Snell XA90ps loudspeaker Measurements
The Snell's estimated sensitivity was to specification at 90dB(B)/2.93V/m, which is higher than average. The worst-case impedance (fig.1) was with the boundary control switched to normal and the rear tweeter on. The magnitude varies between 3 ohms and 5 ohms throughout the midrange and low treble, which will mandate a good 4 ohm-rated amplifier be used, particularly as the electrical phase angle becomes increasingly capacitive in the upper bass due to the passive high-pass crossover filter that splits off the upper-range array from the powered subwoofers. Turning off the rear tweeter raises the impedance to an average 7 ohms above 3kHz, while switching in the boundary compensation adds about half an ohm in the low midrange.
Fig.1 Snell XA90ps, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 shows a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of the cabinet side panel. Four resonant modes can be seen, the lowest at 280Hz, but these are quite low in level and might not contribute to the speaker's perceived character.
Fig.2 Snell XA90ps, cumulative spectral-decay plot of accelerometer output fastened to center of side wall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
The nearfield responses of the ports, woofers, and lower- and upper-midrange units are all shown in fig.3. The port output is the bandpass curve centered on 33Hz. The acoustic crossover between the powered woofers and the lower-midrange units appears to be around 90Hz, and around 350Hz between the lower- and upper-midrange units—both a little lower than specified.
Fig.3 Snell XA90ps, (L-R) nearfield responses of port, woofers, lower-midrange units, upper-midrange units.
The XA90ps' overall response, averaged across a 30 degrees lateral window on the tweeter axis and spliced to the complex sum of all the lower-frequency nearfield responses, is shown in fig.4. The powered subwoofer rolls off very steeply below 32Hz. As LG commented, the upper midrange is balanced to be a little forward—an alternative interpretation of the graph is that the mid and high treble are plateau'd down by 2-3dB—but the big Snell's response is otherwise smooth and flat. This graph was taken with the HF switch set to its lower level. Switching it with the remote to the boost setting increased the level of the range between 7kHz and 20kHz by a maximum of 1.6dB, which will add compensatory high-treble energy in dead or overdamped rooms.