PSB Alpha A/V loudspeaker Measurements

Sidebar 3: Measurements

Despite its diminutive size, the Alpha A/V was quite sensitive, my B-weighted estimate coming in at 88.5dB/2.83V/1m. It is also a kind load for an amplifier or receiver to drive, its impedance (fig.1) dropping below 6 ohms only in the lower midrange and at the port-tuning frequency of 45Hz.

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

The small wrinkles visible in the impedance plots at 200Hz and between 400 and 500Hz imply the presence of some kind of resonant behavior at these frequencies. Fig.2, a cumulative spectral-decay plot calculated from the output of a simple accelerometer fastened to the center of the cabinet sidewall, doesn't show anything untoward at 200Hz, but does feature a strong resonant mode at 460Hz. This could be heard as an added "hoot" on a swept sinewave, but was otherwise innocuous on music. The original Alpha's cabinet had even stronger modes at 240Hz and 400Hz, so the current version is better in this respect. Some "pumping" of the cabinet wall can also be seen at the port tuning frequency. All things being equal, this will soften the bass a little.

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

Figs.3 and 4 show the quasi-anechoic responses of 1992 and 1999 samples of the Alpha, both averaged across a 30 degrees lateral window on the tweeter axis. Considering the eight years' difference in manufacturing time and the revisions to the A/V version, the curves are very similar. Both feature a basically flat balance, with a slight excess of upper-midrange energy (or lacks of energy in the midrange and mid-treble, if you look at this in a different way). Both feature an underdamped LF alignment with similar bass extension, though the current sample's port covers a rather wider passband.

Fig.3 PSB Alpha, 1992 sample, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz.

Fig.4 PSB Alpha A/V, 1999 sample, anechoic response on-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.

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