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KEF R400b Subwoofer

also a ‘female’ type, so all the electrical pins are hidden beneath the surface of the rear control plate. KEF provides the necessary adaptor as standard (it’s packed in with the detachable 240V cable).

KEF’s R400b subwoofer has rotary volume and crossover control knobs, and they’re very high-quality ‘click-stop’ types that allow precise and repeatable adjustments to be made, with 40 clicks between the maximum and minimum settings for each one. In addition to the tactile click-stops themselves, both the volume control and the crossover frequency control have additional white dot markings to further assist with control settings. The crossover control also has a position of 60Hz marked midway, as well as settings at 140Hz and 40Hz. There is a phase control, but it has only two positions: 0° and 180°. The manual notes—somewhat cryptically, I thought—that ‘the phase control will normally be set at 0° but more output level can be sometimes achieved in the 180° position, particularly when the subwoofer is far away from the satellite speakers.’

By far the most interesting control on the rear of the R400b is a three-position ‘Bass Boost’ control which allows you to boost the frequency response at 40Hz by +6dB or +12dB (the third position is a bypass setting that applies no boost at all, or 0dB). KEF says of this control that it is intended to: ‘adjust the response of the subwoofer to compensate for room conditions and user preferences.’

KEF provides two line-level inputs in the form of gold-plated RCA connectors marked ‘Left’ and ‘Right’. The right-most is additionally labelled LFE, so it’s this one that should be used if you connect your AV amplifier or receiver to the subwoofer via its LFE output. These connectors are additionally labelled as being a ‘Smart Connect’ input. KEF’s R400b manual says very little about this,

KEF R400bSubwoofer

Brand: KefModel: r400bCategory: Powered SubwooferRRP: $2,699Warranty: five/one YearsDistributor: Advance Audio AustraliaAddress: unit 8, 509–529 Parramatta road Leichhardt NSw 2040

(02) 9561 0799 sales@advanceaudio.com.au www.advanceaudio.com.au

readers interested in a full technical appraisal of the performance of the Kef r400b Subwoofer should continue on and read the LAborATorY rePorT published on page 82. readers should note that the results mentioned in the report, tabulated in performance charts and/or

displayed using graphs and/or photographs should be construed as applying only to the specific sample tested.

R400b SubwooFER

Lab Report on page 82

of preventing subwoofer cabinet vibration entirely—is to use a well-known driver configuration known as ‘force-cancelling’.

It is exactly this driver configuration that KEF has used in its newest subwoofer, the R400b.

The equipmenTThe cones in the R400b are made from paper pulp that has been reinforced by aluminium, according to KEF. In fact there’s a lot of aluminium in these new drivers, because it’s not only in the cones: the chassis are manufactured from it and so are the voice coils. KEF says in its promotional literature that ‘satin anodised elements contribute to the unit’s elegance’ … and indeed they do, except that because the drivers are hidden inside the cabinet, all this beautiful satin anodising is hidden from view! The only satin anodising that is visible are the wide—and admittedly very attractive—trim rings around the woof-ers’ circumferences. These trim rings not only look great—and make the R400b look very modern and stylish indeed—they also make the drivers appear to be much larger than they really are.

Since I am speaking of size, KEF puts the driver diameter at “9 inches” which my calculator turned into 225mm. However

Subwoofers have a crazy habit of mov-ing around your room when you’re not looking. Not all of them… just the ones that are haunted by evil spirits.

Some audiophiles call it the ‘subwoofer shuf-fle’, others call it ‘walking the subwoofer’ and a few mention the ‘subwoofer creeps’. Whatever your name for it, it is the result of the vibration of the large, powerful subwoof-er cone(s) causing the cabinet to vibrate so much that it moves across the floor. One way to stop a subwoofer walking is to anchor it to the floor with rubber feet… or use the old vampire method of spiking it!

