KEF RDM Two loudspeaker Measurements part 2
Fig.4 KEF RDM Two, horizontal 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.
It might be thought that the coaxial drive-unit arrangement results in time-coherent performance. With its high-order crossover, however, the RDM Two's tweeter leads its woofer slightly in time, as can be seen from the step response (fig.5). Both units are connected with the same positive acoustic polarity, however. The speaker's cumulative spectral-decay plot on the tweeter axis (fig.6) is generally very clean, implying a freedom from grain and a smooth balance. However, the response step in the upper midrange is associated with a degree of resonant decay that I suspect is actually the cause of the nasality that WP found bothersome. That neither MF nor ST was too bothered by this coloration, however, suggests that this sonic character will, to some degree, be system- or room-dependent.
Fig.5 KEF RDM Two, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.6 KEF RDM Two, cumulative spectral-decay plot at 50" (0.15ms risetime).
Finally, I examined the speaker's cabinet vibrational behavior with a simple plastic-tape accelerometer. The main resonant mode I found was on the side panel at 310Hz (fig.7). It was well down in level, however, implying a rigid, optimally braced enclosure. But the RDM Two's cabinet did show a little flexing at the port tuning frequency, which might be associated with the warm-sounding low frequencies.—John Atkinson
Fig.7 KEF RDM Two, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)