Focal Chorus 807V loudspeaker Measurements

Sidebar 3: Measurements

I measured the Focal Chorus 807V with its grille removed. Its voltage sensitivity on the tweeter axis was significantly higher than the average I have measured over the years, at an estimated 90dB(B)/2.83V/m, though this is a little lower than the 92dB specified. Though its impedance drops to 4.1 ohms at the bottom of the midrange, and there is a combination of 5.8 ohms magnitude and –40° electrical phase angle at 120Hz (fig.1), overall the speaker is a fairly gentle load for the partnering amplifier to drive.

Fig.1 Focal Chorus 807V, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The small wrinkles between 20kHz and 30kHz in the impedance traces are due to the tweeter's primary dome resonances; because they lie above the normal range of human hearing, these resonances will be benign. Of more concern is the sharp discontinuity just above 800Hz in the traces, which indicates the presence of some sort of cabinet resonance in that region. The cabinet is tapered toward the speaker's rear to minimize acoustic resonances. Nevertheless, investigating the cabinet walls' vibrational behavior with a simple accelerometer, I did find a major resonant mode at 836Hz on all surfaces. Fig.2 is a cumulative spectral-decay plot showing the behavior of the top panel; some other lower-level modes can also be seen. Predicting the subjective effect of this behavior is not easy, and as Bob Reina noted nothing untoward going on in the upper midrange, I assume this cabinet resonance looks worse than it sounds.

Fig.2 Focal Chorus 807V, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Nevertheless, a small peak can be seen between 800 and 900Hz in the reflex port's nearfield response (fig.3, red trace), as well as one a little lower in frequency. Other than those, however, the port is well-behaved, its bandpass response peaking between 35 and 85Hz. The woofer's minimum-motion frequency is 45Hz (fig.3, blue trace), the same as the low-frequency saddle in the impedance-magnitude trace. Both the woofer's output and the sum of the woofer's and port's outputs (black trace, taking into account acoustic phase) peak higher in the upper bass than is to be expected from the effect of the nearfield measurement technique. I suspect that Focal's design team has chosen a rather underdamped woofer tuning to balance the rise in high-frequency output that can also be seen in this graph, in the two octaves below the response peak due to the tweeter's ultrasonic resonance. Bob did note that "the Chorus 807V's bass was extended and forceful for a 'bookshelf' speaker," which is what I would expect from this measured behavior.

Fig.3 Focal Chorus 807V, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the port (red) and woofer (blue) plotted below 1kHz and 300Hz, respectively, and the complex sum of the nearfield responses plotted below 300Hz.

Though the Chorus 807V's midrange is impressively flat, the region covered by the tweeter can be seen to be shelved up by a couple of dB. All things being equal, this will lead precisely to what BJR described in his auditioning notes: "the extreme high frequencies seemed a bit emphasized. The sibilants of all closely miked female vocalists on familiar recordings...seemed a bit more prominent than I remembered hearing them through other speakers."

The Focal's plot of lateral dispersion (fig.4) is wide and even, though with a touch of flare at the bottom of the tweeter's passband, which will contribute to the speaker's sounding "revealing." The response above 10kHz doesn't fall off as quickly to the speaker's sides as is usually seen with a 1" dome; perhaps this is due to the drive-unit's being mounted at the top of the baffle, above the V-shaped grille. In the vertical plane (fig.5), suckouts develop in the crossover region more than 5° above or below the tweeter axis, suggesting that the Focal's owner choose stands that place his ears reasonably close to the height of the tweeters.

Fig.4 Focal Chorus 807V, 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 Chorus 807V, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Turning to the time domain, the Chorus 807V's step response (fig.6) is a little unusual: the tweeter's output arrives at the microphone well ahead of the woofer's. However, because the tweeter is connected in inverted acoustic polarity, the decay of its step smoothly integrates with the positive-going start of the woofer's step, this correlating with the good frequency-domain integration seen in fig.3. The Focal's cumulative spectral-decay plot (fig.7) is superbly clean for a speaker costing just $1000/pair.

Fig.6 Focal Chorus 807V, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 Focal Chorus 807V, cumulative spectral-decay plot at 50" (0.15ms risetime).

The Focal Chorus 807V offers excellent measured performance, though its treble balance will require care in choosing source and amplification components if it is not to sound too revealing.—John Atkinson

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