BG Z1 loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the BG Z1's voltage sensitivity was 88.5dB(B)/2.83V/m, which is within experimental error of the specified 88dB figure. This speaker will go surprisingly loud with relatively low-powered amplification, but it is fair to point out that for a speaker this small to have this high a sensitivity, it invariably will have restricted low-frequency performance. The Z1's impedance is higher than normal, with a minimum midrange magnitude of 6.4 ohms at 244Hz and a value that drops below 6 ohms only in the top audio octave (fig.1). This speaker will work well with low-cost amplifiers.

Fig.1 BG Z1, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There are some wrinkles in the impedance traces in the midrange, notably at 300Hz and between 700Hz and 800Hz, which imply the presence of cabinet vibrational resonances of some kind. When I investigated the behavior of the panels with a simple plastic-tape accelerometer, the primary resonant mode, very strong and detectable on the top panel and both sidewalls, lay at 750Hz (fig.2).

Fig.2 BG Z1, 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).

The saddle centered on 57Hz in the impedance magnitude trace suggests that this is the tuning frequency of the rear-panel–mounted port. The nearfield output of this port, plotted to the left of fig.3, peaks between 30Hz and 100Hz, but its midrange rolloff is marred by two peaks: a mild one at 500Hz and a severe one centered at 800Hz. The latter is strong enough, in fact, to give rise to a small peak at the same frequency in the Z1's farfield response. As expected, the woofer's output has its minimum-motion point at the port tuning frequency, and the unit's response rises through the upper midrange before crossing over to the tweeter at 2kHz. The tweeter actually comes in a little early for optimal integration with the woofer; its on-axis output also rises a little at the top of its passband.

Fig.3 BG Z1, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the port and woofer plotted below 1kHz and 300Hz, respectively.

Fig.4 shows how the individual drive-unit responses sum in the farfield, spatially averaged across a 30° horizontal window on the ribbon-tweeter axis. The Z1's output is gently sloped up from the upper bass to the top audio octave, with a slight energy excess in the crossover region. There appears to be a slight boost in the upper bass, but this is the effect of the nearfield measurement technique used for the speaker's calculated output below 300Hz. The Z1 is actually maximally flat to about 85Hz, with then the expected fourth-order rolloff. This balance suggests that the Z1 really does need to be used with a subwoofer—or, if not, close to the room boundaries to bring up the upper-bass output, as BJR noted in his auditioning.

Fig.4 BG Z1, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

The Z1's lateral dispersion (fig.5) is generally wide and even, but the dramatic narrowing of the speaker's radiation pattern above 10kHz might make the speaker sound a bit lifeless in large, overdamped rooms. This was unexpected, given the narrow ribbon, but I imagine it must be due to the waveguide surrounding the unit. This restricted off-axis output in the top octave can also be seen in the vertical dispersion plot (fig.6), which suggests that the Z1 is best used on tall stands so that the listener's ears are on or just below the tweeter axis.

Fig.5 BG Z1, 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.6 BG Z1, 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.

In the time domain, the Z1's step response (fig.7) indicates that the drive-units are both connected in positive acoustic polarity, but with the tweeter's output leading that of the woofer by a couple of hundred microseconds. More important, the overshoot of the tweeter's step smoothly hands over to the woofer's step, confirming the good frequency-domain integration seen in fig.4. The Z1's cumulative spectral-decay plot (fig.8) is very clean overall, particularly so in the region covered by the ribbon tweeter.

Fig.7 BG Z1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 BG Z1, cumulative spectral-decay plot at 50" (0.15ms risetime).

The BG Z1's measured performance strongly suggests that it be used with a subwoofer, or at least close to the room boundaries. Its tonal balance might then still sound lightweight, but its clean, detailed presentation echoes that of, for example, the best-selling ProAc Tablette minimonitor, from the early 1980s.—John Atkinson

BG Corp.
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Carson City, NV 89706
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