KEF LS50 Meta loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the KEF LS50's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. My estimate of the LS50 Meta's voltage sensitivity was 84.5dB(B)/2.83V/m, which is the same as that of the original LS50 and within experimental error of the specified 85dB.

The solid trace in fig.1 shows how the Meta's impedance magnitude varies with frequency. Below 200Hz, it is almost identical to that of the 2012 LS50 (fig.2, solid trace), but at higher frequencies the impedance traces are quite different. The Meta remains closer to 4 ohms for longer in the midrange but has a considerably higher magnitude in the crossover region. The new speaker's electrical phase angle (fig.1, dashed trace) is occasionally high, and I used the formula in a 1994 JAES paper to calculate the "equivalent peak dissipation resistance" (EPDR, footnote 1). The LS50 Meta has minimum EPDRs of 1.66 ohms between 135Hz and 140Hz and 1.7 ohms between 660Hz and 725Hz, both regions where music can have high energy. The partnering amplifier needs to be comfortable driving 4 ohms.

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Fig.1 KEF LS50 Meta, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

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Fig.2 KEF LS50 (original), electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The traces in fig.1 are free from the small discontinuities that would suggest the presence of cabinet resonances. Cumulative spectral-decay plots of the cabinet walls' vibrational behavior, calculated from the output of a plastic-tape accelerometer, confirmed that the enclosure was effectively inert (fig.3), even more so than the original, which had performed well on this test.

1220KEF50fig03

Fig.3 KEF LS50 Meta, 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).

The port is tuned to 51Hz, confirmed by the minimum-motion notch at that frequency in the woofer's nearfield output (fig.4, blue trace). The port's nearfield response (red trace) peaks sharply between 40 and 70Hz, and though some upper-frequency output is visible, this is well down in level. The behavior of the woofer and port is effectively identical to that of the original LS50.

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Fig.4 KEF LS50 Meta, anechoic response on HF axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue) and port (red) and their complex sum (black), respectively plotted below 312Hz, 1kHz, 312Hz.

The LS50 Meta's farfield output, averaged across a 30° horizontal window centered on the tweeter axis (fig.4, black trace above 300Hz), is superbly even from the lower midrange through the treble—better, in fact than that of the original version. The tweeter's output remains at full level almost to the 20kHz limit of this graph. As with other recent loudspeakers from KEF that use the coaxial Uni-Q drive-unit, including the earlier LS50, there are no discontinuities in the mid-treble that might be due to destructive interference between the direct radiation from the tweeter and reflections of that radiation from the circumference of the woofer cone. The Meta's dispersion, like that of the original LS50, is very uniform in both horizontal (fig.5) and vertical (fig.6) planes, with well-controlled narrowing of the radiation pattern in the top octaves.

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Fig.5 KEF LS50 Meta, 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.

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Fig.6 KEF LS50 Meta, vertical response family at 50", normalized to response on tweeter axis, from back to front: responses 45–5° above axis, reference response, responses 5–45° below axis.

The LS50 Meta's spatially averaged response in my listening room is shown as the red trace in fig.7. (I average 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually with a 96kHz sample rate, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. This largely eliminates the room acoustics' effects and integrates the direct sound of the speakers with the in-room energy to give a curve that I have found correlates reasonably well with a speaker's perceived tonal balance.) For reference, the blue trace in fig.7 shows the original LS50's spatially averaged response with the speakers in exactly the same positions as the Metas.

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Fig.7 KEF LS50 Meta, spatially averaged, 1/6-octave response in JA's listening room (red), and of original LS50 (blue).

The two models offer identical in-room responses below 300Hz. There are two small peaks and dips that have not been eliminated by the spatial averaging, and the midbass region is shelved down, though the extension is helped by the lowest-frequency mode in my listening room. Both the LS50s and LS50 Metas would have benefited from being placed closer to the wall behind them than is possible in my room. Both speakers have a little too much in-room energy in the upper midrange, but the Meta's response is flatter in the presence region and there is a little more output in the high treble than with the Anniversary Edition. The top two audio octaves slope down with both models, due both to the increased absorption of the room furnishings and the speakers' increasing directivity in this region.

