KEF Blade Two loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the KEF Blade Two's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. The Blade Two's voltage sensitivity is specified as 90dB/2.83V/m; my estimate was somewhat lower than this, at 87.1dB(B)/2.83V/m. The Blade Two's nominal impedance is 4 ohms, with a minimum value of 3.2 ohms. My measurement (fig.1) revealed a minimum magnitude (including cable) of 3.4 ohms at 177Hz (solid trace). Though the impedance remained below 6 ohms in the midrange, the electrical phase angle (dashed trace) was low in the same region. The phase angle was greater in the treble region but the impedance was also higher, ameliorating any drive difficulty.


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

Other than one at 30kHz, presumably due to the tweeter's fundamental dome resonance, the traces in fig.1 are free from the small discontinuities that would indicate the presence of enclosure panel resonances. When I investigated the behavior of the cabinet with a simple plastic-tape accelerometer (similar to a piezoelectric acoustic-guitar pickup), I found very little untoward. The only resonant mode I could find was at 200Hz (fig.2), and although it was present on the curved baffle and sidewalls, it was very low in level. The Blade Two's enclosure is extremely inert, and the force-canceling woofer arrangement seems to work as advertised.


Fig.2 KEF Blade Two, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of front baffle below bottom woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered just below 40Hz in the impedance-magnitude trace in fig.1 suggests that the two ports on the rear panel are tuned in this region. The minimum-motion notch at 37Hz in the sum of the woofers' nearfield outputs (fig.3, blue trace; all four woofers behave identically) confirms that this is the port tuning frequency. Both ports appear to behave identically, and the sum of their nearfield responses (fig.3, red trace) peaks sharply between 30 and 40Hz, with a smooth rollout above that region broken only by some very low-level peaks in the midrange. Unusually, the woofers and ports appear to roll off below the port resonance with closer to 18dB/octave slopes rather than the 12dB/octave slope typical of a ported design. This is because the ports are tuned lower than with a textbook B4 alignment, which gives an initial third-order rolloff that shallows off to 12dB/octave at very low frequencies.


Fig.3 KEF Blade Two, acoustic crossover on tweeter axis at 50", with nearfield responses of midrange unit (green), woofers (blue) and ports (red).

Higher in frequency in fig.3, the woofers roll off smoothly, crossing over to the midrange section of the Uni-Q driver (green trace) between 300 and 400Hz. The midrange unit also rolls off below the crossover to the woofers with an 18dB/octave slope, and its crossover to the coaxial tweeter is seamless. While there is a small peak at 10kHz, note how smooth, even, and extended the Blade Two's upper-frequency response otherwise is in this graph. Fig.4 shows the KEF's farfield response on the tweeter axis, averaged across a 30° horizontal window and spliced at 300Hz to the complex sum of the nearfield responses of the midrange unit, woofers, and ports (taking into account acoustic phase and the different distance of each radiator from a nominal farfield microphone position). The Blade Two offers a superbly flat, even response on the tweeter axis. Wow!


Fig.4 KEF Blade Two, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

That doesn't in itself mean that the speaker will sound neutral, as the perceived quality will also depend on the radiation pattern. The KEF's horizontal pattern is shown in fig.5, normalized to the response on the tweeter axis, which is therefore presented as a straight line (which is not too different from the actual response). Other than the cancellation notch between 300 and 400Hz and the boost centered on 1kHz at extreme off-axis angles, due to destructive interference between the woofers on opposed sides of the enclosure, the contour lines in this graph are superbly smooth and even. The horizontal dispersion is narrower in the midrange and lower treble than with a conventional dynamic loudspeaker, but wider in the top octave. In the vertical plane (fig.6), the Blade Two's response hardly varies at all over a wide (±10°) window centered on the tweeter axis, which itself is 39" from the floor.


Fig.5 KEF Blade Two, 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.6 KEF Blade Two, 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 5–10° below axis.

