Focal Electra 1007 Be loudspeaker Measurements

Sidebar 3: Measurements

The Electra 1007 Be was of average voltage sensitivity, at an estimated 88dB(B)/2.83V/m. Its impedance (fig.1) covered a wide range, with a minimum value of 4.3 ohms at 200Hz and a maximum value in the low treble of 21.5 ohms at 3kHz. With amplifiers having a high source impedance, such as typical tubed designs, this will tend to tilt up the speaker's response at high frequencies. The saddle in the impedance-magnitude trace at 52Hz indicates the tuning frequency of the rectangular, rear-facing port.

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Fig.1 Focal Electra 1007 Be, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces are free from the small wrinkles that would point to the existence of enclosure resonances. Nevertheless, an investigation with a simple plastic-tape accelerometer uncovered a reasonably strong vibrational mode at 350Hz on all surfaces. Fig.2, for example, is a cumulative spectral-decay plot calculated from the accelerometer's output when it was fastened to the center of a side panel. The mode is clearly visible, as is one at 600Hz. These were audible with a stethoscope pressed against the cabinet, but not at the listening position.

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Fig.2 Focal Electra 1007 Be, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

As expected from the impedance graph, the woofer's minimum-motion point—where its cone is held stationary by the back pressure of the port resonance—lies at 52Hz (fig.3, red trace). The port's output (blue trace) peaks between 40Hz and 80Hz, and while a couple of peaks are evident in the midrange, these are well down in level. The black trace below 300Hz in this graph is the sum of the woofer and port outputs, taking into account acoustic phase and the distance of each sound source from a nominal farfield microphone position. Some of the upper-bass boost evident is the result of the nearfield measurement technique, but the speaker overall does have a rich balance.

Higher in frequency in fig.3, the Electra 1007 Be is impressively flat throughout the midrange and low treble, but the middle of the treble region is raised by 3–4dB compared with the level at lower frequencies. The beryllium-dome tweeter has an impressively wide bandwidth: it is still giving out energy at the 30kHz upper boundary of this graph.

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Fig.3 Focal Electra 1007 Be, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the woofer (red) and port (blue), and their complex sum (black).

The Focal speaker's dispersion in the vertical plane is shown in fig.4. A strong suckout develops in the crossover region immediately above the tweeter axis and below the woofer axis. The speaker needs to be placed on high stands, such as Focal's own models, to produce a neutral perceived balance. In the lateral plane (fig.5), the Electra's dispersion is wide and even, even above 10kHz. However, there is a significant off-axis flare at the base of the tweeter's passband in the same region where the on-axis response has an excess of energy.

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Fig.4 Focal Electra 1007 Be, 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.

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Fig.5 Focal Electra 1007 Be, 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.

The result of this behavior, as can be seen in the plot of the speaker pair's spatially averaged response in my listening room (fig.6), is an excess of energy between 3kHz and 10kHz. This is why the speaker tends to sound brilliant (good) under the right circumstances, and bright (bad) in suboptimal conditions. But note the generally flat midrange and the extended, if shelved-down, low frequencies.

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Fig.6 Focal Electra 1007 Be, spatially averaged, 1/3-octave response in JA's listening room.

In the time domain, the 1007 Be's step response on the tweeter axis (fig.7) indicates that the tweeter is connected in inverted acoustic polarity, the woofer in positive polarity. However, the step of the former smoothly hands over to the step of the latter, correlating with the good frequency-domain integration of the two units' outputs. The farfield cumulative spectral-decay plot (fig.8) is superbly clean, with virtually no trace of resonant decay evident, correlating with the speaker's grain-free presentation.

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Fig.7 Focal Electra 1007 Be, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.8 Focal Electra 1007 Be, cumulative spectral-decay plot at 50" (0.15ms risetime).

The Focal Electra 1007 Be's measured performance is generally superb. However, that elevated mid-treble will make the speaker intolerant of inadequate electronics and over-reverberant rooms alike. Large, acoustically well-damped rooms will be more to this speaker's liking than small, lively rooms.—John Atkinson

COMPANY INFO
Focal-JMlab
US distributor: Audio Plus Services
PO Box 3047
Plattsburgh, NY 12901
(800) 663-9352
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