Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the KEF Reference 5's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and in-room responses. The Reference 5's specified sensitivity is 90dB/2.83V/m; my estimate was slightly lower, at 89.1dB(B)/2.83V/m. Its impedance is specified as 8 ohms with a minimum magnitude of 3.2 ohms. Fig.1 shows how the impedance and electrical phase vary with frequency. While the impedance lies above 8 ohms in the low treble, it remains between 4 and 5 ohms throughout the midrange and bass and in the top octaves. The minimum value was 3.3 ohms between 90 and 100Hz, but as the phase angle is generally benign, the Reference 5 should work well with tube amplifiers from their 4 ohm output transformer taps.
As I mentioned in the review, Jack Oclee-Brown eventually decided on using the long ports for the top two woofers and the short ports for the bottom two woofers. The short port gives a slightly higher tuning frequency than the long—45 vs 36Hz—but in fig.3, the blue trace shows the sum of both ports' outputs. It peaks between 27 and 70Hz, and although some midrange output can be seen, this is both low in level and will be ameliorated by the fact that the ports are on the speaker's rear panel. The red trace is the sum of the woofer outputs; it covers the range of 60–300Hz, and the upper-frequency rolloff appears to be around 12dB/octave and free from any spurious response spikes. The midrange unit rolls in with closer to a third-order acoustic slope and the Reference 5's upper-frequency response is relatively smooth, which can also be seen in the response averaged across a 30° horizontal window centered on the tweeter axis (fig.4). As is usual with a reflex design, the overall low-frequency response rolls off rapidly below the port's tuning frequency.
The KEF's plot of lateral dispersion, normalized to the tweeter-axis response (fig.5), reveals that the speaker smoothly and evenly becomes more directional throughout the treble, though the Reference 5 has a somewhat more restricted dispersion than the Blade Two. In the vertical plane, however, the Reference 5's response doesn't change appreciably over quite as wide a window (fig.6).
Fig.7 compares the spatially averaged response of the Reference 5 (red trace) with that of the Blade Two (blue). (Using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, I average 20 1/6-octave–smoothed spectra, individually taken for the left and right speakers, in a rectangular grid 36" wide by 18" high and centered on the positions of my ears.) As one might expect from the similar drive-unit arrays, the two speakers behave very similarly in my room, with a relatively even response from the midbass through the low treble, and with the output around 30Hz boosted by the lowest-frequency resonant mode in my room. However, the Reference 5's greater directivity above 5kHz results in a response that slopes down more in the top two octaves than the Blade Two's response, and actually resembles that of the Magico S5 Mk.II, which I reviewed in February 2017 (fig.8). While a flat treble response is not what you want to see in a graph like this—a room's furnishings are more absorbent at high frequencies than they are lower in the audioband—this graph does explain why I felt the Reference 5 to sound a little sweet.
Turning to the time domain, the KEF's step response on its tweeter axis (fig.9) indicates that its tweeter and woofers are connected in positive acoustic polarity, the midrange unit in inverted polarity. However, the decay of the tweeter's step smoothly blends with the start of the midrange unit's step, and the decay of that unit's step smoothly blends with the rise of the woofers' step. This suggests optimal crossover design. Finally, like the Blade Two, the Reference 5's cumulative spectral-decay plot on the tweeter axis (fig.10) demonstrates superbly clean decay from the midrange up.
Footnote 1: I had bought this PVDF accelerometer from Jesse Klapholz when I was investigating speaker-panel resonances for a 1992 article in Stereophile (see). Sadly, Mr. Klapholz passed away in June 2017, aged 63.
Fig.1 KEF Reference 5, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.).
The traces in fig.1 are free from the small discontinuities that would suggest the presence of panel resonances. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer (footnote 1), I did find some modes present at various places on the sidewalls (fig.2), but these are both very low in level and sufficiently high in frequency not to have audible consequences.
Fig.2 KEF Reference 5, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall level with third-highest woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 KEF Reference 5, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of midrange unit (green), woofers (red), and ports (blue), respectively plotted in the ratios of the square roots of their radiating areas below 400Hz, 2.1kHz, and 900Hz.
Fig.4 KEF Reference 5, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield midrange, woofer, and port responses plotted below 300Hz.
Fig.5 KEF Reference 5, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.6 KEF Reference 5, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 20–5° above axis, reference response, differences in response 5–20° below axis.
Fig.7 KEF Reference 5, spatially averaged, 1/6-octave response in JA's listening room (red); and of KEF Blade Two (blue).
Fig.8 KEF Reference 5, spatially averaged, 1/6-octave response in JA's listening room (red); and of Magico S5 Mk.II (blue).
Fig.9 KEF Reference 5, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.10 KEF Reference 5, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
As always with speakers from KEF, the Reference 5's measured performance reveals excellent speaker engineering.—John Atkinson
Footnote 1: I had bought this PVDF accelerometer from Jesse Klapholz when I was investigating speaker-panel resonances for a 1992 article in Stereophile (see). Sadly, Mr. Klapholz passed away in June 2017, aged 63.