The problem with both these simple solutions is that the subwoofer itself will still vibrate, and it will transmit these vibrations to the air via the cabinet walls—and to the floor through the feet—no matter how flexible the rubber feet, or how sharp the spikes. This vibration can then inveigle its way into your phono cartridge to interfere with the sound of your LPs, or into your CD (or DVD, or Blu-ray) player, where it will almost certainly cause the laser to mis-track, which will in turn introduce errors into the signal being recovered from the disc and in turn affect the sound quality.

The most technically elegant method of preventing subwoofer ‘creep’— and in fact

when I measured the driver to confirm this imperial to metric calculation, I discovered it has a unique design which made it impossible for me to make my usual measurements of the overall and Theile/Small diameters, because the peripheral driver suspension (the roll surround) is not visible, being an ‘inverse’ type. However I was able to measure the useable diameter of the cone as being 183mm, while the diameter of the cone itself was 166mm.

Because of these measurement difficulties I had to estimate the effective cone area (Sd), and my best guess came in at 227cm². So, because two cones are used (and they’re both driven) the cone area available to deliver bass is 454cm². So if KEF had wanted the same cone area from a single bass driver (the most usual format for powered subwoofers) it would have had to fit a cone with a Thiele/Small diameter of 240mm, which would had an overall diameter of around 254mm (10-inches). The point of this mathematical exercise is to prove to you that simply adding a second bass driver with the same diameter does not ‘double’ the cone area, which is the assumption most people make. For example, if you wanted to use 200mm-diameter drivers to get the same cone area as you would from a single 400mm-diameter driver, you’d

• force-cancelling• bass boost switch• Small and smart

• remote• Signal sensing• Phase switch

LAb rePorT

actually have to use four of the smaller-coned drivers to equal the area of the single 400mm-diameter driver.

You will also note that earlier I wrote ‘driven’ drivers. This is because in some subwoofers that appear to have TWO bass drivers, if you actually opened the cabinet, you’d find that only ONE of the two drivers was connected to the driving amplifier. The second cone being just a ‘drone cone’ or ‘passive radiator’ that moves only because the air inside the cabinet is pushing it. A passive radiator looks like a driver from outside the cabinet, but when you look behind it you won’t see any voice-coil or magnet…it’s literally just a ‘cone’. When a subwoofer is using such a drone cone, you can’t add its diameter to that of the driven driver, because such a duo doesn’t move more air per se, they instead move exactly the same amount of air as a single driver, but with each one working best at different frequencies.

That isn’t the case here with the KEF R600b, because each of its two drivers is powered by its own 250-watt Class-D power amplifier and the wiring is arranged so that both drivers move in unison, so that when one is moving ‘outwards’, so too is the other, and vice versa. Inside the cabinet, the backs of the two drivers are joined by a thick section of aluminium rod. This means that the vibrational forces generated by one driver are cancelled by the other, because the action is exactly equal and exactly ‘opposite’. This is KEF’s application of the ‘force-cancelling’ technique that I mentioned previously.

The control panel on the rear of the R400b is beautifully uncluttered, in part because KEF has done something I can’t recall seeing before on any subwoofer. Instead of providing large ‘speaker’ style terminals for the high-level (speaker level) inputs, KEF has instead provided its own custom connector which is not only extremely compact, but

KEF

32 Australian

KEF R400b SubwooferON TEST

the R400b exactly as I would any other subwoofer, because even if one driver is only a few centimetres from a wall and the other driver’s output is unimpeded, it seemed to make no difference to the sound quality. However, positioning the KEF R400b close to vertical surfaces (i.e. walls) does increase the overall output significantly, so if you do this you’ll be able to keep the volume control more counter-clockwise than if the subwoofer were to be placed in the middle of the room. I found that corner positioning works particularly well with the KEF R400b, which will be very useful if you’re using it in a large room, where you’ll get considerably more output than you would otherwise due to the fact that you have two nearby vertical surfaces to reflect the sound.

