Sidebar 4: Measurements
I followed the instructions in the manual to set up the KEF LS60s, first installing the KEF Connect app on my iPhone 11 then connecting the Primary speaker to my Wi-Fi network. The speaker was identified as "LS60 Wireless-1a05a2," and its firmware was up to date. I used the app to select the input and apply DSP settings. Although Roon recognized the KEF as a Roon Ready Airplay device, I didn't use Roon for the testing. Instead, I performed a complete set of measurements with DRA Labs' MLSSA system using the Primary LS60's Aux analog input. I used a calibrated DPA 4006 microphone for the farfield measurements and an Earthworks QTC-40 mike for the nearfield responses.
KEF doesn't specify the speaker's sensitivity. With the speaker's sensitivity set with the app to the control's midway position of "5" and its volume control set to its maximum, white noise with a magnitude of 330mV fed to the analog input gave a B-weighted spl at 1m of 92.1dB(B). This is 19dB below the speaker's specified maximum spl of 111dB at this distance, so the LS60's Aux input has sufficient headroom.
Footnote 1: The LS60's manual says that this control can be adjusted up to ±4dB, but the KEF Connect app indicated ±3dB. Footnote 2: In this respect the effect of the LS60's Phase Control is very similar to that used by the KEF LSX wireless loudspeaker. See figs.1 & 2 here.
Fig.1 KEF LS60, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall below woofers (MLS driving voltage to speaker, 250mV; measurement bandwidth, 2kHz).
I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer. The cabinet was commendably inert. The only resonant mode I found was on the sidewalls, at 469Hz (fig.1); it was very low in level and won't have audible consequences.
Fig.2 KEF LS60, quasi-anechoic response on HF axis at 50", corrected for microphone response, with the nearfield response of the woofers set to Standard (black trace), Extra (red), and Less (blue), all plotted below 300Hz.
I left Wall mode off for the measurements. The black trace below 300Hz in fig.2 shows the nearfield response of the woofers set to Standard Bass extension. The 3dB peak in the upper-bass region will be due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle, ie, one that extends to infinity in both planes. Referred to the level at 100Hz, the LS60's low frequencies roll off with the sealed-box, 12dB/octave slope and are down by 3dB at 40Hz. Setting the Bass extension to Extra (red trace), the setting preferred by KR, lowers the –3dB frequency to 30Hz. Setting it to Less (blue trace) increases the –3dB frequency to 46Hz.
Fig.3 KEF LS60, Effect of the Treble trim set to +3dB (red) and to –3dB (blue) (1dB/vertical div.)
With the LS60's Treble trim set to 0dB with the app, the speaker's farfield response on the tweeter axis (fig.2, black trace above 300Hz) is superbly even and flat up to 8kHz, with then a slight lack of energy. Setting the Treble trim to the maximum of +3dB increased the speaker's response above 2kHz by up to 2dB (footnote 1). Setting it to –3dB reduced the level above 2kHz by approximately 2dB (fig.3). Fig.2 was taken with the Phase Control turned on. The LS60's farfield response was identical with the Phase Control off.
Fig.4 KEF LS60, lateral response family at 50", normalized to response on HF axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.5 KEF LS60, vertical response family at 50", normalized to response on HF axis, from back to front: differences in response 20–5° above axis, reference response, differences in response 5–20° below axis.
The LS60's horizontal dispersion is shown in fig.4, normalized to the response on the tweeter axis, which therefore appears as a straight line. The contour lines in this graph are evenly spaced, indicating uniform dispersion, though the speaker's radiation pattern narrows in the mid-treble region. The small suckout between 8kHz and 12kHz in the on-axis output tends to fill in to the speaker's sides, however, which will make the suckout less audible. Fig.5 shows the speaker's dispersion in the vertical plane plotted up to 20° above and be low the tweeter axis, which is 32" from the floor with the speaker sitting on its spiked feet. The speaker's balance doesn't change appreciably over a wide range of listener ear heights. Neither the horizontal nor vertical radiation patterns were affected by the Phase Control.
Fig.6 KEF LS60, step response on HF axis at 50" with Phase Control off (blue trace) and on (red trace) (10ms time window, 30kHz bandwidth).
I have shown the LS60's step response with the Phase Control off (fig.6, blue trace) and on (red trace). Without the Phase Control, the Uni-Q driver's tweeter and midrange sections can be seen to be connected in inverted acoustic polarity, with the tweeter's output arriving first at the microphone. The positive-going decay of the midrange section step smoothly blends with the start of the woofers' step. (Ignore the interruptions in the response at 17.6ms and later; these are due to reflections of the Uni-Q driver's output from the room boundaries.) Note the scaling of the horizontal axis in this graph. With a passive loudspeaker, it takes 3.7ms for sound to arrive at a microphone 50" away. The KEF's output arrives at 13.75ms, implying 10ms of extra latency from the loudspeaker's DSP. The Phase Control adds an additional 4.5ms of latency, aligning the negative-going tweeter and midrange steps so that they coincide to give a perfect right-triangle shape (footnote 2). Again, the decay of their time-coincident step smoothly blends with the woofers' step.
Fig.7 KEF LS60, Treble trim set to –3dB, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).
Fig.7 shows the LS60's cumulative spectral-decay plot on the tweeter axis, taken with the Phase Control on. It is superbly clean and wasn't significantly different when I turned Phase Control off. (I had inadvertently left the Treble trim set to –3dB for this measurement.)
The LS60 offered excellent measured performance. The Treble trim control's 0dB setting will be optimal for small rooms; Kal Rubinson had set it to +1.25dB for his auditioning. But given the absence of measurable differences in frequency response and dispersion with the Phase Control on and off and its positive effect on time alignment, I remain puzzled why KR expressed a preference for some kinds of music with it turned off.—John Atkinson
Footnote 1: The LS60's manual says that this control can be adjusted up to ±4dB, but the KEF Connect app indicated ±3dB. Footnote 2: In this respect the effect of the LS60's Phase Control is very similar to that used by the KEF LSX wireless loudspeaker. See figs.1 & 2 here.
