overall tonal balance trend is smooth and relatively flat throughout the midrange and low treble, spoiled only by a slight plateau in the top octave. While I found this audible in my own auditioning of the Amator as a slight emphasis to tape and microphone hiss and rather exaggerated vocal sibilants, it was not enough to interfere with the music and, indeed, added a degree of "air" to the speaker's sound.

Fig.5 Sonus Faber Amator, anechoic response on tweeter axis at 45", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

To the left in fig.5 are shown the individual responses of the port and woofer, taken in the nearfield (ie, with the microphone just 1mm away from the woofer dustcap or port opening). The level matching between these two curves and the quasi-anechoic curve to their right can only be approximate, but they reinforce JE's feeling that the Amator's bass went quite low.

One alarming thing to note is the peak in the port's output just above 600Hz. As can be seen from the cumulative spectral-decay or "waterfall" plot of the port's output (fig.6), this is due to a resonance of some sort, possibly a pipe mode of the reflex port itself. The fact that the port is on the speaker's rear will probably minimize the effect of this resonance on music. Indeed, JE noted no veiling or one-note character in the midrange that could be laid at the feet of this behavior.

Fig.6 Sonus Faber Amator, cumulative spectral-decay plot calculated from the nearfiled port output.

The wide-range waterfall plot for the Amator (fig.7) shows an exceptionally fast, clean decay in the treble: this is one good tweeter! The frequency resolution of this graph is not sufficient to do more than hint at problems in the midrange. Nevertheless, with the exception of a couple of possible modes near 1kHz and 2kHz which might lend the sound a slight nasality, the Amator appears to be an excellent performer here.

Fig.7 Sonus Faber Amator, cumulative spectral-decay plot at 45" (0.15ms risetime).

Jack noted some criticality to the speaker's vertical listening axis, preferring the sound with his ears level with the woofer. I explored this aspect of the speaker's behavior in fig.8, which shows the changes in measured response to be expected on different axes compared with those on the tweeter axis. (This is shown as a straight line—there are no changes.) By comparison with fig.4, it can be seen that listening too high results in a hollow-sounding low treble, due to a suckout appearing in the crossover region. Otherwise, fig.8 implies that Amator is not critical regarding the listening height, which is at odds with JE's auditioning. This is also at odds with the specified 6dB/octave crossover slopes, as first-order crossovers tend to result in a vertically critical listening axis at this close a microphone position.

Fig.8 Sonus Faber Amator, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-7.5 degrees above axis, reference response, differences in response 7.5 degrees-22.5 degrees below axis.

Horizontally, as can be seen from fig.9, the Amator's dispersion is also well-controlled, moving to the speaker's side producing an increasing amount of top-octave rolloff and a slight boost in the mid-treble. If the Amator's on-axis excess of extreme HF is problematic in your system, not toeing them in all the way to the listening seat will usefully tame it. All in all, these excellent measured results confirmed both JE's and my good opinions of this little gem.—John Atkinson

Fig.9 Sonus Faber Amator, lateral response family at 45", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

Sonus Faber
US distributor: Sumiko
2431 Fifth Street
Berkeley, CA 94710
(510) 843-4500
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