The usefulness of the controls being click-stop and the use of dots around the controls became immediately obvious when I read KEF’s set-up instructions for connecting to an AV receiver which (somewhat paraphrased) say: ‘Set the receiver to 80Hz, set the R400b to 140Hz and set the volume on the R400b to the seventh dot.’ You can’t get much more specific than the ‘seventh dot’! Well, actually, with the R400b it turns out that you can get more specific, because there are four click-stop positions between each dot, so when I was setting up the R400b and comparing different settings, I was setting controls to the ‘23rd click-stop’, which is even more precise than using the dots. My advice is that once you

achieve a setting you like the sound of, you should count how many ‘clicks’ the control is from either Min (for the volume control) or from 40Hz (for the crossover control) and write these settings down (perhaps on a piece of paper you could attach to the bottom of the subwoofer) so that if someone accidentally changes any of your control settings, or you want to experiment with other settings, you will be able to use your note to save a lot of time by using the paper to instantly regain an exact calibration.

I found that setting the volume and crossover controls on the subwoofer was actually made a little more difficult because of the added flexibility provided by the KEF’s unusual bass boost control, so it was several hours before I finally decided that I didn’t much care for either the 0dB or +12dB settings and instead concentrated on using the +6dB position and then optimising the subwoofer’s crossover and volume controls to get the best integration between my main speakers and the subwoofer. (Note, however, that my perception of the +6dB setting being the best was in my room and with the various main speakers I trialled with the R400b—in your room and with your own loudspeakers, one of the other two positions may turn out to provide superior performance.)

For me, the setting process was made more time-consuming again because, as usual, I trialled the KEF R400b with both large floor-standing loudspeakers and with small stand-mount loudspeakers, using both two-channel hi-fi sources and multi-channel soundtracks from DVD. However, I can’t emphasise enough that no matter what subwoofer you own, it is essential that you locate the best spot in the room to put it, and then tune it to best match whatever main speakers you’re using. You’ll find a guide to

but what the ‘Smart Connect’ circuit does is automatically sense whether you’re using one or both of the RCA line-level inputs. If you’re only using one of them, the Smart Connect circuit adds 6dB of gain to compensate for the level being lower than it would be if you were using both inputs.

Since I have mentioned KEF’s instruction manual, I have to say that despite its brevity (there’s only a single page in English, not counting the specification page), it’s remarkably informative and includes useful diagrams (six in all) that will enable you to correctly set up the R400b even if you’ve never before owned a sophisticated powered subwoofer. However, elsewhere in its promotional literature, KEF confusingly says that the R400b is ‘just 31 inches square’ which would make it as flat as a pancake! In fact, the subwoofer has no dimension that is 31-inches, as the subwoofer measures 14.4×13×13.8-inches (HWD), which in Aussie metric works out at 365×330×351mm…or about the size of a plastic milk crate. These external dimensions give an internal volume of only 22-litres. But don’t be fooled into thinking that because the R400b is small, it doesn’t weigh much. This little KEF sub tips the scales at just over 21kg, so you’ll need to engage a second pair of hands to help you if you intend to lift it safely.

in use and LisTening sessionsWhen connecting the R400b I was surprised to find there was only a single power on/off rocker switch, which I initially assumed meant that the subwoofer would stay on all the time, consuming energy from the mains supply even when it wasn’t being used. However, once I had everything connected and powered-up, I realised that my assump-tion was incorrect, because it turned out that the R400b does have auto-signal sensing circuitry built-in, so that it switches itself on automatically whenever it detects a signal at its input(s) and switches itself off after a period during which there’s been no signal (around 30 minutes, so far as I could tell… certainly long enough that cycling won’t be an issue!). Although I would have preferred to have manual control over the signal-sensing circuitry by being able to disable it (via an additional switch with positions for ‘Perma-nently On’ and ‘Signal Sensing’), I confess that I cannot justify this preference even to myself… in point of fact I always use the ‘signal sensing’ position exclusively on my own subwoofers!

