KEF Reference Series Model Four loudspeaker:
Measurements part 2
The impulse response on the tweeter axis is plotted in fig.6, the step response in fig.7. The Model Four's drivers are all connected with the same acoustic polarity, though the speaker is not time-coherent. The cumulative spectral-decay or "waterfall" plot is shown in fig.8. This is a very clean result, with noticeably less decay hash apparent across the spectrum than is usually seen. The only significant mode is associated with the on-axis rise above 10kHz.

Fig.6 KEF Reference 4, impulse response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 KEF Reference 4, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 KEF Reference 4, cumulative spectral-decay plot at 50" (0.15ms risetime).

Finally, fig.9 shows the cabinet vibration measured on one of its sidewalls, 12" from the floor. As I'd suspected, this is a very well-braced, well-damped cabinet. Measurements taken at other locations were similarly lacking in significant panel resonances.

Fig.9 KEF Reference 4, cumulative spectral-decay plot of accelerometer output fastened to side of enclosure 12" from floor (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

This is a superb set of measurements of what is clearly a well-engineered loudspeaker. A little less output in the upper-bass/lower-midrange and a little greater EHF horizontal dispersion are the only areas where I feel improvements might pay big dividends. But, clearly, most loudspeakers can only envy the Reference Four's test-bench results.—Thomas J. Norton