Focal Sopra No.3 loudspeaker Measurements

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.

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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.

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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.

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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).

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.

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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.

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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.

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.

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Fig.6 Focal Sopra No.3, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

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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

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COMMENTS
Supperconductor's picture

The older I get the more I hear the room in most systems. I can't help but wonder if judicious use of room treatment/EQ/correction might be a topic you guys could give more attention as part of your speaker reviews.

Given all the time, expense, and attention paid to electronics, system matching, cables, tweaks, etc., it seems to me that digital processing should merit higher consideration.

Most of my listening is with Audirvana+ at my desktop workstation. Minor EQ bumps of just a few dB here and there can give bigger bang than a component upgrade IMHO. I'm in the process (AT LONG LAST) of setting up a dedicated listening/viewing room (a strictly 2.0 system). And I will certainly be experimenting with Dirac, room treatments, and the like.

The demos at audio shows of room correction have always struck me as the most impressive. I'm sure this review system is fantastic so why not take that last step at addressing the biggest component? THE ROOM!

Kal Rubinson's picture

If you know me, you know that I agree with you entirely. In the past, I have mentioned that I think it is necessary to review speakers without applying any room correction because the vast majority of stereo listeners do not use EQ; in fact, they reject it. OTOH, one can justify a reviewer using EQ to attempt to minimize the bias introduced by the unique influence particular to his space.

I do both but I only mention the effect of the EQ if it is significant. In most cases, the effect is an improvement but rarely transforming. Such was the case here. The leopard didn't change his spots.

mrkaic's picture

I agree 100% with both of you and have a related question. I have a Micromega M-One without the room correction option. Does anyone have any experience with the Micromega room correction upgrade option? I am thinking about getting it, since my listening room is quite small and my speakers (Focal Aria 936) too big for the small room.

Grateful for any advice.

Best,

MM

Kal Rubinson's picture

I am afraid that I have not heard of this device nor can I find any technical description of the room correction system on the Web (in English).

mrkaic's picture

Thank you anyway. I think that many readers would be grateful for a review of the M-One. I think it is a really nice machine: http://micromega.com/en/products/mone-range/

Best,

MM

Supperconductor's picture

Good to know that EQ was tried. Yeah, I used to be one of the EQ/DSP NIMBY's but lately the rooms I've found myself in, have issues.

The new house has a media room and I've got carte blanche (wife's idea). I'm picking the color scheme and light control (to serve an ISF calibrated front projector), speaker placements, acoustic treatments, seating, and also evaluating DSP options. On more than one occasion, I've witnessed absolute transformations with speaker placement, surface treatments & EQ/DSP.

HansRamon's picture

You don't like the french?

Kal Rubinson's picture

Why would you think that?

HansRamon's picture

Dear Mr. Herb Reichert, i will mail you a couple of "GaueHippies" when available..........

Kal Rubinson's picture

Who is Herb Reichert? ;-)

Dazgum's picture

Nice review! I'm surprised that you didn't do any comparison with your B&W 802 D3's as they are very similar in specification been 3 way 2 x 8inch woofers exoctic tweeters and fancy midrange drivers and price not to far apart. Would be very interesting to know what the main differences are.

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