System Audio 1070 loudspeaker Measurements

Sidebar 3: Measurements

The floorstanding 1070 was sensitive, at an estimated 89dB(B)/2.83V/m. However, with an impedance plot (fig.1) that dropped to just below 4 ohms in the middle of the midrange, it will need to be driven by a good solid-state amplifier or a tube amplifier with a generously specified 4-ohm output tap. The saddle in the impedance trace at 45Hz indicates the tuning of the twin ports, this usually equivalent to a speaker's anechoic –6dB frequency.

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Fig.1 System Audio 1070, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

As also is usual with a reflex-loaded design, this is also the minimum-motion point of the woofers, as can be seen from fig.2, which shows the individual outputs of (from left to right) the two ports, the two woofers, and the tweeter. A vicious resonance spike can be seen in the ports' output between 800 and 900Hz. While this could be heard on pink noise when I was standing behind the speaker, the fact that the ports face away from the listener should significantly reduce its audibility. A very slight lack of energy in the woofers' farfield output can be seen at the same frequency, however.

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Fig.2 System Audio 1070, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 1kHz, respectively.

Fig.1 suggests that the electrical crossover is set to 2.5kHz. From fig.2, however, it appears both that the acoustic crossover point is somewhat higher than this and that the woofers roll out a little too early. The effect of these can be seen in fig.3, which shows the System Audio 1070's overall response averaged across a 30° horizontal window on the tweeter axis at a microphone distance of 50". (Other than impedance, all measurements were performed with the latest version of DRA Labs' MLSSA system and a calibrated B&K 4006 microphone.) Although the overall balance is very flat, there is a broad lack of energy in the presence region. All things being equal, this would be expected to make the speaker sound rather laid-back and distant, as well as lacking a little life and sparkle.

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Fig.3 System Audio 1070, anechoic response on tweeter axis at 50" without grille, averaged across 30° horizontal window and corrected for microphone response, with complex sum of the nearfield woofer and port responses plotted below 300Hz.

But all things are rarely equal with speakers, and the plot of the 1070's lateral dispersion reveals that there is a slight response "flare" off-axis in the same region, indicated by the cursor position at 3kHz. In a typical, not too large room, the excess of presence-region energy in the reverberant field will counteract the on-axis depression in the same region, leading to a flat perceived balance—which is what BW found in his auditioning. The otherwise well-controlled off-axis behavior that can be seen in fig.4 will contribute to accurate, stable stereo imaging.

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Fig.4 System Audio 1070, horizontal 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 speaker's vertical dispersion is shown in fig.5, with the off-axis behavior normalized to the tweeter-axis response. It can be seen that if the listener sits above the tweeter, which is some 34" from the floor, the presence-region suckout becomes markedly accentuated. System Audio 1070 owners should sit so that their ears are between the centers of the twin woofers.

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Fig.5 System Audio 1070, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 20°–5° above HF axis; reference response; differences in response 5°–10° below HF axis.

In the time domain, the step response (fig.6) reveals that the tweeter's output leads the woofers' by a fraction of a millisecond, and that all the drive-units are connected with positive acoustic polarity. The associated cumulative spectral-decay or waterfall plot (fig.7) is relatively clean throughout the upper midrange and treble, although some low-level HF hash can be seen, as well as a small ridge of resonant energy at 2kHz. This latter might be expected to add a slight hardness at high levels, but at moderate levels might add a pleasing sense of attack.

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Fig.6 System Audio 1070, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.7 System Audio 1070, cumulative spectral-decay plot at 50" (0.15ms risetime).

Finally, the tall cabinet was rather live between 300 and 400Hz, as can be seen in fig.8, a waterfall plot calculated from the output of a simple accelerometer fastened to a sidewall 12" from the floor. The resonant modes are reasonably low in level, however, and I note that BW did not remark on any subjective problems in the lower midrange.—;John Atkinson

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Fig.8 System Audio 1070, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall 12" from the bottom. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

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