PSB Synchrony One loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the PSB Synchrony One's voltage sensitivity was 88.3dB(B)/2.83V/m, which is within experimental error of the specified 88dB. The speaker's impedance magnitude remained below 4 ohms throughout the midrange (fig.1, solid trace), with minimum values of 2.6 ohms at 1160Hz and 2.65 ohms at 73Hz. Though the electrical phase angle is generally low within the audioband (fig.1, dashed trace), the combination of 4.1 ohms and –45° phase angle at 29Hz suggests that the PSB be used with a good amplifier rated at 4 ohms.

The traces in fig.1 are free from the small discontinuities that would imply the existence of cabinet resonances of various kinds. Investigating the panels' vibrational behavior with an accelerometer revealed just two resonant modes on the sidewalls, at 340 and 420Hz (fig.2). While detectable with a stethoscope as the speakers reproduced the half-step–spaced tonebursts on Editor's Choice (CD, Stereophile STPH016-2), these modes are relatively low in level and well damped. The Synchrony One's tall cabinet is sensibly braced, though one of the speakers did develop a narrowband buzz in the upper bass after several weeks of use. However, this was detectable only with a stethoscope; I couldn't hear it with music during normal listening.

Fig.1 PSB Synchrony One, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 PSB Synchrony One, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The small saddle around 40Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the three reflex ports. However, as each of the three woofers is loaded with its own subenclosure and port and is driven by a different crossover filter, the low-frequency behavior of the PSB will be more complex than usual. This is revealed by fig.3, which shows the nearfield outputs of the upper woofer and its port (red traces), the middle woofer and port (blue), and the bottom woofer and port (green), all taken with DRA Labs' MLSSA system. Each woofer has a slightly different minimum-motion notch in its response between 37 and 44Hz, and the three ports each cover a slightly different region. But more important, each woofer also covers a different bandpass. The bottom woofer's output (green) peaks at 70Hz but rolls off rapidly in the upper bass. The top woofer's output (red) peaks around 100Hz but shelves down in the midrange. Only the middle woofer's output (blue) extends upward in frequency to cross over to the midrange unit.

Fig.3 PSB Synchrony One, nearfield responses of top woofer and port (red), middle woofer and port (blue), and bottom woofer and port (blue), all plotted in the ratios of their radiating diameters.

The two traces to the left of fig.4 show the complex sums of the individual port and woofer responses; the traces to the right show the farfield responses of the woofers and of the midrange/tweeter array on the midrange axis. The acoustic crossover point between the woofers and midrange occurs at 600Hz, a little higher than the specified 500Hz. Notable in this graph is how flat each set of drive-units is within its passband, and how well-controlled the rolloffs are. The woofer output does peak up a little in the region covered by the bottom woofer, but there is little sign of the usual nearfield boost in the upper bass, which suggests that the Synchrony One's woofers are somewhat overdamped.

Fig.4 PSB Synchrony One, acoustic crossover on listening axis, corrected for microphone response, with farfield responses of midrange/tweeter and woofers, with the summed nearfield responses of ports and woofers.

Fig.5 shows how these individual responses sum in the farfield, averaged across a 30° horizontal angle on the midrange axis, with the grille removed. The port can be seen to extend the bass to –6dB at 30Hz, a low frequency considering the speaker's small footprint. Again the low-frequency output peaks up a little in the region covered by the bottom woofer, and a small discontinuity can be seen at 3.8kHz. Overall, however, this is an extraordinarily flat response. A couple of small peaks can be seen close to the upper edge of the audioband, and I do wonder if these were the reason Erick Lichte was less tolerant of the Synchrony One's top-octave performance than I was. My hearing cuts off above 15.5kHz these days, while Erick's extends to 19kHz. Then again, he's half my age.

Fig.5 PSB Synchrony One, anechoic response on listening axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

There is nothing in fig.5 to indicate why I felt the Synchrony One's balance to be a bit forward in the mid-treble. However, looking at the speaker's plot of lateral dispersion (fig.6), while the contour lines are commendably even and well-controlled overall, a slight off-axis flare can be seen at the base of the tweeter's passband. This speaker may work best in rooms where it can be placed well away from the sidewalls, or where the sidewalls are absorptive rather than dispersive as they are in my room. In the vertical plane (fig.7), the Synchrony One's balance remains stable over a reasonably wide range of listening axes between the tweeter and the top woofer; ie, 29–39" from the floor.

Fig.6 PSB Synchrony One, lateral 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.

Fig.7 PSB Synchrony One, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

To look at how this quasi-anechoic behavior translates into the Synchrony One's behavior in the listening room, I took ten 1/6-octave–smoothed spectra for each speaker individually in a rectangular grid 40" wide by 18" high and centered on the position of my ears in my listening chair. (I used an Earthworks omni microphone and a Metric Halo ULN-2 FireWire audio interface in conjunction with SMUGSoftware's Fuzzmeasure 2.0 running on my Apple laptop.) The spatial averaging reduces the influence of position-specific room-acoustic effects in the bass and lower midrange; the result is shown in fig.8. The PSB's superbly flat anechoic behavior and even dispersion translate into an equally flat response in-room, with useful bass extension evident down to almost 20Hz. The usual floor-bounce suckout in the lower midrange is very much reduced in amplitude, but I conjecture that the slight excess of presence-region energy evident correlates with my feeling that the Synchrony One sounded a touch forward at times.

Fig.8 PSB Synchrony One, spatially averaged, 1/6-octave response in JA's listening room.

Turning to the time domain, the PSB's step response is shown in fig.9. All the drive-units are connected with positive acoustic polarity, each one's step smoothly handing over to that of the next lower in frequency. This correlates with the excellent frequency-domain integration of their outputs noted earlier. The speaker's cumulative spectral-decay plot (fig.10) is clean overall, though a slight amount of delayed energy is apparent at the frequency of the on-axis step in the treble.

Fig.9 PSB Synchrony One, step response on midrange axis at 50" (5ms time window, 30kHz bandwidth).

Fig.10 PSB Synchrony One, cumulative spectral-decay plot at 50" (0.15ms risetime).

The PSB Synchrony One offers superb measured performance, as I have come to expect of Paul Barton designs.—John Atkinson

PSB Speakers International
633 Granite Court
Pickering, Ontario L1W 3K1, Canada
(905) 831-6555
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