The fact that the KEF R400b delivers equal output from both drivers made me re-consider the subwoofer placement options available to me in my listening room, but after considerable experimentation with positioning I discovered that I could position

There’s a lot of high technology packed inside this little subwoofer.

Close RelationsIf you’re looking for speakers to partner with the Kef r400b you should be made aware of the fact that the r400b is a part of Kef’s latest ‘r Series’ speakers all of which, says this uK company, ‘use trickle-down technology’ from the company’s state-of-the-art blade speaker. In addition to the r400b, this r Series comprises three floorstanding loudspeakers (the r500, r700, r900) and two standmount speakers (the r100, r300). There are also two centre-channel models available (the r200c and r600c), as well as a pair of small dipole surround speakers known as the r800ds. bar the smallest standmount r100—and, of course, the r400b!—all these speakers are three-way designs that

use a new magnesium/aluminium version of Kef’s famous uni-Q driver. This driver features large voice coils for high power handling, and a new ‘Z-flex’ surround that results in fewer baffle interactions for improved dispersion. Dispersion is also improved via the ‘Tangerine’ waveguide that sits forward of the tweeter. The three floorstanding models have three different bass driver diameters: 133mm, 165mm and 203mm bass driver and share the same uni-Q midrange driver. The r100 has only a single uni-Q driver, while the r300 adds a 165mm diameter dedicated bass driver. The centre channels differ in having either 133mm or 165mm bass drivers plus a uni-Q, while the dipole surrounds have the uni-Q and two 133mm bass drivers.

CoNTINuED oN PAGE 50

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ON TESTON TEST KEF R400b Subwoofer

positioning a subwoofer in a room at www.tinyurl.com/sub-pos

Even during the setting-up process it was immediately obvious that the KEF 400b would be something special, because the bass was not only impressively loud, but also because it was immediately obvious that the subwoofer had considerable low-frequency extension. Not having a grille in front of the drivers meant I could see how far the drivers

could move back and forth, and it’s a looong way! However once I had the subwoofer set up and its output correctly balanced against that of my main front speakers, I could barely see the cones move—even when playing at quite high volume levels—so there’s obviously a fair bit of headroom available.

When operated with a pair of large floor-standing speakers, the KEF 400b subwoofer easily ‘filled in’ the very lowest octave in the range of human hearing (from 20Hz to 40Hz), letting the floor-standers take over the music delivery above 40Hz. In this mode, I could not hear when the sub/main speaker transition took place, even when I played music with bass lines that danced around the transition point. The effect was uncanny… it really was

as if I’d magically extended the low frequency response of the main speakers. I also found that with my eyes closed, I could not ‘localise’ the position of the subwoofer with my ears, which proved to me that its distortion is so low that I was not able to hear any of the higher-order harmonic distortion products that would have allowed such localisation. Also uncanny was the total lack of vibration from the subwoofer… uncanny because it’s

so contrary to expectations, even if it is exactly what force-cancelling theory predicts.

Integrating the KEF R400b into a multi-channel system that used small two-way loudspeakers in all channels meant that I had to wind the crossover control up to what I calculated to be around a 100Hz crossover point, but once again the R400b surprised me by still seamlessly taking over the bass duties from the front-channel speakers. Again, there was no sense of the sound being ‘split’ between the

subwoofer and the satellites—the front sound stage was exactly as if I were listening to just a stereo pair. I did feel that with the extended upper bass response I was more able to localise the subwoofer though, so I compensated for this by re-locating the subwoofer exactly midway between the front left and right loudspeakers, which in turn also necessitated a slight increase in the subwoofer’s volume level, which the KEF R400b also took easily in its stride.

ConCLusionIt’s not inexpensive, but there’s a lot of high technology packed inside this little subwoofer, and if you want to get both big bass and deep bass from a small cabinet, it’s only technology such as this that will get you there. But the other great thing about the KEF R400b (other than the great sound, the small size and the usefully-adjustable controls… particularly that bass boost!) is that it really is a superbly stylish-looking subwoofer… so stylish that its very presence will enhance the décor of any room with modern furnishings. There’s not too many subwoofers you can say that about! greg borrowman

LAb REPoRT oN PAGE 82

So stylish that its very presence will enhance the décor of any room with modern furnishings.

