Sonus Faber Minuetto loudspeaker Measurements

Sidebar 3: Measurements

The Sonus Faber's B-weighted sensitivity, at a calculated 85dB/W/m, was moderate for a small speaker. Its impedance (fig.1) is kind to amplifiers, remaining above 6.5 ohms at low frequencies, and above 12 ohms in the treble. The port is tuned to 42Hz, the frequency of the saddle between the two low-frequency impedance peaks, though the lower peak is much lower in frequency than is usually the case.

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

The individual response plots for the drivers and port (fig.2) reveal the port output—the bandpass centered on 50Hz—to be significantly down in level from that of the woofer. It also has a significant peak in its output at 900Hz, though this will probably have no subjective consequences, due to its low level and the fact that the port faces the rear. The woofer's output is humped up in the upper bass, though relatively flat through the midrange and rolling off smoothly above 2kHz. The tweeter comes in slowly between 1 and 3kHz, rising to its maximum output level above 8kHz.

Fig.2 Sonus Faber Minuetto, acoustic crossover on tweeter axis at 45" corrected for microphone response, with nearfield woofer and port responses below 300Hz and 1kHz, respectively.

The integration of these drive-unit responses on the tweeter axis at a microphone distance of 45" and averaged across a 30° horizontal window is shown in fig.3. The midrange and lower midrange are shelved-up by up to 5dB, while the low treble is shelved-down by 2dB. Here is much of the reason for Corey's finding the Sonus Faber to have a "polite," over-dull balance. The relative lack of energy in the presence region (an octave below the suckout in the Audio Physic Step's mid-treble, a speaker reviewed by Jack English elsewhere in this issue) will result in a balance that will be too uninvolving for many tastes. The hump up in the midbass, however, will endow the Minuetto with impressive low frequencies, as CG commented.

Fig.3 Sonus Faber Minuetto, anechoic response on tweeter axis at 45" averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses below 300Hz.

It's possible that the Minuetto's limited HF dispersion also contributes to its mellow balance. Fig.4, which shows just the differences to be found off-axis to the speaker's sides, reveals that the off-axis fall-off in the Minuetto's tweeter output occurs more rapidly than is normal with a 1" dome tweeter. This will give the listening room's reverberant field an HF-less balance which will exacerbate the recessed on-axis sound quality.

Fig.4 Sonus Faber Minuetto, horizontal response family at 45", normalized to response on tweeter axis, from back to front: differences in response 90°–5° off-axis; reference response; differences in response 5°–90° off-axis.

Corey remarked upon the Minuetto's sensitivity to vertical listening height due to its use of first-order crossover filters. This is confirmed by its vertical-dispersion graph (fig.5). As well as the top octaves rolling off more than a few degrees off the tweeter axis, the presence-region depression is exaggerated by a crossover suckout appearing both above and below this axis. In addition, if you sit too high, the upper midrange becomes exaggerated in level; sit too low and the upper midrange starts to develop a suckout.

Fig.5 Sonus Faber Minuetto, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 45°–5° off-axis above; reference response; differences in response 5°–45° off-axis below.

In general, a designer will choose to use first-order filters to give time-coherent performance. However, with conventional dynamic drive-units, this will also involve the tweeter having to be physically stepped-back somehow, due to its smaller chassis depth. This is normally achieved with a sloping baffle, as in the Spica and Thiel designs, or by staggering separate driver modules (Vandersteen and Dunlavy Audio Labs). Without either of these strategies being adopted, the speaker will not be time-coherent, as is revealed by the Minuetto's step response (fig.6). Both drivers have the same positive acoustic polarity, but the tweeter's output pulse leads that of the woofer by 300µs or so. The good news is that the Sonus Faber's cumulative spectral-decay, or waterfall, plot (fig.7) is very clean, with only some low-level hash present in the treble.

Fig.6 Sonus Faber Minuetto, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

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

Finally, while the speaker's solid-feeling cabinet has a couple of resonant modes noticeable—fig.8 shows a waterfall plot calculated from the impulse response of a simple PVDF-tape accelerometer fastened to the center of the rear panel—these are high enough in frequency that they should not have too much of a subjective effect.—John Atkinson

Fig.8 Sonus Faber Minuetto, cumulative spectral-decay plot of accelerometer output fastened to center of enclosure back panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth 2kHz.)
Sonus Faber
US distributor: Sumiko
3101 Telegraph Ave.
Berkeley, CA 94705
(510) 843-4500

PredatorZ's picture

For a moment or two I was happy to think that Corey was back writing for Stereophile, but sadly this was just a reprint. I always enjoyed Corey's irreverent style and sense of humor. The good old DIY amps and PBJ days... good times !

Nellomilanese's picture

Systema di reviewini

That was funnycool and i'm italian :D