PSB Alpha B1 loudspeaker Measurements

Sidebar 3: Measurements

All the measurements were performed with the speaker's grille in place. Despite its small size, the PSB Alpha B1 had an estimated voltage sensitivity of 87dB(B)/2.83V/m, which is right on the average of the speakers I have measured over the years. Though its impedance drops to a minimum value of 3.7 ohms at 235Hz (fig.1), the PSB will be relatively easy to drive overall, which is important given the inexpensive amplification with which it will be required to operate much of the time.

Fig.1 PSB Alpha B1, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The traces in the impedance graph are free from the small discontinuities that would indicate the presence of mechanical resonances of various kinds. Investigating the cabinet panels' vibrational behavior with an accelerometer did reveal a fairly strong mode present on all surfaces at 560Hz (fig.2), but this is too high in frequency to have much of an effect on sound quality. There is also some kind of resonant behavior apparent just below 200Hz, and I suspect it was this that led to the feeling of congestion at high levels.

Fig.2 PSB Alpha B1, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 57Hz in the impedance-magnitude plot (fig.1, solid trace) suggests that this is the tuning frequency of the rear-facing port. This was confirmed by the nearfield responses of the port (fig.3, red trace), which peaks between 40Hz and 80Hz, and of the woofer (fig.3, blue), which has the expected minimum-motion notch in its output at 57Hz. The port's upper-frequency output is refreshingly free from any resonant peaks.

Fig.3 PSB Alpha B1, anechoic response with grille on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the port (red), woofer (blue), and their complex sum (black), plotted below 500Hz, 300Hz, and 300Hz, respectively.

The sum of the woofer and port outputs (taking into account acoustic phase and the different distances from a nominal farfield microphone position) is shown as the black trace below 300Hz; even with the usual upper-bass boost from the nearfield measurement technique, the speaker's output is down 6dB at 60Hz. The Alpha is definitely a small speaker; however, the clean, well-damped nature of its bass alignment will allow it to be used relatively close to room boundaries, to get some low-frequency reinforcement.

Higher in frequency in fig.3, the Alpha B1's response on its tweeter axis in the midrange and treble is superbly flat and even. Yes, there are very slight peaks between 1 and 2kHz and at 10kHz—the latter probably due to the effect of the grille—and there is a narrow suckout at the crossover frequency, but overall, this kind of performance would be commendable in an expensive speaker, let alone one that costs just $279/pair.

I did find the PSB's presentation to be occasionally a touch bright, and this is not explained by the trace in fig.3. However, the speaker's horizontal radiation pattern (fig.4) does show a slight off-axis flare at the base of the tweeter's passband that might well have contributed to my feeling. The speaker's ¾" tweeter maintains its top-octave output to the sides to a greater degree than would a 1" unit. In the vertical plane (fig.5), a deep suckout centered on 3.4kHz develops more than 5° above or 10° below the tweeter axis, suggesting that sufficiently high stands be used to place the Alpha B1's tweeters level with the listener's ears.

Fig.4 PSB Alpha B1, 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.5 PSB Alpha B1, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Fig.6 shows the Alpha B1's in-room response, plotted with 1/6-octave resolution and averaged across a rectangular grid centered on the position of my ears. The small peaks and dips in the lower midrange are room effects that have not been completely eliminated by the spatial averaging. But other than that, the Alpha B1's in-room response smoothly and gently slopes down from the middle of the midrange to the top of the audioband in an almost perfect depiction of how a speaker needs to behave. Almost, because there is slight hint of there being too much energy in the presence region, which would give the speaker a slightly bright presentation, though this would also bring a benefit: recorded detail would be accentuated. The low frequencies extend in-room to the 80Hz band at almost full level, but roll off below that region. My room, unfortunately, does not give small speakers much in the way of boundary reinforcement.

Fig.6 PSB Alpha B1, spatially averaged, 1/6-octave response in JA's listening room.

In the time domain, the Alpha's step response (fig.7) indicates that both of the speaker's drive-units are connected in positive acoustic polarity, with the short, sharp step of the tweeter smoothly handing over to that of the woofer. The PSB's cumulative spectral-decay plot (fig.8) is perhaps the only graph that indicates any compromises due to the speaker's low price, in that the decay of the impulse is marred by some low-level resonant modes throughout the treble.

Fig.7 PSB Alpha B1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 PSB Alpha B1, cumulative spectral-decay plot at 50" (0.15ms risetime).

Overall, however, this is superb measured performance for such an affordable speaker, and demonstrates that some excellent engineering has gone into its design.—John Atkinson

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