82 Australian

LAB REPORT KEF R400b Subwoofer

We test this hugely powerful subwoofer!

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TesT ResuLTsKEF specifies the frequency response of the R400b as extending from 26–140Hz –6dB and you can see from the measurements made by Newport Test Labs that it met this specifica-tion exactly. Graph 1 shows the response measured at 2-metres, using pink noise as the test stimulus and you can see the black trace, which is the smoothed trace taken with the crossover control set at the 140Hz position (and with the bass boost control set to +12dB) extends from 26Hz to 140Hz –6dB. Another, more familiar way of stating this same curve would be as ‘26Hz to 140Hz ±3dB.’ As you can see, the response is extraordinarily flat across the region from around 35Hz up to 100Hz, being within ±0.5dB!

When the crossover control is set to 40Hz (and boost maintained at +12dB), the upper frequency response is necessarily restricted, which has the effect of ‘extending’ the low-frequency response, so in this setting, the KEF R400b’s frequency response was measured by Newport Test Labs at 20Hz to 75Hz –6dB. With this control combination the response is fairly ‘peaky’ with the peak occurring at 40Hz. My guess is that reducing the boost level to –6dB would remove this peak and flatten the response so that is would extend flat to around 75Hz. Unfortunately, this measurement was not made, but you can see

that I’m correct by looking at the lower of the two black traces shown on Graph 2, which shows the subwoofer’s performance during near-field testing (a test which is performed on only one of the two drivers). You can see that in the +6dB boost position the response is very flat from 20Hz to 40Hz, then rolls off at around 24dB per octave as frequency increases.

Looking at the effects on the frequency response of the Bass Boost control, it appears KEF would have been better to label the Bass Boost Settings as ‘+6dB’, ‘0dB’ and ‘–6dB’, as this more accurately describes their effect on the R400b’s frequency response. However, KEF’s nomenclature is technically correct, since you can’t have –6dB of ‘Boost’. This is actually one case where I think the advertising guys should have over-ruled the technical boffins!

Graph 3 shows the Boost Control effect when the crossover control is set to 60Hz, using a nearfield measurement technique, while Graph 4 shows all the nearfield responses on the same graph. You can easily see from this graph that you will be able to find a combination of settings that will enable you to integrate the R100b perfectly with any pair of speakers, from the smallest of satellite speakers through bookshelf models and right down to large floorstanders. Overall, my opinion is that this is a brilliantly designed—and brilliantly executed—subwoofer. Steve Holding

CoNTINuED FRoM PAGE 30

Overall, my opinion is that this is a brilliantly designed — and brilliantly executed — subwoofer.

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Graph 4: Nearfield sine frequency response of bass driver with crossover control set to 140Hz, 60Hz and 40Hz showing response with Bass Boost Control set to +12dB (red traces), +6dB (black traces) and 0dB (green traces). [KEF R400b Subwoofer]

Newport Test Labs

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Graph 2: Nearfield sine frequency response of bass driver with crossover control set to 140Hz showing response with Bass Boost Control set to +12dB (red trace), +6dB (black trace) and 0dB (green trace) and also to 40Hz (same trace colours). [KEF R400b Sub]

Newport Test Labs

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Graph 3: Nearfield sine frequency response of bass driver with crossover control set to 60Hz showing response with Bass Boost Control set to +12dB (red trace), +6dB (black trace) and 0dB (green trace). [KEF R400b Subwoofer]

Newport Test Labs

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Graph 1: Pink noise frequency responses at 2.0 metres with crossover control at 140Hz smoothed (black trace) and unsmoothed (red trace) and at 40Hz smoothed (green trace) and unsmoothed (blue trace). [KEF R400b Subwoofer]

Newport Test Labs