Stand Loudspeaker Reviews

Sidebar 5: Mk.II Measurements

I did some preliminary listening to the PC80 Mk.IIs before I measured them. This is a pretty good speaker for the price: relatively neutral with a surprising amount of bass. My estimate of its B-weighted sensitivity was a little below specification at 88.2dB/2.83V/m, but this is still on the high side for a small speaker. The PC80's impedance magnitude and phase are shown in fig.1: although it drops to 5 ohms in the lower midrange and to 6 ohms or so in the high treble, it generally approximates the specified 8 ohm load. The port tuning is revealed by the "saddle" centered at 43Hz.

Fig.1 Phase Technology PC80 II, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Note, however, the wrinkles in the traces at 160Hz and 245Hz. These indicate the presence of resonances of some kind (footnote 1). Listening to the panels with a stethoscope while I swept a sinewave up and down in this frequency region, I found a very strong resonance present at 245Hz on the back and top of the speaker, while the side panels vibrated strongly at 160Hz. Predicting the subjective effects of these resonances is not easy, as, being of high "Q" or Quality Factor, they have to be excited by a musical note coinciding more or less with their exact frequency. Note, however, that LB was not aware of any congestion or lack of clarity in the lower mids.

The PC80 has an impressively flat response on its tweeter axis (fig.2). Other than a slight energy excess in the upper midrange between 500Hz and 900Hz, and a slight lack in the crossover region, the balance looks pretty neutral. The woofer's output (the trace with the notch at the port tuning frequency) rolls off below 100Hz, crossing over to the port (the sharp bandpass response centered on 45Hz). The top trace to the left of the graph is the complex sum of the woofer and port nearfield outputs, weighted in the ratio of their diameters with the distance between the woofer and the rear-mounted port taken into account. Interestingly, though designer Ken Hecht's own ground-plane MLSSA measurements don't feature any rise in the low frequencies, this graph does suggest that the PC80's low frequencies appear to be a little exaggerated in level. This is something I found typical of the speaker in my own auditioning, though I suspect the one-note character of the bass that LB noted is perhaps due more to the highish-Q port tuning. The bass extension is excellent, however, the –6dB point lying at a low 37Hz.

Fig.2 Phase Technology PC80 II, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield woofer and port responses plotted below 300Hz and 800Hz, respectively, and the complex sum of nearfield woofer and port responses (top trace below 300Hz).

The PC80 is best auditioned with the grille in place, as LB found. This measurement was made with the grille in place. Without it, sharp peaks and dips of up to 3dB appeared in the speaker's treble response.

Fig.3 shows how the Phase Technology's response changes as the listener moves up or down from the tweeter axis. As long as you sit within a ±5° window centered on the tweeter, the balance should not change appreciably. Sit much below the tweeter, or stand up, and a suckout appears in the low treble. Horizontally (fig.4), the PC80 II offers excellent dispersion, even with its tweeter in its normal position. Perhaps because of the use of a flat-diaphragm woofer, the PC80 lacks the usual beaming at the top of the woofer passband.

Fig.3 Phase Technology PC80 II, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45°–5° above tweeter axis; reference response; differences in response 5°–45° below tweeter axis.

Fig.4 Phase Technology PC80 II, horizontal response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90°–5° off-axis; reference response; differences in response 5°–90° off-axis.

In the time domain, the PC80 II's step response (fig.5) shows the expected lack of time coherence, while the associated cumulative spectral-decay, or waterfall, plot (fig.6) reveals a very clean decay in the treble, marred only by a slight bit of resonant behavior at 2.2kHz, this associated with a slight on-axis peak that appears in both Ken Hecht's and my measurements. It's possible that this contributed to LB's feeling that the PC80 had a bit of upper-midrange emphasis, though this speaker's on-axis response is also a bit forward.

Fig.5 Phase Technology PC80 II, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Phase Technology PC80 II, cumulative spectral-decay plot at 50" (0.15ms risetime).

All in all, the PC80 Mk.II appears to be a well-engineered loudspeaker. Yes, a couple of problem areas are revealed by the measurements; but this is, after all, a very affordable loudspeaker.—John Atkinson

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Footnote 1: I use a swept sequence of a large number of spot sinewave tones to produce my loudspeaker impedance plots. Other techniques, such as MLSSA stimulation or fast swept sinewaves, aren't as revealing of cabinet problems, I've found.—John Atkinson