Wilson Audio Specialties Sophia loudspeaker Measurements

Sidebar 3: Measurements

Other than impedance, for which I used an Audio Precision System One, all acoustic measurements were made with the DRA Labs MLSSA system and a calibrated B&K 4006 microphone. To minimize reflections from the test setup, the measuring microphone is flush-mounted inside the end of a long tube. Reflections of the speaker sound from the mike stand and its hardware will be sufficiently delayed not to affect the measurement.

My estimate of the Sophia's voltage sensitivity came in a little below the specified figure, at 88.3dB(B)/2.83V/m, but the difference is negligible. The speaker's impedance drops to a minimum value of 3.26 ohms at 220Hz (fig.1), but the electrical phase angle is very low at the same frequency, meaning that a good 4-ohm-rated amplifier will have no problem driving the speaker. The phase angle does reach a fairly high value of 40 degrees, capacitive, in the midbass, but the impedance is high enough in the same region to ameliorate any drive difficulty.

Fig.1 Wilson Sophia, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Other than a wrinkle at the tweeter's ultrasonic resonance frequency, the traces in fig.1 are free from the glitches that would indicate the presence of mechanical resonances in the enclosure. I investigated the panels' vibrational behavior with a plastic-tape accelerometer and found little that could be considered sonically significant. As shown by fig.2, a waterfall plot calculated from the accelerometer's output when it was attached to the back panel, there were two modes present, at 360Hz and 539Hz, but these are low enough in level and high enough in frequency to be not worth worrying about.

Fig.2 Wilson Sophia, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's rear panel level with the midrange/woofer transition. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The saddle centered at 26Hz in the magnitude trace indicates the tuning frequency of the big, rear-facing port, which in turn implies good low-frequency extension. The colored traces in fig.3 show the nearfield responses of the lower port (green), the woofer (red), and the midrange unit (blue). (I haven't shown the response of the upper port, as it appeared to be a clone of the midrange unit's output but lower in level.) The minimum-motion point in the woofer's output is a little higher in frequency than I expected from the impedance graph, and, most unusually, the midrange unit has a notch in its output at almost the same frequency. (This is well down in level, however, due to the action of the crossover.) The port itself covers the bandpass between 18Hz and 70Hz, but its output is a little suppressed compared with the woofer, which peaks up sharply between 50Hz and 100Hz.

Fig.3 Wilson Sophia, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses (black), the nearfield midrange response (blue), the nearfield woofer response (red), and the nearfield port response (green) plotted below 300Hz, 500Hz, 1kHz, and 500Hz, respectively.

Because of this peakiness, the Sophia's overall output in the bass (fig.3, black trace below 300Hz) is boosted somewhat in this midbass octave. This woofer tuning will lead to problems in some rooms—Martin Colloms noted a problem with it in his review of the Sophia in the May 2002 issue of Hi-Fi News—but it was not an issue in my room, which has a general lack of energy in the 63Hz band. Fig.3 also looks a little worse than it should because of the 3dB boost given low frequencies by the nearfield measuring environment compared with a true anechoic measurement.

The acoustic crossover frequency between the midrange unit and the woofer appears to be set at around 150Hz. The inverted-dome tweeter has a small rise apparent above 15kHz, but rolls off sharply above the audioband, while the speaker's overall response in the midrange and treble is basically flat, though broken up by small peaks and dips. As these tend to be equally spaced, with peaks balanced by dips, the perceived balance will tend to be neutral.

Aiding this is the fact that dips tend to fill in and the peaks become suppressed to the speaker's sides (fig.4). In typical rooms, therefore, the Sophia will tend to sound evenly balanced in the highs. In the vertical plane (fig.5), the speaker's balance doesn't change much as long as the listener sits with his or her ears below the top of the enclosure. Stand, however, and a large suckout appears at the upper crossover frequency, which appears to be just under 2kHz.

Fig.4 Wilson Sophia, 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.5 Wilson Sophia, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees$bn5 degrees above axis, reference response, differences in response 5 degrees-10 degrees below axis.

In-room, the Sophia's spatially averaged response (fig.6) is impressively smooth and flat, though a slight excess of presence-region energy is apparent at the bottom of the tweeter's passband. I suspect that this contributes both to the speaker's clarity and to its impatience with less-than-stellar recorded balances. The slightly sweet top octave I noted in my auditioning is probably associated with the tweeter's limited dispersion above 12kHz. Note that this graph reveals the Sophia to offer superb low-frequency extension in my room, which does offer good support in the lowest audio octave, and that the nearfield excess in the Wilson's midbass is not apparent.

Fig.6 Wilson Sophia, spatially averaged, 1/3-octave, freefield response in JA's listening room.

In the time domain, the Sophia's step response (fig.7) reveals that its tweeter and woofer are connected in positive acoustic polarity, the midrange in inverted polarity—which is what is needed, in conjunction with the phase shift provided by the crossover, to ensure that the outputs of the drive-units add to give a flat response in the farfield in the crossover regions.

Fig.7 Wilson Sophia, on-axis step response at 50" (5ms time window, 30kHz bandwidth).

The cumulative spectral-decay plot (fig.8) is generally clean. Though some delayed energy is apparent through the presence region, it doesn't appear as the ridges typical of resonant behavior, which suggests that it might instead be due to reflections.

Fig.8 Wilson Sophia, cumulative spectral-decay plot at 50" (0.15ms risetime).

All in all, this is excellent measured performance for which no apologies need be made, though the woofer alignment suggests that the Sophia will work better in some rooms than others.—John Atkinson