Sonus Faber Minima FM2 loudspeaker Measurements
As might be expected from such a small speaker, the Sonus Faber's B-weighted sensitivity was on the low side, the same as the BBC LS3/5A: 82.5dB/W/m, a little lower than specified. As can be seen from the plots of impedance magnitude and phase (fig.1), however, the Minima is very easy to drive, even low-powered tube amplifiers being suitable. The port is tuned to a high 65Hz.
Fig.1 Sonus Faber Minima FM2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The acoustic crossover between the two drive-units is shown in fig.2, and it can be seen that there is a region about an octave wide where the tweeter and midrange/woofer overlap. Above crossover, the tweeter's on-axis response tilts up a little, while the woofer's output is a bit peaky at the top of its passband. To the left of fig.2 are the nearfield responses of the woofer and port. The over-damped woofer starts to roll off below a high 200Hz, with the port covering a narrow bandpass centered on 60Hz. The Minima is definitely a minimonitor compared with the other two speakers reviewed this month by LG. A couple of port resonances can be seen in fig.2, at 465Hz and 800Hz, but these are well down in level.
Fig.2 Sonus Faber Minima FM2, 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.
Averaged across a 30 degrees lateral window on the tweeter axis, the overall response (fig.3) is slightly ragged throughout the midrange and treble. LG did find its balance rather midrange-forward; this type of response is prone to become fatiguingly peaky at high levels, again as LG found during his auditioning.
Fig.3 Sonus Faber Minima FM2, 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.
LG noted that he found the Minima to sound a little rolled-off in the high treble. There's plenty of energy on-axis (fig.3), but to the sides of that axis, the speaker rolls off quite rapidly in the top two octaves (fig.4), which means that the room-reverberant field will be duller than it would be with either the Totem Model 1 or B&W Matrix 805, this contributing to LG's impression. In heavily furnished rooms, the Minimas might sound too dull, therefore. Vertically (fig.5), while the Sonus Faber is slightly more fussy than the other two speakers regarding its optimum listening axis—that overlap between the drive-units—it still is not too critical as long as you listen on or below the tweeter axis.
Fig.4 Sonus Faber Minima FM2, 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.
Fig.5 Sonus Faber Minima FM2, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-7.75 degrees above axis, reference response, differences in response 7.5 degrees-22.5 degrees below axis.
Turning to the time domain, the Minima's impulse response on the tweeter axis (fig.6) doesn't appear to be as time-coherent as the crossover specification might suggest, this reinforced by the step response (fig.7), which shows a positive-polarity tweeter followed a fraction of a millisecond later by an inverse-polarity midrange/woofer. The cumulative spectral-decay, or "waterfall," plot calculated by the MLSSA system from the impulse response can be seen in fig.8. While the decay of the impulsive sound is overall quite clean, there is some residual woofer character noticeable around 2.5kHz, and a tweeter mode at 16kHz, though this will be too high for most people to hear.
Fig.6 Sonus Faber Minima FM2, impulse response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).
Fig.7 Sonus Faber Minima FM2, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).
Fig.8 Sonus Faber Minima FM2, cumulative spectral-decay plot at 45" (0.15ms risetime).
Finally, though the Minima's superbly rigid cabinet construction doesn't eliminate cabinet resonances, it does push them up in frequency to where they will have less effect on the music. Measured with a PVDF accelerometer, the main sidewall mode, for example, though high in level at -4dB for a 7.5V input, lay at an innocuous 390Hz.—John Atkinson