The red trace in fig.7 is the Blade Two's spatially averaged, 1/6-octave response at the listening position in my room. (Using an Earthworks QTC-40 microphone, I average 20 1/6-octave–smoothed spectra, individually taken for the left and right speakers using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, in a rectangular grid 36" wide by 18" high and centered on the positions of my ears.) Other than the large peak between 30 and 40Hz, due to the coincidence of the port tuning frequency and the lowest mode in my room, and which I couldn't eliminate by experimenting with the speaker positions, the KEFs' in-room response is impressively smooth and even from the midbass through the high treble. The blue trace in this graph is a similar spatially averaged response for the DALI Rubicon 8 speakers, which I reviewed in the March issue. That Danish speaker produced more upper-bass energy in-room, which balanced its higher level of mid-treble energy, and extended slightly lower in frequency.


Fig.7 KEF Blade Two, spatially averaged, 1/6-octave response in JA's listening room (red); and of DALI Rubicon 8 (blue).

While capturing these in-room responses, I was impressed by how closely the two review samples matched. Other than the left speaker having slightly more output in a narrow band in the presence region, the two channels matched to within 0.5dB from 600Hz to 30kHz (fig.8).


Fig.8 KEF Blade Two, 1/6-octave response in JA's room at listening position (left speaker blue, right red).

In the time domain, the Blade Two's step response on its tweeter axis (fig.9) indicates that the tweeter is connected in inverted acoustic polarity, the midrange and woofers in positive acoustic polarity. More important, the decay of the tweeter's step smoothly blends with the rise of the midrange unit's step, and the decay of that unit's step smoothly blends with the rise of the woofers' step. This suggests optimal crossover design. Finally, the KEF's cumulative spectral-decay plot on the tweeter axis (fig.10) demonstrates superbly clean decay from the midrange upward.


Fig.9 KEF Blade Two, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).


Fig.10 KEF Blade Two, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

KEF's Blade Two offers superb measured performance that is close to the state of the art.—John Atkinson

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

audiodoctornj's picture

As a Blade dealer who has both the LS 50 and the Blades and the Ref 1 and 3 and the R series, I feel I know the line really, really well.

I agree with many of the comments that were made, however, I was really taken back by the comparison to the LS 50 saying that other than increased bass, and dynamics, you seem to be implying that there is some similarity between the LS 50 and the Blade II.

We love the LS 50 and even with the matching KEF R400 sub woofer the sound is really great, and in fact will challenge a lot of $5k plus speakers, but the resulting sound doesn't come close to amazing resolution, and incredible holographic sound stage, which comes out of the Blades, so it is not just about the increased bass and dynamics, there is a wholeness and organic quality to the sound that you just don't get out of the LS 50 or many other speakers.

We had a setup at the New York High End Show in 2013 with the original Blades setup with state of the art electronics, a Merrill Williams Turntable, and an EMM Labs Digital setup and the system was amazing! Yes this system was over a $100k: $30k speakers, and 50k electronics, and $15k front end plus $20k plus on cables, so the system was up there in price but I did listen to the $107k Yg the $54k Wilson's plus some other very expensive speakers and I would say that a well setup pair of Blades offers the performance of speakers in the $50k-100k price range and that is an amazing value proposition that the Kef Blades and Blade II represent.

w1000i's picture

How do you compare Ref 3 to blades ?

dfwatt's picture

Agree with you completely AudioDoctorNJ, about the Blades (both original and their baby brother).

We just picked up a pair of the Blades I, after owning (and loving each in their own way) the 104/2, the Reference 107s, the 201/2s, R300s, and several others. But the Blades are just in a different league, exactly in the way you describe in terms of the holographic sound stage they create, with a seamlessness and coherence that is just so addicting. Each generation of the KEF Uni-Q driver system has been better, and in the 201/2 generation you could begin to see this approach and technology really hitting its stride. Those 201/2 were the best small speaker anyone had created up to that point in time, while the R series might be the best value in a speaker for those with reasonable budgets, and offering an amazing amount of the technology of the previous gen Reference series for a whole lot less money.

