Phase Technology PC60 CA loudspeaker Measurements
I measured the Phase Technology PC60 CA's frequency response in the farfield with DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. I used an Earthworks QTC-40 for the nearfield response. All measurements were performed with the grille removed.
My estimate of the PC60 CA's B-weighted voltage sensitivity was 85dB(B)/2.83V/m, which is significantly lower than the specified 87dB. The impedance is specified as 4 ohms, which I think appropriate, the PC60 CA's impedance magnitude varying between 3.3 and 6 ohms in the midrange (fig.1, solid trace), though it remains above 7 ohms in the midhigh treble. The electrical phase angle (fig.1, dotted line) is generally high, though its more extreme values never coincide with low magnitudes. A good 4 ohmrated amplifier or receiver will have no difficulty driving this speaker.
The traces in fig.1 are free from the small discontinuities that would imply the existence of panel resonances. Nevertheless, when I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer (similar to a piezoelectric guitar pickup), I found a series of resonant modes on the top panel and both sidewalls (fig.2). Though some of these are relatively high in level, their frequencies are sufficiently high that I wouldn't expect them to add midrange coloration.
The single impedance peak in the bass, centered on 55Hz, suggests that this is the tuning frequency of the woofer's sealed-box alignment. This was confirmed by the PC60 CA's nearfield response (fig.3), which indicates that the low-frequency output is down by 6dB at that frequency. The apparent boost in the upper bass in this graph is entirely an artifact of the nearfield response technique; the PC60 CA is tuned to be maximally flat in the bass. Higher in frequency in this graph, the speaker's midrange output is impressively flat, though the tweeter appears to be balanced a couple of dB too high in level. (I note that Bob Reina did find the speaker unforgiving of recordings that had problems in the treble.) There is a suspicious-looking peak and dip in the presence region, though BJR didn't remark on any coloration in this area. The grille introduced some unevenness, the variations reaching ±2dB in the low and mid-treble.
A speaker's on-axis response can't be examined in isolation; the dispersion will also affect the speaker's perceived balance. The PC60 CA's horizontal radiation pattern is shown in fig.4. Comparison with fig.3 indicates that the dip in the presence region fills in to the speaker's sides, while the limited dispersion in the top two audio octaves will work against the excess of energy in the same region on the tweeter axis. Fig.5 shows the vertical dispersion. The speaker maintains its response over a relatively narrow (±5°) window, suggesting that listeners should be sure to use stands that place their ears close to the tweeter axis. Though the crossover frequency is specified as 2kHz, a notch at 2.5kHz appears more than 15° above or below the tweeter axis, implying that that is the acoustic crossover frequency.
Turning to the time domain, the step response on the tweeter axis (fig.6) reveals that both drive-units are connected with positive acoustic polarity, while the smooth blend of the decay of the tweeter's narrow step and the rise of the woofer's step implies optimal crossover design. Some suspicious-looking undulations follow the peak of the woofer's step; the cumulative spectral-decay, or waterfall, plot (fig.7) shows a significant ridge of delayed energy at 2750Hz. This might be due to a problem with the termination of the flat woofer diaphragm by the surround. This resonance might also contribute to the speaker's intolerance of bright recordings. But other than that, the decay of the PC60 CA's sound is impressively clean, especially in the midrange. I'm not surprised that BJR used the word transparency to describe the PC60 CA's sound quality.
Its measured performance indicates that, other than that resonant problem in the crossover region, this is a well-engineered little speaker.John Atkinson