Spica Angelus loudspeaker 1993 Measurements

Sidebar 2: 1993 Measurements

As my original review of the Angelus in February 1988 was published well before I had established my standardized loudspeaker test procedure, I thought that including this well-established and generally recommended speaker in this survey would give me the chance to perform a thorough set of measurements.

First, the Angelus's impedance (fig.1) only drops below 7 ohms above 4kHz, reaching a minimum value of 5.2 ohms at 8kHz. It therefore presents an easy load to the amplifier driving it, coupled with a moderate B-weighted sensitivity of 86.5dB/W/m. The single peak in the bass, due to the sealed-box tuning, lies at 38.5Hz, indicating reasonably good low-frequency extension for a speaker with a single 8" woofer. The slight wrinkles in the traces at 250Hz and 320Hz are probably due to cabinet resonances, though I have no idea what causes the unusual shape of the magnitude trace between 100 and 200Hz. Normally, a speaker's impedance drops smoothly from the height of the bass peak to a minimum value in the upper bass without a sharp discontinuity like that apparent in fig.1.

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

This unusual impedance trace is probably tied-in with the double-humped nature of the speaker's nearfield woofer response, shown to the left of fig.2. Note, however, the excellent bass extension: a -6dB point of 28Hz referenced to the level at 100Hz (or a still excellent 33Hz referenced to the maximum level at 136Hz). That the listeners generally found the Angelus to sound lean is probably due to the rather over-damped nature of the bass alignment, this exacerbated by the need in the panel tests to raise the speaker slightly from the floor.

Fig.2 Spica Angelus, anechoic response on listening axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer response plotted below 300Hz.

Other than the excess of energy in the transition region between the upper bass and lower mids, the midrange shown in fig.2 is pretty flat. The speaker does have a slight peakiness in the lower treble, which may be due to a woofer cone mode of some sort, what Martin Colloms refers to as "cone cry." The mid-treble, and what there is of the top octave before the soft-dome tweeter's output rolls off sharply above 16kHz, are raised above the midband level by an average of 4dB or so. This measured plateau contributes to the panelists' feeling that the speaker had too much HF energy, or "soft-dome grain," as TJN called it. In my original review, I, too, was bothered by the HF emphasis given to record ticks and surface noise.

Laterally, the Angelus maintains its response to 15 degrees off-axis, greater angles progressively rolling off the top octave and introducing a suckout in the crossover region. Fig.3 shows the differences in response to be found off-axis on the tweeter side of the strange-shaped, asymmetric baffle. (This notch is broader but less deep on the other side of the baffle.)

Fig.3 Spica Angelus, lateral response family at 50", normalized to response on listening axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

Vertically, the response only changes a little as the listener moves his or her ears down from the tweeter, the speaker progressively acquiring more low-treble energy (fig.4). But if you sit so that you are above the tweeter, not only does the speaker acquire more top-octave energy, but a large suckout appears in the mid-treble which will both make the speaker sound laid-back and emphasize the high treble. Finally, if you listen to the speaker while standing, the suckout fills in again and you hear a smooth but significantly treble-heavy balance. Ceiling reflections of this speaker's sound will therefore be very toppy, which presumably added to the mid-treble excess on-axis to contribute to the listening panel's impression of "too much highs." If you like what the Angelus does but are bothered by its excessive highs, it would be worth experimenting with patches of, say, Sonex foam on the ceiling at the primary reflection points.

Fig.4 Spica Angelus, vertical response family at 50", normalized to response on listening axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-10 degrees below axis.

In the time domain, the Angelus's impulse response on the tweeter axis (fig.5) appears time-coherent, due to the combination of its crossover's performance—first-order high-pass to the tweeter, fourth-order Bessel low-pass to the woofer—and the time alignment of the drive-units due to the sloping front baffle. This is confirmed by the step response (fig.6), which shows an excellent right-triangle shape due to the same-polarity outputs of the tweeter and woofer arriving at the microphone at the same time, spoiled only by a leading-edge overshoot associated with the treble plateau noted in fig.2.

Fig.5 Spica Angelus, on-axis impulse response at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Spica Angelus, on-axis step response at 50" (5ms time window, 30kHz bandwidth).

The cumulative spectral-decay plot calculated from fig.5 (fig.7) shows some minor resonant modes in the low- and mid-treble, these presumably correlating both with the hardness noted at high levels and the "splashy," "raspy" treble coloration that bothered some of the panelists.

Fig.7 Spica Angelus, cumulative spectral-decay plot at 50" (0.15ms risetime).

Other than its side panels, the Spica's large cabinet seemed quite lively to the knuckle-rap test, this confirmed by accelerometer measurements. Fig.8, for example, shows the cumulative spectral-decay plot calculated from the accelerometer's output when fastened to the center of the cabinet's lower back panel. A number of resonant modes can be seen, as well as the large mode at 184Hz. TJN noted a slight "boxiness" to the Angelus's sound in the blind auditioning, while both in my 1988 review and in my blind 1993 auditioning, I found the speaker to be a little "hooty" in the 200Hz region. These comments might well be laid at the feet of this cabinet behavior.—John Atkinson

Fig.8 Spica Angelus, cumulative spectral-decay plot, calculated from the output of an accelerometer fastened to the cabinet's rear panel opposite the woofer. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)