Spica TC-50 loudspeaker 1989 Measurements
Fig.1 shows the way in which the TC-50's impedance changes in amplitude (solid line) and phase (dotted line) from 10Hz to 50kHz. [Note that the phase scale is the opposite way around from what the magazine adopted in 1991, with negative (capacitive) angles at the top.—Ed.] Although its magnitude drops to a hair under 4 ohms in the low bass, lower midband, and mid-treble, the speaker shouldn't present any drive problems. It would be unwise, however, to connect more than one pair in parallel with a typical low-powered receiver. The single LF resonant peak due to the woofer tuning can be seen at 65Hz.
Fig.1 Spica TC-50, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The spatially averaged in-room response is shown in fig.2. Note the rather overdamped bass, with the response starting to drop around 125Hz. The slow rate of roll-out typical of a well-damped sealed-box alignment, however, results in there still being appreciable output in-room at 40Hz. Measured in the nearfield, the -6dB point was also a reasonably low 39Hz. Other points of interest about the room-averaged curve, which is overall very smooth, are the slight excess of energy in the lower midrange, which correlates with my feeling that the speaker has a somewhat "cupped-hands" signature, and the peak in the high treble, again audible but this time as a slight lispy quality to sibilants. Above the treble peak, the HF drops off rapidly.
Fig.2 Spica TC-50, spatially averaged, 1/3-octave response in JA's Santa Fe room.
The individual spectra taken to make up fig.2 show that for a listener more than 30 degrees off the optimum horizontal axis, a significant suckout develops between 2 and 5kHz. It is essential, therefore, for the TC-50s to be toed-in toward the listening seat if the sound is not to lack presence and sound rather peaky in the highs.
Turning to the time behavior of the speaker on the optimum listening axis, it can be seen from the impulse response (fig.3) that nearly all the energy arrives within a very short time period. The tweeter output starts to rise just to the right of the center line, but note that 150µs earlier, the start of the woofer output can be seen in the same direction, confirming the time alignment of the two drivers. Some 1.2ms after the initial pulse, a small reflection can just be made out, as can what is probably another 1ms later. As there was nothing in the room near the speaker that would provide such closely spaced reflections, I can only assume that even with the heavy damping of the front baffle provided by the felt blanket, these are reflections of the tweeter output from the baffle edge.
Fig.3 Spica TC-50, impulse response on tweeter axis at 1m (5ms time window).
Looking at the FFT analysis of the impulse response, averaged across the ±15 degrees horizontal window (fig.4), which represents the anechoic response of the TC-50 measured with 100Hz resolution, it can be seen that this pretty much agrees with the in-room measured response in fig.2. Again the slight midrange dominance to the balance can be noted, as can the excess of energy in the top octave.
Fig.4 Spica TC-50, FFT-derived anechoic response on tweeter axis at 1m, averaged across 30 degrees horizontal window (not corrected for microphone response).
That the TC-50 treble balance is extremely dependent on listening axis can be seen in fig.5, which is the FFT response of the speaker taken with the microphone perpendicular to the baffle directly on the tweeter axis. Not only did the impulse response (not shown) now show that the outputs of the two drivers were no longer time-coincident, but the tweeter peak is significantly raised in amplitude, and left isolated by a lack of energy in the octave below. No wonder the TC-50 sounds so thin and aggressive in the treble to a standing listener.—John Atkinson
Fig.5 Spica TC-50, FFT-derived anechoic response on axis perpendicular to the baffle at 1m on tweeter axis (not corrected for microphone response).