Review: Kef Kube 12B Subwoofer

Review Kef Kube 12B SubwooferUK speaker builder Kef has a great reputation in the hi-fi field. But here, Stephen Dawson looks at one from a range of Kef subwoofers which are clearly targeted at the installation market. The use of terminal blocks for the high level or speaker level inputs, rather than spring clips or binding posts, makes that clear.

There are three models in the new KEF Kube range: the KEF Kube8b, Kube12b and Kube12b. No one will be surprised to be informed that the digits in the names refer to the nominal driver sizes. Here, I’m looking at the biggest of the three, the KEF Kube12b.

In addition to the terminal block speaker inputs (a suitable connector is included in the box) they have normal RCA sockets for line level. Normally a subwoofer has one of these (typically the left) marked for mono LFE use. These models use something called ‘KEF smartConnect’ to detect the presence of stereo or mono signals and set the volume level to match.

Also built in is a ’56-bit 50 MIPS DSP preamplifier’. KEF’s DSP programming is used both to provide EQ curves – there is a switch that allows it to be set for corner, free standing or against wall/in-cabinet.

It also delivers KEF’s ‘Intelligent Bass Extension’ or iBX. This seems to be one of those systems that modifies the performance to deliver the greatest power and bass extension that can be safely delivered, based on things like the signal itself, the level and of course the physical capabilities of the subwoofer itself.

KEF says this allows the sub “to play louder and remain dynamic at any listening level”.

Power is provided by a 300W Class D amp (the same is used in all three models). Class D means relatively cool running, which also permits in-cabinet placement.

In addition to the filter control and location EQ switch, there’s a level control on the back. The frequency response of the KEF Kube12b is rated at 22Hz to 140Hz ±3dB and KEF rates its maximum output at 114dB.

It’s a medium sized unit, measuring 410mm or less on all dimensions. It weighs just a little over 20kg.

9

Listening

I was using Denon’s modestly priced but surprisingly effective AVR-X2400H home theatre receiver with this subwoofer, with KEF R300 bookshelf sized front speakers and older, but high quality, centre and surrounds and overheads. I turned the filter control knob on the sub to the ‘LFE’ position and let the receiver manage crossover matters. The subwoofer went into my favoured subwoofer corner with its EQ switch to the relevant position. The Audyssey auto calibration put the crossover frequency a little low on the front speakers, in my judgement, so I changed it to 80Hz all round.

I hadn’t been clear on whether or not Audyssey applies ‘corrections’ to the subwoofer as well as the full range speakers. So I ran some pink noise through the subwoofer alone and switched the Audyssey EQ on and off. It soon became clear that the EQ was affecting the sub. That raised the question: is it more realistic, for review purposes, to listen with the Audyssey EQ off or on? It was tempting to have it off (and that’s what I did later for measurement purposes), but that would just mean that my impressions were tied to my particular room. These days virtually all systems will be equalised with some system or other, so Audyssey stayed on.

Starting with the ridiculous, I loaded up an old Telarc DTS surround sound sampler CD which, in addition to some rather fine music, has a track called Jurassic Lunch which contains copious levels of infrasonic bass, much of it under 10Hz. As this opened, the very first dinosaur footsteps, which are deepest, were only hinted at. The range of frequencies increases as the footsteps ‘near’ and then the subwoofer came into its own, delivering prodigious levels of bass in the low 30s and 20s of hertz. Just not in the ultra deep stuff.

Another surround go-to is the child abandonment and destruction-of planet-Earth opening in the kid’s (!) movie Titan A.E. This is distinguished as full of amazing, throbbing and powerful bass. I’ve measured some of the peaks at just 13Hz. The subwoofer provided the earthquake-like underpinning throughout this scene in the lead-up to the destruction, and first class impact as it hit.

Upgrading somewhat in taste, I switched over the Wendy Carlos’ Switched on Bach. This is stereo and so rather than being fed LFE, the sub was handling the below-80Hz bass handed off by the receiver’s settings. The superb bass line in the opening movement of the third Brandenburg Concerto was clean, fast and musical. It melded perfectly with the R300 speakers. It could have been them, had I not known the settings and that they can’t do that depth of bass with that kind of power.

That music’s pretty old, so I went over to Beyonce’s most recent, Lemonade, from last year. The opening track features a slow, very deep and very powerful bass line, sustained in way that can draw down amplifier reserves. But the Kube12b held its own, providing clean power at very high levels. I’d forgotten that the next track – Hold Up is similar, but more so. And it’s not just sustained, it attacks. There was a great visceral delivery, a real slam on each note, along with shifting items in my listening room. That slam again speaks of speed, and of good time coherence, keeping the range of frequencies within its coverage in proper alignment.

I also dipped into a Spotify Playlist called Subwoofer Songs which appeared to have been selected on the basis of their impressiveness with car subwoofers. El Chapo by The Game and Skrillex certainly seemed that way, with a mightily powerful subwoofer blast, comfortably handled by the Kube12b, that was entirely confined to one note. I guess that’s a note that car subs are particularly good at.

But other tracks on the same playlist – Really Really by Kevin Gates, for example, or Coke Like the ‘80s by Rick Ross – reinforced the impressions so far. The KEF Kube12b is an extremely competent subwoofer, capable of delivering enormous levels of very deep bass. I spent way more time in that list that was probably good for me, the security of fittings in my listening room, and my power bill.

10

Testing

I checked out the frequency response of the subwoofer, using pink noise fed from the home theatre receiver with the receiver’s crossover set to 250Hz, and the receiver’s EQ switched off. The measurement microphone was placed close to the subwoofer’s driver to minimise room effects and the level was set first to 80dB at one metre, then 90dB at one metre. The subwoofer itself had its EQ setting set to corner, since that was its location.

Despite the close placement, some allowance has to be made for the real world measurement conditions, so I typically use a ±4.5dB envelope rather than manufacturer’s ±3dB envelope.

Within that, I measured the output at 21.5Hz to 180Hz at 80dB, and 22Hz to 146Hz at 90dB. What can I say other than that these clearly confirm KEF’s own specifications.

But I’d add that what happens below those lower limits is interesting. The output doesn’t effectively disappear, but continues at a reduced rate. The 16Hz to 18Hz region was only a further 3dB down in level. That’s a characteristic of traditional sealed enclosure design. Bass reflex (and even more so passive radiator) designs lose oomph very quickly as they go below their design lower limit.

But KEF could easily have designed the DSP to cut off the bass. Instead, it seems to have left it to the enclosure to provide a natural 6dB per octave attenuation.

Conclusion

The KEF Kube12 is powerful, musical and equally capable with both music and movie LFE played at reference levels in a medium to large room. The price seems to me to be remarkably low for what it offers.

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