Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Focal Sopra No.3's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. (I measured serial number '000325.)
The Focal will play loudly when fed only a few watts, my estimate of its voltage sensitivity being 89.2dB/2.83V/m. However, as fig.1 shows, the Sopra No.3 is a difficult load for an amplifier to drive, with a minimum magnitude of 2.75 ohms at 96Hz and a combination of 4 ohms and a –56° electrical phase angle at 68Hz, both frequencies where music can have high levels of energy. Because the magnitude is considerably higher in the treble, the speaker will sound brighter when driven by tubed amplifiers, with their typically high output impedances.
The Sopra No.3's plot of lateral dispersion, normalized to the response on the tweeter axis (fig.4), reveals that the speaker's off-axis behavior is well controlled. However, a slight excess of energy at the bottom of the tweeter's passband might make the speaker sound a little bright in small or undamped rooms. With the wide baffle, it's not surprising to see the Sopra No.3's top-octave output become relatively directional. I also suspect that its wide baffle results in the relatively restricted soundstage depth that Kal Rubinson noted. In large or overdamped rooms, the speaker might sound a bit lacking in "air." In the vertical plane (fig.5), the Sopra No.3 maintains its tonal balance over a ±5° window centered on the tweeter axis, which is a sensible 38" from the floor.
Turning to the time domain, the step response on the tweeter axis (fig.6) indicates that the tweeter and midrange drive-units are connected in inverted acoustic polarity, the woofers in positive polarity. I checked this by applying a 9V battery to the speaker terminals: both woofer cones moved forward, as expected from the step response. The cumulative spectral-decay plot (fig.7) reveals a relatively clean decay in the upper midrange and treble, though with some interference effects and low-level delayed energy visible.
Fig.1 Focal Sopra No.3, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.).
The traces in fig.1 are free from the small discontinuities that would suggest the presence of panel resonances. However, there was a strong mode at 668Hz on the sidewalls level with the midrange drive-unit (fig.2). This is too high in frequency, and of too high a Quality factor (Q), to have audible consequences. There were other modes at lower frequencies on the sidewalls adjacent to the woofers (not shown), but these were at a much lower level and, again, will probably have no audible consequences.
Fig.2 Focal Sopra No.3, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel adjacent to midrange unit (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The two woofers behave identically in having a minimum-motion notch in their outputs at 32Hz (fig.3, blue trace), as predicted by the impedance plots. The downward-firing port (red trace) behaves in textbook fashion, its bandpass covering the range of 20–70Hz. Its output is free from any midrange peaks. The woofers are crossed over to the midrange drive-unit (green trace) just below 200Hz, and the combined response of the midrange and tweeter in the farfield is impressively flat (black trace above 300Hz). Though the Focal's overall nearfield response (black trace below 300Hz) peaks by up to 4dB in the mid- and upper bass, this is almost entirely an artifact of the nearfield measurement technique, which assumes a 2pi or "half-space" acoustic environment for the drive-units. The Sopra No.3 is tuned to have a maximally flat low-frequency response, with the –6dB point at the port tuning frequency of 32Hz. In-room, with the usual boundary reinforcement, the Focal will have a flat response down to 20Hz.
Fig.3 Focal Sopra No.3, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of midrange unit (green), woofers (blue), and port (red) plotted in the ratios of the square roots of their radiating areas, and complex sum of nearfield responses plotted below 300Hz (black).
Fig.4 Focal Sopra No.3, lateral response family at 50", 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.
Fig.5 Focal Sopra No.3, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, difference in response 5° below axis.
Fig.6 Focal Sopra No.3, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Focal Sopra No.3, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Overall, this big speaker measured very well.—John Atkinson
