Monitor Audio Silver S2 loudspeaker Measurements
The Monitor Audio Silver S2's sensitivity was slightly above average, at an estimated 88dB(B)/2.83V/m, but this is a significant 3dB lower than the specified figure. The speaker's plot of impedance magnitude and electrical phase (fig.1) shows that it drops to 4 ohms in the midbass and lower midrange, but is otherwise a reasonably easy load for the partnering amplifier to drive. The saddle centered at 42Hz in the magnitude trace reveals the tuning frequency of the rear-facing port, while the small blip in the curve just below 20kHz suggests that the metal-dome tweeter's diaphragm resonance lies a little lower in frequency than is usual for the type.
Fig.1 Monitor Audio Silver S2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Although not visible at the scale this graph is reproduced in the magazine, some minor glitches in the traces between 250Hz and 600Hz indicate the presence of cabinet vibrational resonances. However, with the exception of one mode at 560Hz, which could be detected at its strongest on the sidewall (fig.2), these were all low in level. The 560Hz mode is high enough in frequency that its audibility will be questionable. However, listening to the cabinet sidewalls with a stethoscope revealed a mode around 260Hz to be continually excited on music, which might lead to the lack of lower-midrange clarity I commented on.
Fig.2 Monitor Audio Silver S2, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
Also of concern in fig.1 was the sharp notch at 1.5kHz in the impedance-magnitude trace, which implies that something peculiar is going on. The double-humped nature of the curve in the region of the crossover frequency also suggests the presence of either some sort of equalization network or, more likely, a high-order filter such as an elliptic.
Looking at the individual responses of the drive-units and the port (fig.3), the woofer's output features a sharp notch between 4kHz and 5kHz, above which a series of peaks mars an otherwise smooth rollout. Nothing untoward can be seen at 1.5kHz, but a broad overlap between the woofer and tweeter outputs is evident between 2kHz and 10kHz. The port's behavior in fig.3 is generally clean, though some energy is apparent in the upper midrange. Fortunately, the port faces away from the listener, so this behavior should not result in any coloration.
Fig.3 Monitor Audio Silver S2, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the woofer and port plotted below 300Hz and 1.5kHz, respectively.
The inevitable metal-dome tweeter peak occurs just below 20kHz, as suggested by the impedance graph. While this will not be an issue for anyone but bats and very young humans, it will be excited some of the time, which might lead to some lower-frequency problems. (When a metal-dome tweeter's resonance lies above 22kHz, the absence of any energy at those frequencies with CD playback means that it is unlikely to be energized.)
The trace above 300Hz in fig.4 shows how these individual responses sum on the S2's tweeter axis in the farfield. Below 300Hz, the graph shows the complex sum of the nearfield responses (added together, taking acoustic phase and the different distance of the woofer and port from a nominal farfield reference point). The low-frequency -6dB point coincides with the port tuning frequency of 42Hz, but a slight energy excess in the upper bass is visible. Together with the overall depressed level of the tweeter, this will give rise to the warm, rather mellow balance I heard.
Fig.4 Monitor Audio Silver S2, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.
Note the peak at 4kHz. This was not apparent in the individual drive-unit outputs, but is real nevertheless, and is evident in Monitor Audio's own measurements of the Silver S2. I wonder if it correlates with the touch of mid-treble "bite" I occasionally noticed.
Lateral dispersion (fig.5) was fairly even, the 7" woofer beaming a little lower in frequency than the smaller-diameter units usually used in two-way designs, and an off-axis flare apparent between 5kHz and 6kHz. The tweeter's top octave is also suppressed to the sides, adding to the rather airless highs. In the vertical plane (fig.6), large suckouts develop in the crossover region if the speaker is listened to much above or below the tweeter axis, and, again, the tweeter gets quite beamy in its top octave. Even so, the Silver S2's spatially averaged room response (fig.7) is remarkably even for such an inexpensive speaker. The slightly suppressed tweeter balance is evident in this graph, as is the somewhat elevated lower midrange, both correlating nicely with my impressions of the speaker's tonal balance.
Fig.5 Monitor Audio Silver S2, lateral response family at 50", 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.6 Monitor Audio Silver S2, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45 degrees-5 degrees above axis, reference response, differences in response 5 degrees-45 degrees below axis.
Fig.7 Monitor Audio Silver S2, spatially averaged, 1/3-octave response in JA's listening room.
The Silver S2's step response (fig.8) holds no surprises. Both drivers are connected in positive acoustic polarity and some ringing due to the 20kHz tweeter resonance is apparent. The cumulative spectral-decay plot on the tweeter axis (fig.9) is marred by ridges of delayed energy at 3.9kHz, 5.4kHz, 7.8kHz, and 20kHz, all but the last presumably stemming from the woofer's metal cone.—John Atkinson
Fig.8 Monitor Audio Silver S2, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 Monitor Audio Silver S2, cumulative spectral-decay plot at 50" (0.15ms risetime).