KEF Q900 loudspeaker Measurements

Sidebar 3: Measurements

I measured the KEF Q900's frequency response in the farfield with DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. The Q900's voltage sensitivity is specified as 91dB/2.83V/m. My estimate was slightly lower, at 90dB(B)/2.83V/m, but this is still usefully higher than average. The speaker's impedance is specified as 8 ohms, but as fig.1 shows, the impedance drops to 4 ohms in the top octaves and to below 4 ohms in the lower midrange, reaching a minimum value of 3.17 ohms at 160Hz. There is also a combination of 5 ohms magnitude and –42° electrical phase angle at 80Hz, meaning that the KEF does need to be used with a good 4 ohm–rated amplifier or receiver.

The traces in fig.1 are free from the small wrinkles that would imply the existence of panel resonances of various kinds. However, investigating the cabinet's vibrational behavior with a simple plastic-tape accelerometer did uncover some pumping of the sidewalls at the tuning frequency of the passive radiator, as well as, on all surfaces other than the baffle, resonant modes at 375, 453, and 512Hz (fig.2). The higher-frequency modes might be of sufficiently high Quality Factor (Q) to have no audible consequences; certainly, Kal Rubinson did not comment on any midrange congestion that might have resulted from their presence.

Fig.1 KEF Q900, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 KEF Q900, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

As KR did mention, the Q900 has four 8" cones covering the lower frequencies: an active midrange driver loaded by the passive radiator below it, and an active woofer loaded by a second passive radiator at the bottom of the front baffle. The saddle at 50Hz in the impedance-magnitude graph (fig.1, solid trace) suggests that this is the tuning frequency of both passive radiators, but looking at the individual nearfield responses (fig.3), the Uni-Q midrange unit's minimum-motion notch lies at 46Hz (black trace), the woofer's at 43Hz (blue trace). In each case, the output of the passive radiator (green and red traces) peaks significantly higher than the active unit's minimum-motion notch. The woofer's output gently rolls off above 120Hz or so, meaning that the midrange driver alone handles the musically critical midrange region (200Hz–2kHz). But the midrange unit's response does also extend down to the upper bass, which is why the speaker is referred to as a "two-and-half-way" design.

Fig.3 KEF Q900, nearfield responses of midrange unit (black), upper passive radiator (green), woofer (blue), and lower passive radiator (red).

Note, by the way, that the ultimate low-frequency roll-off of each radiator in fig.3 is closer to 18dB/octave rather than the usual 12dB/octave. This suggests the presence of a large series capacitor in the feed to both the midrange unit and the woofer, presumably to limit cone excursion at subsonic frequencies.

Usually, I calculate the nominal low-frequency farfield response of a speaker with multiple diaphragms by adding the nearfield responses of those diaphragms and weighting each response in the ratio of the diaphragm's diameter (or, if the diaphragm is not circular, the square root of the radiating area), taking into account each unit's acoustic phase. As I always say, this gives rise to a boost in the speaker's apparent output in the upper bass, due to an assumption made by the nearfield measurement technique: that the diaphragm is mounted in a true infinite baffle; ie, one that extends to infinity on all direction.

With the Q900, this calculation gave a large boost centered on the frequency of the peak outputs of the passive radiators (fig.4, green trace below 300Hz). This raised my eyebrows, so I attempted to measure the speaker's true farfield low-frequency response by placing it 8' above the ground—easier to say than do! —in the center of my back yard, and positioning the microphone to get the maximum path-length difference between the Q900's direct sound and the reflections from the walls and ground. This response is shown, in fig.4, as the red trace below 300Hz. Though accurate down to only 70Hz, it suggests that the sum of the nearfield responses is over-generous. In the listening room, due to boundary reinforcement, the Q900's low-frequency performance will be somewhere between the two extremes shown in fig.4, depending on the size of the room, but this does suggest that Kal's initial impression, that the balance was bit lightweight, was correct.

