PSB Alpha B loudspeaker Measurements

Sidebar 3: Measurements

Despite its modest price, the PSB Alpha B offers slightly greater than average sensitivity, at an estimated 88.5dB(B)/2.83V/m. In addition, its plot of impedance magnitude and phase (fig.1) suggests it is a fairly easy load for an amplifier to drive. Although the minimum magnitude is 3.9 ohms at 250Hz, the electrical phase angle at that frequency is very low.

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

Other than a small glitch just below 30kHz, associated with the tweeter's ultrasonic dome resonance, the impedance traces are smooth and free from any wrinkles that would indicate the presence of panel resonances. Nevertheless, a waterfall plot calculated from the output of a plastic-tape accelerometer fastened to the enclosure top panel (fig.2) indicates the presence of some resonant modes. The two highest in frequency, at 530Hz and 1.2kHz, are probably above the range where they might have subjective consequences. However, the lowest-frequency mode, at 180Hz, is right in the middle of the region where it might be thought to obscure lower-midrange clarity.

Fig.2 PSB Alpha B, cumulative spectral-decay plot of accelerometer output fastened to center of top panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Nevertheless, as Bob Reina was impressed by the speaker's lower-frequency definition, it is probable that this cabinet resonance looks worse than it sounds. It was lower in level on the Alpha's side panels, and, as was recently pointed out to me by PSB's Paul Barton, it is important to remember that resonant vibrations on one panel may be canceled at the listening position by similar but out-of-phase vibrations on other panels.

The "saddle" centered at 58Hz in the impedance magnitude plot (fig.1) indicates the tuning of the rear-facing reflex port, and suggests, as might be expected from the Alpha's diminutive size, that the speaker doesn't go very low in frequency. The trace to the left of fig.3, with a notch in it at the port tuning frequency of 58Hz, is the woofer response; the trace with the broad peak centered in the same region is the port response. Adding these together in the ratio of their radiating diameters, taking into account both acoustic phase and the fact that the port is on the rear of the cabinet, gives the top trace to the left of the graph. The broad, shallow peak in the upper bass will be mainly due to the nearfield measurement technique, below which the speaker's output is down by almost exactly 6dB at the port tuning point.

Fig.3 PSB Alpha B, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and port responses and their complex sum plotted below 300Hz, 1kHz, and 300Hz, respectively.

Moving higher in frequency, BJR did note the PSB's slightly midrange-forward balance, and it can be seen from fig.3 that although the overall response, averaged across a 30 degrees horizontal window on the tweeter axis, is basically quite flat, the 800Hz-2.5kHz region is slightly higher in energy in the regions immediately above and below. (The grille was left off for these measurements.) Above the audioband is a sharp peak due to the tweeter's primary dome resonance, though this is too high in frequency to have any audible consequences.

(Note: I actually measured two pairs of Alpha Bs for this review. The matching, both between the members of each pair and between samples from different pairs, was superb, which is particularly commendable considering the Alpha's $250/pair price.)

The dispersion also affects the PSB's perceived tonal balance in a real room. Fig.4 shows the speaker's response as the measuring microphone moves from 90 degrees on one side of the tweeter axis to 90 degrees off-axis on the other side. While a few bumps and dips are visible, the overall smoothness of the off-axis curves and the even spacing of the contour lines in this graph suggest a well-controlled radiation pattern, this giving accurate, stable stereo imaging and, in conjunction with the relatively flat on-axis response, a natural in-room tonal balance.

Fig.4 PSB Alpha B, lateral response family at 50", from back to front: responses 90 degrees-5 degrees off-axis, reference response on tweeter axis, responses 5 degrees-90 degrees off-axis.

In the vertical plane (fig.5), steep suckouts in the crossover region develop above the tweeter axis and below the woofer axis. The Alpha B is best used on stands high enough to keep the top of its cabinet just above the listener's ears.

Fig.5 PSB Alpha B, vertical response family at 50", from back to front: differences in response 45 degrees-5 degrees above tweeter axis, reference response, differences in response 5 degrees-45 degrees below tweeter axis.

In the time domain, the Alpha's step response (fig.6) indicates that both drive-units are connected with the same, positive acoustic polarity, while the waterfall plot (fig.7) is surprisingly clean for such an inexpensive model. Overall, its measured performance gives no hint of the PSB Alpha B's low price.—John Atkinson

Fig.6 PSB Alpha B, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 PSB Alpha B, cumulative spectral-decay plot at 50" (0.15ms risetime).

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