If any $32k speaker can be considered a bargain, in the rarefied air of speakers attempting to be 'ultimate' loudspeakers, the Blades are a steal. They are worth every penny. And they frankly embarrass some of the esoteric house brands pushed by the audiophile fringe, which can easily go for three times their cost.

JA's measurements show why they sound so good. KEF technology and understanding of psycho-acoustics is a step ahead of the field, and they focus on what research shows you can actually hear, and not stuff that the audiophile community finds fashionable, trendy, etc. Science really does work, and their commitment to empirically based design shows that right now, they are making the best speakers in the world for the money.

brian_pdx's picture

Would love to hear these speakers although esthetics will take time to get used to. Similar to TT dust covers are speaker grills going away? Dust covers serve a need, make total sense and a well designed speaker grill keeping your 5 year old from putting a PBJ through a woofer makes even more sense.

brian_pdx's picture

Went to an event last Saturday at Echo Audio in Portland where the big Blades were featured by KEF. I was completely taken by the sound, one I had never heard from a home speaker before. For that price you expect it but KEF also demo'd the Reference 1 and the LS50s both of them simply outstanding but the Ref 1 was also driven by the 400W Chord monoblocks. The KEF people said the Ref. 1 could be well driven by 200 W. They also said, to answer my above question, the drivers are very "tough" and hard to damage even without grills. I agree with the reviewer that these speakers are very neutral, source material is important but I cannot get over the way the sound of all three speakers embraced me. At 1/4 the price, the Reference series is actually attainable. But the Blades make me want to buy a lottery ticket. Highly recommended.

Venere 2's picture

Although this review is about the KEF Blade, I have to mention another KEF speaker that is highly recommended.
I had the chance to audition the KEF LS50 speakers. The hype and fanfare surrounding the LS50s is out of this world. The performance is anything but… They are very good, but they don't come anywhere close to what all the hype would have one believe. They are a good buy for their price, nothing more. They absolutely do not belong in Class A!

If the 1500$ KEF LS50 belongs in class A, then what Class for the 25 000$ blades? The first Class A++++ ever for Stereophile?

makarisma's picture

I think they are great as desktop speakers.

klosterman's picture

Hi John,
Chatted with a KEF dealer recently about the Blade Two's. He emphasized they absolutely require top flight amps to make them sing properly. Didn't see that mentioned in this review.

Perhaps the unspoken assumption that pricey speakers = pricey amps? Did you try the Blades with other amp combos out of curiosity?

spudnik's picture

Having read your review I have been left with the frustrating belief that it was incomplete. The problem, as you have no doubt been informed, is that there is no comparison between Blades I and II. The ommission is also obvious: Sterephile did not reveiw the Blade I. I do not know the rationale for that choice but as I have complained before, it was a bad one. You are Stereophile. You ought to have found a way.

WM's picture

I'm guessing the slopes are steep with the side firing woofers, but just wondering if there is any issue with that crossover point and 2pi etc.

Axiom05's picture

Although this review is somewhat old, the speakers are still current, so I thought I would comment. First, the in-room response must be one of the smoothest that I have ever seen. I would imagine that this contributes to the superb imaging. I am surprised that KEF did not provide alternative port tubes like they do for the Reference series, these might have allowed you to tweak the low end a bit more, although the lowest frequency room mode is always difficult to modify with ported speakers. Second, KEF's dealer network is really awful in the US. I would love to give these speakers a serious audition but there is not a single Blade-carrying dealer in the state of Florida. The same was true when I lived in Philadelphia, Pennsylvania. Basically, I have been trying to hear these speakers since their release.

davidrmoran's picture

What is up w/ that 500Hz peak? Looks nasty-sounding. Not real?

Allison effect unpleasant lower-midrange dip ~160-260Hz can be partly addressed w/ placement experimentation closer to front wall. Probably not possible to get rid of it altogether cuz the woofers are up too high.
Nice that they are sidemounted.

remlab's picture

By looking at the lateral response family on the Uni-Q, the crossover point appears to be around 3.5 kHz rather than the specified 2.3 kHz.

steve59's picture

Thanks for the review. Now that the blade 2 meta is out I’m interested in how kef improves on these speakers.