KEF R107 loudspeaker 1987 Measurements
As part of the preliminary test work, the KUBE was subjected to some basic lab tests for crosstalk, headroom, distortion, and noise. These were all passed with no trouble at all; we then ran off some response curves at the various settings. For example, fig.1 (one horizontal division equals 5dB) shows the effect of the bass-extension control for a constant Q of 0.5, Contour set at Zero. Given that the speaker has a falling low-frequency response, increasing its extension requires various degrees of bass boost. On line 1, with the minimum extension of 50Hz, the boost is negligible at 20Hz, while for the most extended 18Hz, -3dB response, the 20Hz point is lifted by 11dB and the 10Hz point by 17dB, relative to the midband. (Note that two constant equalization features are present on the graph, namely the dip at 50Hz to account for the underdamped intrinsic bass resonance cancelled by the KUBE, and the small dip near the upper crossover point at 2kHz, together with an associated 2.5dB of treble shelf cut.)
Fig.1 KEF R107 KUBE, Extension at Q = 0.5, Contour = 0 (5dB/vertical div.).
Fig.2 shows the effect of the Contour control at Q 0.5, Extension 18Hz (Setting 4). Operating below 300Hz, the contour applied a gentle, broad lift to the bass/low midrange, allowing control of the tonal balance to account for taste and variation in room acoustics. Typically, a range of ±3dB is available relative to the mean setting. In practice, no more than ±1.5dB should be required for such adjustment, so the range KEF offers is satisfactorily generous.
Fig.2 KEF R107 KUBE, Contour at Q = 0.5, Extension = 18Hz (5dB/vertical div.).
Fig.3 explores the effect of the synthesized system resonance Q over the allowed range of 0.3 to 0.7, with Extension set to 18Hz and Contour set at the mid or zero position. At the maximum Q of 0.7, the 50Hz box "excess" is barely nulled, while the rate of boost is at its most extreme, with +18dB at 10Hz. For a Q of 0.5, close to the optimum, the rate of boost is more gentle, with increased control at 50Hz. For Q of 0.3, the slope is shallower still, approximately 7dB/octave at the transition, while the 50Hz lift is fully cancelled by 3-4dB of equalization dip.
Fig.3 KEF R107 KUBE, Q at Extension = 20Hz, Contour = 0 (5dB/vertical div.).
Given the wide combination of equalization options, fig.4 was prepared to show the maximum and minimum limits. With a minimum 50Hz extension, an overdamped Q of 0.3, and Contour at -3, the 50Hz point is subject to a substantial cut of 12dB. Conversely, with everything at "full wick" the 10Hz boost rises to 22dB, requiring considerable extra headroom in the matching amplifier. Suppose a disc with relatively heavy bass requires an spl at 30Hz 3dB below the peak program level. In the fully equalized position, the bass requires 10dB of lift, less 3dB, for the equivalent bass level. Thus a 100W amplifier clipped in the bass at this 30Hz level could only drive the mid to a level 7dB lower, or approximately 20W. This is where the well-above-average sensitivity of the R107 performs a service, still managing to generate a substantial 98dB-plus for a stereo pair in a typical room from this "20W" midband input.
Fig.4 KEF R107 KUBE, Maximum.minimum equalization possible (5dB/vertical div.).
In practice, the bass power and equalization demands are generally lower than this, at typically +6dB at 30Hz, this providing little restriction on maximum level. The R107 load impedance checked out as an essentially pure 4 ohms resistance, within 5% from 20Hz to 20kHz—a relatively straightforward amplifier load. This resistive loading is compatible with many tubed amplifiers connected on their 4 ohm taps; eg, the ARC D115 or a Quicksilver mono. With a verified 90dB/W (8 ohm, 2.83V) sensitivity (within ±0.5dB), and a 200W (8 ohm) power capacity on program, the R107 will provide high maximum sound levels of up to 110dBA in typical rooms. Amplifiers as small as 25W will still offer quite high levels.
A plot of the forward frequency characteristics (semi-anechoic third-octave) was taken at 1 meter (fig.5). This showed the speaker to have a superbly uniform axial response, typically holding within ±2dB limits over the 40Hz-20kHz test range. Given its physical height, the 15 degrees off-axis result in the vertical plane above this speaker is an unlikely one, and shows some irregularity around the crossover point. Over the 30 degrees and 40 degrees lateral off-axis responses, the main frequency range is extremely well controlled. Some minor off-axis unevenness exists in the upper treble.
Fig.5 KEF R107, forward frequency characteristics (semi-anechoic 1/3-octave) at 1 meter.
Fig.6 shows the predominant axial frequency response at a rather close 0.5m measuring distance. This fine result also indicates the excellent integration of the driver outputs in this system.
Fig.6 KEF R107, 1/3-octave axial frequency response at 0.5m.
Fig.7 gives the averaged room response, with the KUBE omitted—a little bright, with a lumpy low bass, but with sufficient extension to 25Hz, in my room at least.
Fig.7 KEF R107, spatially averaged, 1/3-octave, free-field response in MC's listening room, no KUBE.
Fig.8 shows the computed room response with the standard settings of Q 0.5, Extension 20Hz, and with the Contour at zero. The low bass is seen to be a touch excessive, from 25Hz to 50Hz, while the upper bass is muted at 80Hz.
Fig.8 KEF R107, spatially averaged, 1/3-octave, free-field response in MC's listening room. KUBE settings: Q = 0.5, Extension = 20Hz, and Contour = 0.
Fig.9 shows the result of one of my attempts to tame the bass, using a Q of 0.4 and an extension of approximately 40Hz. Subjectively, this was considered respectably "dry" in the low bass, but somewhat lightweight in terms of midrange balance.
Fig.9 KEF R107, spatially averaged, 1/3-octave, free-field response in MC's listening room. KUBE settings: Q = 0.4, Extension = 40Hz, and Contour = 0.
Fig.10 shows the result of a good compromise in terms of bass balance and midrange tonality, though with some inevitable excess in the low bass. This result employed a reduced Q of 0.3, full 20Hz extension, and some +1 of contour lift to "warm" the tonal balance. The 50Hz point is several dB high, but not unacceptably so, while the overall frequency balance is pretty good.
Fig.10 KEF R107, spatially averaged, 1/3-octave, free-field response in MC's listening room. KUBE settings: Q = 0.3, Extension = 20Hz, and Contour = 1.
Specific distortion tests were not conducted; however, no perceptible distortion was noted during the tests, except in the case of gross bass overdrive.—Martin Colloms