PSB Image B25 loudspeaker Measurements

Sidebar 3: Measurements

The very nicely finished PSB Image B25 offered a voltage sensitivity slightly higher than average, at an estimated 88dB(B)/2.83V/m, which is within the margin of error of the specified figure. Its impedance was also moderately high, dropping below 6 ohms only in the lower midrange and mid-treble, with a minimum value of 3.7 ohms at 177Hz (fig.1). Good 4-ohm-rated amplifiers and receivers will have no problems driving this speaker.

Fig.1 PSB Image B25, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The wrinkle at 22kHz in the impedance traces is due to the tweeter's "oil-can" dome resonance, which is sufficiently high above the audioband to have no subjective consequences. The impedance graph is free from the small, sharp discontinuities that would imply the existence of resonances in the cabinet panels. Nevertheless, investigating the behavior of these panels with an accelerometer revealed the presence of some resonant modes. Fig.2, for example, is a waterfall plot calculated from the accelerometer's output when it was placed on the center of the sidewall. A mode at 387Hz is indicated by the ridge of delayed energy at that frequency, and a mode at 220Hz was also present on the back panel. However, as Bob Reina didn't comment on any coloration in the lower midrange, I must assume that these resonances do not couple to the air in any meaningful manner.

Fig.2 PSB Image B25, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered at 40Hz in the impedance-magnitude trace indicates that this is the tuning frequency of the flared port beneath the woofer on the front baffle. This is confirmed by the nearfield measurement of the woofer's output (fig.3, red trace), which has the expected minimum-motion point at this frequency, and of the port's output (fig.3, blue), which broadly peaks in the same region. The port's upper-frequency rolloff is commendably free from resonant behavior, and other than showing the usual upper-bass hump due to the nearfield measurement, the sum of the woofer and port responses (fig.3, black trace below 300Hz) shows a classic reflex alignment. The bass extends to -6dB at the port tuning frequency, good enough to reproduce the lowest notes of the double bass in good measure.

Fig.3 PSB Image B25, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer (red) and port (blue) responses plotted below 350Hz and 1kHz, respectively, as well as their complex sum, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.

Higher in frequency, the Image B25 is superbly flat on the tweeter axis (fig.3), the trace broken only by some slight wrinkles in the crossover region and a slight excess of energy at around 10kHz. Considering the B25's low price, this reveals some excellent speaker engineering on the part of PSB's design team. Of course, whether a speaker with a flat on-axis response actually sounds flat in a room will depend on its dispersion, but the Image B25 excels in this area also. The contour lines in its lateral dispersion plot (fig.4) are smooth and even, with just the slightest off-axis flare in the bottom of the tweeter's passband. The tweeter does get a bit directional above 10kHz, but in-room this will tend to balance the slight energy excess at 10kHz mentioned above.

Fig.4 PSB Image B25, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

In the vertical plane (fig.5), a big suckout develops in the crossover region more than 5 degrees above the tweeter axis, suggesting that high stands will work better than lower ones. However, given the slight flare in the tweeter's off-axis output between 3kHz and 7kHz, it's possible that the listener could ameliorate the effect of this on the perceived balance in very live rooms by sitting just above the tweeter axis.

Fig.5 PSB Image B25, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45 degrees-5 degrees above axis, reference response, differences in response 5 degrees-45 degrees below axis.

The B25's step response (fig.6) indicates that the tweeter and woofer are both connected with positive acoustic polarity. Other than a bit of low-level nonsense at 3kHz, the farfield cumulative spectral-decay plot (fig.7) is superbly clean.

Fig.6 PSB Image B25, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 PSB Image B25, cumulative spectral-decay plot at 50" (0.15ms risetime).

In the e-mail that accompanied his review text, Bob Reina enthused that PSB's Image B25 was a "very special speaker. A new benchmark at the $450 price point." Its measured performance indicates much the same!—John Atkinson

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