Fig.4 KEF Q900, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response (red trace), with farfield tweeter response (blue), farfield midrange/woofer response (green above 700Hz), farfield low-frequency response plotted below 300Hz (red), and complex sum of nearfield midrange, woofer, and passive radiator responses (green below 200Hz).

Higher in frequency in fig.4, the crossover between the Q900's midrange unit and woofers (green trace) and its coaxial tweeter (blue trace) lies approximately at the specified 1.8kHz, but there is broad overlap between the outputs, suggesting the use of low-order crossover filters. There are some peaks in the midrange unit's output between 5 and 10kHz; while these were audible on pink noise with the tweeter disconnected, they may well be sufficiently low in level when the tweeter is operating to add little to no treble coloration. However, when KR wrote "It was only when I really stressed the Q900 with very high levels that the Uni-Q tweeter became somewhat intolerant," I wonder if it was actually this misbehavior of the midrange driver that contributed to what he heard. The Q900's overall response on the tweeter axis is relatively even, but with a broad excess of energy in the low treble that will add to the sense of recorded detail. Overall, however, the treble is smoother and more uniform than that of some of the earlier Uni-Q speakers I have measured, suggesting that KEF's Tangerine waveguide works as advertised. The tweeter does have some very high-Q peaks present just below 30kHz, though of course these will be subjectively benign.

All the responses mentioned so far were taken with the Q900's grille removed. The effect of that grille is shown in fig.5: It introduces some small ripples in the response, and without it the Q900 does perform better, with a smoother treble. The Tangerine waveguide contributes to well-controlled dispersion in both the horizontal (fig.6) and vertical (fig.7) planes, the former correlating with the excellent stereo imaging noted by KR. The tweeter has wider top-octave dispersion than you'd expect from a 1.5" dome, and this will tend to compensate for the slight shelving down of the output in the same region in fig.4. Fig.7 suggests that the Q900 will not be fussy about the exact listening axis.

Fig.5 KEF Q900, effect of grille on anechoic response on tweeter axis at 50" (2dB/vertical div.).

Fig.6 KEF Q900, 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.7 KEF Q900, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

In the time domain, the Q900's step response on the tweeter axis (fig.8) suggests that the tweeter is connected in inverted acoustic polarity, the midrange and woofer in positive polarity. The fact that the decay of the tweeter's step blends smoothly with the start of the low-frequency units' step suggests optimal crossover design. Other than the low-level peaks in the mid-treble, the cumulative spectral-decay or waterfall plot on the tweeter axis (fig.9) is superbly clean throughout the midrange and treble.

Fig.8 KEF Q900, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 KEF Q900, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Despite its affordable price, KEF's Q900 offers superb measured performance, and the new Uni-Q driver, with Tangerine waveguide, gives much more uniform dispersion than have earlier Uni-Q designs.—John Atkinson

US distributor: GP Acoustics
10 Timber Lane
Marlboro, NJ 07746
(732) 683-2356

tiberius1023's picture

I listened to these speakers and they do sound quite well...very balanced. I'm curious as to how the q500 compares to the Kef q900 performance wise. I read another review here

They also gave the 900 a favorable review, although they reviewed it not with music but strictly for watching movies.

commsysman's picture

You say that the width is 14 inches in the text.

The manufacturer says 9.6 inches.

It looks more like 9.6" in the pictures.

Kal Rubinson's picture

One measurement is without the plinth.

Other measurement is with the plinth.


Tubejunkee's picture

The spikes did not come pre-installed and its up to the user to either screw them in from the top to take advantage of the spike,  or screw them in from the bottom for a wood friendly foot.

MhtLion's picture

I purchased KEF Q900 11 years ago based on Kal's review here. And, they still serve me with great musical satisfaction. I think my electronics are now about 10x in cost vs what I paid for the pair of Q900. So far these speakers scaled up nicely with each equipment upgrade. Sure, I will eventually upgrade. But, for casual listening while working from home, they are great.