Turning to the time domain, the LS50 Meta's step response on the Uni-Q axis (fig.8) indicates that the tweeter and woofer are both connected in positive acoustic polarity, with the tweeter's output arriving first at the microphone. This is different from the original LS50, where the tweeter was connected in negative polarity (fig.9). In both cases, the decay of the tweeter's step smoothly integrates with the start of the woofer's step, which implies optimal crossover design. Like the Anniversary Edition LS50, the LS50 Meta's cumulative spectral-decay plot on the tweeter axis (fig.10) features astonishingly clean decay at all frequencies. (Ignore the black ridge of delayed energy just below 16kHz in this graph, which is due to interference from the computer's video-display circuitry.)

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Fig.8 KEF LS50 Meta, step response on HF axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.9 KEF LS50 (original), step response on HF axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.10 KEF LS50 Meta, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).

This is superb measured performance, even without taking into consideration the LS50 Meta's affordable price.—John Atkinson


Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads" by Eric Benjamin, JAES, Vol.42 No.9, September 1994, and Keith Howard's article here.
COMPANY INFO
KEF, GP Acoustics (UK) Ltd.
US distributor: GP Acoustics (US) Inc.
10 Timber Lane
Marlboro, NJ 07746
(732) 683-2356
ARTICLE CONTENTS

COMMENTS
latinaudio's picture

It´s written on my Santa´s letter this year...

Anton's picture

https://images-na.ssl-images-amazon.com/images/I/71P6kaBRLnL._AC_UX385_.jpg

Charles E Flynn's picture

The "Introducing the Meta" section of the review gives the price as "a penny less than $1500/pair", as does the KEF.com website, but the Specifications section has " $1599/pair".

tonykaz's picture

If you can wait for a few months, these Meta versions ( previously loved ? ) will probably sell for about half of MSRP.

The earlier versions are selling for $700 approximately which is a good thing for a Chinese product.

Schiit gear sells on eBay for closer to 75% of MSRP but it's all made in USA.

A used pair of British made LS3/5a will sell for Considerably More than these KEFs but they are all the exact same loudspeaker as the BBC design mandates. ( the venerable LS3/5a remains a Gold Standard Transducer )

Tony in Venice

John Atkinson's picture
Charles E Flynn wrote:
The "Introducing the Meta" section of the review gives the price as "a penny less than $1500/pair", as does the KEF.com website, but the Specifications section has " $1599/pair".

A slip of my typing finger. Have corrected the error and my thanks for finding it.

John Atkinson
Technical Editor, Stereophile

Charles E Flynn's picture

You are welcome, and thanks for another fine review and excellent photography.

Dan5000's picture

Thanks John - Because of your rec, among other reasons, I just ordered an M10 as a first (easy) step into this world. Do you think these pair well overall? A significant upgrade in sound (for the price) against something like the B&W 606 Anniversary?

Thanks.

John Atkinson's picture
Dan5000 wrote:
Thanks John

You're welcome.

Dan5000 wrote:
Because of your rec, among other reasons, I just ordered an M10 as a first (easy) step into this world. Do you think these pair well overall?

Yes, especially as the M10 incorporates Dirac Live EQ, which can compensate for room acoustic problems and extend the KEF's low frequencies. See how Dirac works with the original LS50 at www.stereophile.com/content/nad-masters-series-m10-streaming-integrated-amplifier-trying-dirac-live-room-correction.

