Acarian Systems Alón Circe loudspeaker Measurements

Sidebar 3: Measurements

The big Alón's sensitivity was to specification at an estimated 87dB(B)/2.83V/m. Its impedance plot of magnitude and phase against frequency (fig.1) dropped below 6 ohms only in the upper bass and upper midrange, with a moderate phase angle across most of the band other than in the bass. The Circe will be relatively easy to drive, which presumably explains why listeners have had success using the speaker with SE triode amplifiers. But note the very high impedance peak at 39Hz: 24 ohms. This will exaggerate the midbass with such amplifiers. It also indicates that the quite large sealed bass bin is tuned relatively high in frequency, which will impact ultimate bass extension.

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

Some wrinkles can be seen in the impedance traces between 180Hz and 240Hz and at 26kHz. The latter is associated with the metal-dome tweeter's "oil-can" resonance and should be subjectively benign. However, the former indicates the presence of some cabinet resonances in the lower midrange. Fig.2, for example, shows a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of a cabinet side panel. A very strong resonance can be seen at 207Hz, with two more just above and below that frequency. All things being equal, I would have expected this behavior to add an over-warm chesty quality to spoken male voice. As Wes Phillips didn't note any problems in this region in his auditioning, I must assume this cabinet problem looks worse than it sounds.

Fig.2 Alón Circe, cumulative spectral-decay plot of accelerometer output fastened to bass-cabinet side wall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Because of the Circe's external crossover, it was possible to do a more through analysis of the crossover behavior than usual. Fig.3, for example, shows the actual electrical drive signals supplied to the three drive-units. The electrical crossover frequency between the woofer and the midrange unit appears to be around 550Hz rather than the specified 400Hz, while that between the midrange and tweeter is 4.2kHz. The actual acoustic crossover points will be different from the electrical ones, due to the differing drive-unit sensitivities, of course. The tweeter has an overall rising drive signal over most of its passband, while the woofer driver peaks by 2dB around the cabinet tuning frequency, suggesting some series resistance in its path. The rolloff slopes are approximately first-order, other than the midrange unit's low-pass slope, which is second-order.

Fig.3 Alón Circe, crossover electrical drive signals to the three drive-units, measured at the speaker terminals with the crossover loaded by the three units.

Figs.4, 5, and 6 compare the raw drive-unit response with the response as filtered by the crossover for the woofer, midrange unit, and tweeter, respectively. Without any equalization, the woofer's output extends up to the presence region (fig.4), with then a very sharp rolloff. The crossover both pads down the woofer's level and starts to roll off its output above 600Hz. The midrange unit, too, has its level dropped by the crossover (fig.5), with its output gently rolled off above 4kHz. A sharp spike in the unequalized output can be seen at 9.5kHz; this is suppressed by only about 8dB compared with the reference level at 1kHz. Note also the peaks and dips between 1kHz and 2kHz in the midrange unit's output. This unevenness will be due to the unit's acoustic environment—the dowels are probably too small in diameter to present an acoustic obstacle in this frequency region, but the top plate and the top of the bass enclosure should produce strong reflections.