Focal Chorus 826W 30th Anniversary Edition loudspeaker Measurements

Sidebar 3: Measurements

I measured the behavior in the farfield of the Focal Chorus 826W using DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. For the nearfield responses I used an Earthworks QTC-40, whose ¼" capsule doesn't present as much of an acoustic obstacle, which can be an issue with port measurements. As Robert Deutsch preferred the Chorus 826W with its grille removed, that's how I measured it, though I left the wire mesh in front of the tweeter in place.

The Chorus 826W offers a higher-than-usual voltage sensitivity, my estimate coming in at 90.1dB(B)/2.83V/m on its tweeter axis, which is close to the specification. The speaker is also specified as having a nominal impedance of 8 ohms, with a minimum magnitude of 2.9 ohms at 118Hz. Fig.1 shows that while the impedance does remain above 8 ohms for most of the bass and treble regions, it varies a lot, with a minimum of 2.6 ohms at 119Hz. There is also a current-hungry combination of 3.83 ohms magnitude and –53° electrical phase angle at 94Hz, a frequency where music can have high energy levels. This speaker should be used with a good amplifier rated at 4 ohms.


Fig.1 Focal Chorus 826W, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There are the expected discontinuities in the impedance traces above the audioband, these associated with the metal-dome tweeter's resonant modes. However, there are also small glitches in the mid- and low treble, and just above 400Hz. The last is associated with a vibrational resonance at 426Hz that I could detect on all the cabinet surfaces but that was particularly strong on the sidewall adjacent to the midrange unit (fig.2). However, it is probable that this mode is high enough in both Quality Factor (Q) and frequency not to lead to coloration. (The higher in frequency a high-Q resonance is, the less likely it is to be excited and the more quickly it will decay once stimulated.)


Fig.2 Focal Chorus 826W, cumulative spectral-decay plot calculated from output of accelerometer fastened to sidewall adjacent to midrange unit (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Although there are minor differences in their upper-frequency behavior, the two woofers appear to be identically tuned. The same thing was true of the two ports. I have plotted the combined outputs of the woofers and ports in fig.3 as the blue and red traces, respectively, scaled in the ratio of the square roots of the total radiating areas of each. The notch at 41Hz in the woofers' output suggests that this is their tuning frequency, as suggested by the low-frequency saddle in the impedance graph at the same frequency. The ports cover a broader-than-usual bandpass but roll off above 100Hz. Though there is some liveliness in their output in the midrange, this is well down in level. The woofers roll off smoothly and cross over to the similarly sized midrange unit (green trace) between 200 and 300Hz.

Below 300Hz, the black trace in fig.3 shows the complex sum of the lower-frequency drive-units' outputs, taking into account acoustic phase. Most of the apparent boost in the upper bass is due to the assumptions made by the nearfield measurement technique, but the Chorus 826W is balanced a bit rich in this region. The black trace above 300Hz shows the Focal's farfield response, averaged across a 30° horizontal window centered on the tweeter axis. Overall, it is as flat as I would have expected from RD's comments about the speaker's neutrality, with small dips in the mid-treble matched by small boosts. There is the usual peak between 20 and 30kHz, due to the metal-dome tweeter's primary "oil-can" resonance, but this will have no subjective consequences. The mesh grille had only a small effect on the tweeter's behavior (not shown), boosting the output between 10 and 15kHz by about 2dB.


Fig.3 Focal Chorus 826W, 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 ports (red), plotted below 600Hz, 700Hz, and 700Hz, respectively.

Though the plot of the Focal's lateral dispersion (fig.4) appears to show some off-axis peakiness in the mid-treble, this is mainly due to the on-axis suckout between 4 and 5kHz filling in to the speaker's sides. The speaker's radiation pattern is otherwise smooth and relatively even, presumably because of its narrow front baffle. In the vertical plane (fig.5), a suckout in the upper crossover region develops for listening axes above the tweeter. However, because the tweeter is a fairly high 40" from the floor when the spikes of its support platform are used, this reflects sensible crossover design.


Fig.4 Focal Chorus 826W, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° off axis, reference response, differences in response 5–45° off axis.


Fig.5 Focal Chorus 826W, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

Turning to the time domain, fig.6 shows the Focal's step response on its tweeter axis. The first, sharply defined up/down spike is the tweeter, which is connected in positive acoustic polarity. This is followed by a lazier up/down pulse that is the output of the midrange unit, also connected in positive acoustic polarity. The decay of the midrange unit's step smoothly blends into the negative-going step of the woofers, these connected in inverted acoustic polarity. (I confirmed this by looking at the individual step responses of the units, not shown.) That each unit's step on this axis smoothly blends with that of the next unit lower in frequency confirms the optimal design of the Chorus 826W's crossover. The speaker's cumulative spectral-decay plot (fig.7) is dominated by the tweeter's ultrasonic resonance but is otherwise respectably clean in the treble and upper midrange.


Fig.6 Focal Chorus 826W, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).


Fig.7 Focal Chorus 826W, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Overall, the Focal Chorus 826W is a well-engineered speaker at an attractive price. I echo RD's commendation of Focal for introducing a 30th-anniversary design at a price that will make it accessible to many more audiophiles than had it cost megabucks.—John Atkinson

US distributor: Audio Plus Services
156 Lawrence Paquette Industrial Drive
Champlain, NY 12919
(800) 663-9352
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