John Atkinson
Technical Editor, Stereophile

Mikk's picture

Good work Kef- a high-performance speaker at a great price. As suitable for audiophiles as for non-audiophiles, and would look great in any room.
The only question now is how they compare with the wireless version, and what sort of gear would be needed to get the passive version to sound on-par with the wireless version!
A pair of either, with well setup subs, should be very impressive.

aweingarten's picture

For those of us not allowed to put speakers 3 feet from the rear-wall, was wondering how far from the rear-wall you would recommend for these?

tabs's picture

One thing to remember is that the LS50 Metas come with two options for stuffing the ports and a guide in the owners manual in how to approach it. If I remember from my setup a few days ago, if the rear panel of the Meta is between 0 and 20cm from the wall, they recommend fully stuffing the port (donut hole foam in donut ring). From 20-50cm they recommend using the hollow donut ring foam piece to partially close it. From 50cm on you run the ports wide open.

Mine sit 25cm from my rear wall in a relatively small room. With the ports open at that position, bass was indeed too prominent. With the hollow donut ring in place that helped perfectly. I’ve since done a bunch of REW measurements and the donut ring brought them perfectly into line with the rest of the frequency response.

JRT's picture

Dutch & Dutch 8C active loudspeaker is much more expensive, but it was designed to work well in close proximity to the room boundary behind the loudspeaker, so it may be a solution to your problem which offers very much better performance.

Alternatively, move the loudspeakers and listening chair into the proper listening position when listening to the loudspeakers, and move them back to where you are "allowed" to keep them when not using them.

Fruff1976's picture

"(I am reminded of something Bob Stuart had told me in the 1980s, that loudspeakers that have the same width as the human head always excel with vocal reproduction due to the similarity between the sounds' intensities."

So this is why all of those gigantic speakers at the audio shows sound ridiculous....I listened to the flagship Borrensen speakers at Axpona that people were raving about and Joe Satriani sounded like a hot air balloon head. Not exactly my cup of tea.

Sam Tellig's picture

Sam has moved on to the LS50 Meta from the original LS50 Anniversary. Resolution is stunning, the best I've heard since I sold my Quad ESL 2805 electrostatics. I hear no cabinet colorations. There is nothing to smear the sound. What this means is you can save money on speakers and buy the best possible electronics. I am using the Chord Qutest DAC, BTW. Cheapskates have never had it so good!

latinaudio's picture

I miss your column and the way you wrote stories about the products and their origins. Greetings to Marina and Merry Christmas.

sdecker's picture

John, what's your guess as to why KEF would rewire the tweeter 'out of phase' versus the previous version? I could find no reference to that or other related crossover changes in their Meta White Paper. If no related changes are made to a major redesign of a classic, wouldn't this be audible, and/or suspicious? (I listen to Thiels, so perhaps this is more important to me than to KEF? Though I just bought a pair of these!)

Jim Austin's picture

It's a good question.

Just a guess, but the answer may be found in Fig.9. Compare the frequency response near the 2.1kHz crossover frequency with the original: A very shallow depression--almost flat--replaces a small but significant rise.

Also note the slight change in the crossover frequency relative to the original: 2.1 vs 2.2 kHz. Hardly surprising. They've apparently found a better way to do the crossover--simple as that.

Jim Austin, Editor
Stereophile

daltonknox's picture

Preferring an MQA recording is not toeing the Stereophile party line - risking banishment to the gulag?

JRT's picture

...and that proprietary MQA audio, directly to the KEF LS50 Wireless II network attached active loudspeakers.

For somebody like your niece or daughter who just wants to listen to some good music, isn't concerned about the proprietary nature of MQA if it works well enough, and isn't interested in fussing with a lot of gear and physical media, that may be a very good solution.

Neil Jetty's picture

Hi John

Is the Meta strong enough vs. the orignal LS50 to make you consider trading up?

John Atkinson's picture
Neil Jetty wrote:
Is the Meta strong enough vs. the original LS50 to make you consider trading up?

I'm seriously considering doing so once my wallet allows it.

John Atkinson
Technical Editor, Stereophile

Charles E Flynn's picture

Just announced: https://us.kef.com/kc62-subwoofer.html

agb's picture

Until recently the Brit audiophile product was off my radar.

On a whim for my Budapest flat I purchased the second pair of LS50's, this time the wireless W-designated model. Simply put, it way exceeded my expectations here in the US where I tested them first. The powered model was significantly better than the base speaker with any amplifier I used it with. I am speaking of expensive amps.

