Acarian Alón IV loudspeaker Measurements

Sidebar 3: Measurements

Looking first at the Alón's impedance magnitude and phase (fig.1), the minimum value agrees with the specification and is shown by the cursor position: 3.03 ohms at 70Hz. This speaker will be moderately hard to drive; although a good tube amplifier with a 4 ohm output should have no problems, a solid-state amplifier will give better control of the speaker's bass region. The sealed-box bass enclosure is tuned to a low 38Hz. The sensitivity on the tweeter axis was also to specification at around 87dB at 1m for 2.83V in.

Fig.1 Acarian Alón IV, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The speaker's three sets of input terminals make it easy to examine the behavior of each of the drive-units. The individual amplitude responses are shown in fig.2. The woofer's output peaks at 50Hz, rolling out at the expected 12dB/octave below that frequency and above the crossover frequency of 400Hz. The midrange unit smoothly rolls in at its lower crossover frequency, but features both a peak and sharp suckout between 1kHz and 2kHz. The tweeter also rolls in smoothly below its passband, but there is an octave region where the tweeter and midrange unit overlap.

Fig.2 Acarian Alón IV, acoustic crossover response on listening axis at 45", corrected for microphone response, with the nearfield midrange and woofer responses plotted below 300Hz.

The right-hand curve in fig.3 shows how these individual drive-unit responses sum on an axis 38" from the ground, a typical listening height midway between the tweeter and midrange that was chosen by RD. The peak and dip in the latter's bandpass are still present, but the tweeter's output integrates beautifully, giving a flat, unexaggerated treble region. Bob Deutsch did comment on an occasionally relentless quality to the 4's upper midrange, and I suspect that the midrange's peak is responsible here. Usually, I have found this kind of response to lend a speaker's sound a touch of nasality. However, RD was not perturbed by any such coloration.

Fig.3 Acarian Alón IV, anechoic response on listening axis 38" form the floor at 45", averaged across 30° horizontal window and corrected for microphone response, with the nearfield woofer response plotted below 200Hz.

To the left of fig.3 is shown the woofer's response measured with the microphone almost touching its dustcap, which gives an approximation to an anechoic response. The level matching between this curve and the FFT-derived one to its right can only be approximate, yet it suggests a powerful, perhaps undercontrolled bass region. That RD could appreciate the Alón IV's low-frequency extension yet was not bothered by any boom suggests that designer Carl Marchisotto has carefully tailored the sealed-box–loaded woofer to work well with the typical boost give by the room.

Fig.4 shows the manner in which the speaker's tonal balance changes as the listener moves to its side. (Only the changes are shown, which means that the on-axis response appears to be a straight line—every speaker designer wishes!) The first thing to note from this graph is that the on-axis suckout fills in off-axis. This is probably why RD was not bothered by any nasal coloration due to the peak/adjacent dip: the dip would only be heard on axis; in a typical room, the reverberant soundfield—which also contributes to a speaker's perceived balance—would not have the dip. However, note that the peak accentuates for extreme off-axis angles as the region below—the center of the dipole midrange's passband—nulls out and the region above, handled by the tweeter, increasingly rolls off. Reflections of the speaker's sound from the sidewalls will be too bright, therefore, and will exacerbate the propensity of the on-axis peak to give the sound somewhat of an "etched" quality. Experimenting with rugs or diffusing surfaces on the sidewalls will be a good idea with this speaker.

Fig.4 Acarian Alón IV, lateral response family at 45", normalized to response on listening axis, from back to front: differences in response 90–15° off axis, reference response, differences in response 15–90° off axis.

Fig.5 shows the changes in tonal balance for changes in vertical listening axis. Below the 38"-high axis, the sound increasingly acquires a deep suckout in the crossover region between the tweeter and midrange unit. The lowest curve shown and the one most afflicted is for a listening axis level with the top of the Alón IV's bass bin, 29" from the ground. This is what a short person in a typical couch would perceive, so be warned: don't sit too low with this speaker. Sit high and the entire tweeter region comes up a couple of dB, but the response remains smooth. Listen with your ears level with or above the top of the tweeter baffle, however, and as well as the crossover suckout reappearing, the low treble becomes significantly peaky.

Fig.5 Acarian Alón IV, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 20–5° above axis, reference response, differences in response 5–20° below axis.

Finally, the Alón's impulse response on the listening axis, taken with DRA Labs' MLSSA system and a calibrated B&K mike, is shown in fig.6. Note a reflection at 4.6ms, about 1ms after the start of the impulse; this might be a reflection from the edge of the bass enclosure. The step response on the same axis is shown in fig.7. The individual impulse responses (not shown) confirm that the tweeter and woofer are connected with positive acoustic polarity, the midrange with negative. Fig.8 is the cumulative spectral-decay or "waterfall" plot calculated from fig.6. Note how smooth and clean the entire treble region is. Here's one of the reasons Bob Deutsch liked the sound of this speaker as much as he did. The cursor, which is a little hard to see, is placed at the bottom of the midrange suckout at 1.7kHz. Note that there is delayed energy at this frequency, which means that the suckout is probably due to the speaker's geometry: ie, the direct sound is canceling with a reflection at that frequency.

Fig.6 Acarian Alón IV, impulse response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

Fig.7 Acarian Alón IV, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

Fig.8 Acarian Alón IV, cumulative spectral-decay plot at 50" (0.15ms risetime).

All things considered, this is an excellent set of measurements, revealing the Alón IV to be as well-engineered as might be predicted from its sound quality. Its only real problem is the slight tendency to brightness in the lower treble, but this can be ameliorated by careful positioning of the speakers in the room, as well as by experimenting with wall coverings.—John Atkinson

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