RA Labs Black Gold Mini Reference loudspeaker Measurements

Sidebar 3: Measurements

Despite its small size, the RA Labs speaker is surprisingly sensitive; my B-weighted estimate was around 89dB/2.83V/m. However, as its impedance (fig.1) drops below 6 ohms for most of the midrange, 8 ohm–rated amplifiers and receivers should be avoided. The tuning of the sealed box is revealed by the peak at 65Hz, implying only moderate bass extension.

Fig.1 RA Labs BG Mini-Reference, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Note the small wrinkle in the traces between 200Hz and 300Hz. These suggest the presence of some kind of cabinet resonance in this region. Fig.2 shows a cumulative spectral-decay plot calculated from the output of a simple accelerometer fastened to the center of the cabinet side wall. Some minor flexing at the cabinet tuning frequency can be seen, but the main mode is moderately high in level at 275Hz, and could be detected on all surfaces; in addition, a couple of high-level higher-frequency modes were found on the back panel. The effect of this cabinet behavior is hard to predict, but RJR noted nothing untoward in the lower midrange.

Fig.2 RA Labs BG Mini-Reference, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the quasi-anechoic responses of both current samples of the Mini-Reference, averaged across a 30° horizontal window on the tweeter axis. Looking at the top trace (S/N B06496), the woofer is peaky at the top of its passband, with then a slight suckout before the tweeter comes in. The tweeter, too, is somewhat peaky. The nearfield bass measurement shows that the –6dB point coincides with the woofer tuning frequency, as expected. The response of S/N B06497) is smoother, with less extreme peaks and dips. But both are disappointing compared with the earlier samples of the speaker that I measured around the time of Sam Tellig's rave review, which measured more smoothly through the treble (fig.4).

Fig.3 RA Labs BG Mini-Reference, 1997 samples, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield woofer response plotted below 300Hz. (S/N B06496, top; B06497, bottom.)

Fig.4 RA Labs BG Mini-Reference, 1994 sample (S/N B0040), anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield woofer response plotted below 300Hz.

You shouldn't attempt to predict a speaker's sound quality just from its on-axis response. But if you look at the Mini-Reference's lateral dispersion plot (fig.5), you can see that the tweeter's mid-treble peak is actually higher in level to the speaker's sides. I suspect that this, rather than the on-axis peakiness between 800Hz and 2400Hz, lies at the heart of Bob Reina finding the speaker's treble so unforgiving. When I measured the speaker, this peak was audible as a "whistley" character added to the sound of the pseudo-random noise signal used by DRA Labs' MLSSA measuring system.

Fig.5 RA Labs BG Mini-Reference, 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.

In the vertical plane (not shown), the speaker appeared to be reasonably unfussy regarding listening axis. As long as you sit with your ears between the top of the cabinet and the woofer—implying the use of quite tall stands—you'll get a reasonably evenly balanced sound from the Mini-Reference. Sit too high, however, or below the speaker, and a suckout appears in the crossover region.

The step response on the tweeter axis (fig.6) reveals the tweeter and woofer to be connected with the same positive acoustic polarity. The cumulative spectral-decay or waterfall plot on the same axis (fig.7) shows that the low-treble peak in the on-axis response is associated with some delayed resonant energy, perhaps from a woofer-cone resonance. However, there is only a slight degree of resonant energy associated with the peak at 11kHz.—John Atkinson

Fig.6 RA Labs BG Mini-Reference, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 RA Labs BG Mini-Reference, cumulative spectral-decay plot at 50" (0.15ms risetime).

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