Almarro M0A loudspeaker Measurements

Sidebar 3: Measurements

As Bob Reina explained, the Almarro M0A is deceptive-looking because what appears to be its stand is actually part of the speaker's acoustic design. I therefore measured it attached to the stand. I estimated the M0A's voltage sensitivity as 88.5dB(B)/2.83V/m. Though this is slightly below the specified 90dB, it is still higher than average. The M0A's impedance magnitude (fig.1) is quite different between the bass, where it averages 5 ohms, and the treble, where it rises from 6 ohms at 1kHz to more than 20 ohms above 12kHz. Using a tube amplifier with this speaker, with its typically high source impedance, will tilt up the speaker's high frequencies by an audibly significant result compared with a typical solid-state design.

Fig.1 Almarro M0A, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There is a wrinkle in the impedance traces at 200Hz, which implies that some sort of resonance exists at this frequency. However, investigating the vibrational behavior of the cabinet's panels with an accelerometer uncovered no resonances other than one at a low level at 94Hz on the rear panel (fig.2). The cabinet's side panels were acoustically inert.

Fig.2 Almarro M0A, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's rear panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 41.5Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the large-diameter port on the speaker's rear panel, which loads the woofer, not the 4" cone "tweeter." This is confirmed by the nearfield responses of the woofer (fig.3, red trace) and port (fig.3, green). The woofer's output features the usual reflex notch at the port tuning frequency, while the port's output peaks appropriately at the same frequency. However, a second peak can be seen in the port's response at 200Hz, the frequency of the impedance anomaly, which coincides with a suckout in the woofer's response. This behavior is classic evidence of some sort of internal air-space resonance and, all things being equal, might be expected to lead to a lack of clarity in the lower midrange. I note that BJR commented on some warmth in this region.

Fig.3 Almarro M0A, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the tweeter (blue), woofer (red), and port (green) plotted below 300Hz, 400Hz, and 1kHz, respectively.

The big surprise for me was to discover that the M0A's two drive-units are not driven by a conventional crossover. Instead, what I referred to as the "tweeter"—the 4" cone unit—extends down to 125Hz (fig.3, blue trace), rolling off below that frequency with the second-order, 12dB/octave slope typical of a sealed-box loading. I expect that this driver's limited cone excursion at lower frequencies contributes to the lack of dynamic range noted by BJR in his auditioning. The woofer and port extend almost two octaves below the "tweeter," with the woofer duplicating its output up to the upper midrange. The woofer's midrange output is uneven, and it rolls off steeply above 1kHz. The 4" unit's output is smooth from the lower midrange through the mid-treble, but its response in the top two octaves is ragged, broken by peaks and dips.

Below 300Hz, fig.4 combines the complex sum of the three nearfield responses, scaled in the ratio of their radiating diameters and allowing for acoustic phase and the different distances of the sound sources from a nominal point in the speaker's farfield. As BJR found in his auditioning, the M0A offers good bass extension, though the lack of the usual nearfield hump in the upper bass suggests a rather overdamped alignment, which trades off bass weight against definition and control.

Fig.4 Almarro M0A, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield tweeter, woofer, and port responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.

Higher in frequency, fig.4 shows the Almarro's actual farfield response, averaged across a 30° window on the "tweeter" axis. The fact that two drive-units cover the region below 1kHz leads to an apparent shelving-down of the treble region, which might have contributed to BJR's comments about the speaker's "relaxed" high frequencies, particularly when compared to more conventional designs. But note how flat the M0A's treble is overall, at least below 10kHz and above the deep, narrow suckout at 1.2kHz.

Fig.5, a plot of the M0A's lateral dispersion, reveals that this on-axis notch tends to fill in to the speaker's sides, though the 4" unit's relatively large radiating diameter gives rise to an uneven, generally poor radiation pattern above 3.5kHz or so. The 1.2kHz notch also fills in for microphone positions below the "tweeter" axis (fig.6) and worsens for positions above that axis, suggesting that it might be due to interference between the overlapping drive-units. However, looking back at fig.4, there are none of the higher-frequency notches that would be present if that were the case.

Fig.5 Almarro M0A, 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 Almarro M0A, 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.

Why would the Almarro M0A's designer use this very unconventional approach to system design? One factor is that, without a crossover, the speaker's step response (fig.7) has an excellent right-angle shape, though this is overlaid with both high- and low-frequency oscillations. Looking at the steps of the individual drive-units (fig.8, taken with an anti-aliasing filter different from that used for fig.7, which exaggerates the attacks of the leading edges), the high-frequency ringing is associated with the step of the "tweeter" (red trace), while the lower-frequency ringing emanates from the woofer alone (blue).

Fig.7 Almarro M0A, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Almarro M0A, step response of tweeter (red) and woofer (blue) on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

The M0A's waterfall plot (fig.9) reveals that the LF ringing is associated with the on-axis notch at 1.2kHz; the suckout must therefore be characteristic of the woofer's behavior, not an interference effect. The Almarro decays cleanly in the low and mid-treble, but its top two octaves are marred by ridges of resonant energy. Fortunately, most of this energy lies above 10kHz, where the ear's sensitivity begins to decrease.

Fig.9 Almarro M0A, cumulative spectral-decay plot at 50" (0.15ms risetime).

The Almarro M0A offers a mixed bag of measurements, but overall its performance mostly overcomes what might be thought to be the limitations of its unconventional design. The exception, of course, will be its limited dynamic range—you can ask for only so much from a 4" drive-unit run full-range—and its poor dispersion in the treble.—John Atkinson

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