The built-in DAC/amps outperformed the nekkid speaker with a far more expensive external amplifier. On many levels. There is no upside to external components and wires not as well matched to the driver as KEF accomplished within the whole package. I think others have had the same experience as I.

But the KEFs didn't realize their performance limits until I dragged them to Budapest in two luggages. My flat there, in an 1847 World Heritage building, has 4 meter high ceilings, meter thick walls, and parquet. Solid that is.

I applied some EQ to take down the bass (along with the speaker's own built-in DSP functions to limit the output, which was overwhelming). I used real time FFT with peak hold on a variety of music streams - with optical wire, no USB here - to get a response almost the same I was getting with two high end headphones. I could superimpose one graph on the other.

It's obvious: KEF got things right with an unusual, but obviously brilliantly-designed and well-built driver in a well-braced cabinet. Pretty much the same driver KEF uses in its $32,000 BLADE. And that tells us a lot.

With the DSP the W model is great; the Meta version probably addresses the issues I had to deal with, with the EQ necessary on the earlier model. John addressed these improvements, and accordingly, I may even buy the newer model for one of my rooms in the US. I am that pleased by the sound of this Brit.

Kick ass - these KEFs are really an unbelievable value at the price.
John probably understated his case for them.

To sum up: At the price, the quality of build and sound, at least speaking for the powered version, KEF made these as "keepers."

Charles E Flynn's picture

From https://us.kef.com/speaker/flagship-hi-fi-speakers/ls50/ls50-meta.html :

LS50 Meta

Driver Unit: 12th Generation Uni-Q with MAT+ 40% frequency response smoothness

Driver rearward Source Absorption: 99% driver rearward sound absorption by MAT

Mid-band Total Harmonic Distortion (THD) at 90dB/1m: THD 0.07%
Extremely low distortion by driver motor redesign

LS50

Driver Unit: 11th Generation Uni-Q

Driver rearward Source Absorption: 60% driver rearward sound absorption by vented tweeter

Mid-band Total Harmonic Distortion (THD) at 90dB/1m: THD 0.1% Mid-band at 90dB/1m

Butter's picture

Maybe I don't know how to read all the measurements, but is there a way to understand how well the LS50 meta would play loud in a larger room. I understand it's a compact insensitive speaker.

I have LS50 Anniv editions in my office (15x15) and don't "blast it" because I'm in the house. But my new office (30x35) will be in another building, so I'm thinking of getting LS50 Metas a sub, and say... a NAD M33. I wouldn't mind getting floorstanders if there was a way to quantify the high SPL capability between different options. The price point for the LS50 is just so attractive and I love my Anniv editions, I'm seeing if I should hit the shops to demo some floorstanders.
Thanks for any advice.

JRT's picture

It would be interesting, informative and useful to see a curve showing EPDR magnitude with respect to frequency as a third curve added to the graphic showing electrical impedance and phase.

This is just a suggestion, and is not a negative criticism of the measurements and associated commentary which you are already providing, all of it worthwhile. Thank you for your efforts.

dc_bruce's picture

Mr. Atkinson: somewhere I recall you praising Karen Carpenter's work in a prior review; and you've done it again here. Just so! Being your contemporary, I recall when the Carpenters were making hits. I pretty much dismissed them at the time for political reasons. (I think President Nixon was a fan.) Courtesy of my wife, I own an LP of their "Singles" which I play often. While I don't care for some of Richard Carpenter's arrangements, his sister shows an unbelievable vocal range and maturity for someone her age. And to think that she supposedly thought of herself as a drummer, not a singer! In my opinion, her vocal command far exceeds any of her contemporaries, especially the over-praised Linda Rondstadt. Linda had a strong voice but unstable pitch, as her foray into the standards with the assistance of Nelson Riddle shows all too well. The original LS-50s are wonderful with Karen Carpenter's voice. Like you, I plan to upgrade to the Metas when the piggy bank is full.

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