KEF LS50 Anniversary Model 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 KEF's voltage sensitivity was 84.5dB(B)/2.83V/m, which is within experimental error of the specified 85dB. This is a little lower than average but 2dB higher than the LS3/5A's.

Somewhat optimistically specified at 8 ohms, the LS50's impedance (fig.1, solid trace) drops to 4 ohms at 200Hz and to 5.4 ohms at the top of the audioband. The electrical phase angle is generally mild, but the combination of 5.3 ohms and –41° at 135Hz, a frequency where music often has high energy, will make the speaker work at its best with a good, 4 ohm–rated amplifier.

The small blip at 39kHz in the magnitude trace in fig.1 indicates that the tweeter's fundamental dome resonance lies at this very high frequency, but the traces in this graph are otherwise free from the small lower-frequency discontinuities that would suggest the presence of cabinet resonances of some kind. Cumulative spectral-decay plots of the cabinet walls' vibrational behavior, calculated from the output of a plastic-tape accelerometer, didn't uncover any midrange resonances on any of the surfaces, though the sidewalls flexed a little at the frequency of the port tuning frequency (fig.2).

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

Fig.2 KEF LS50, 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 52Hz, confirmed by the minimum-motion notch at that frequency in the woofer's nearfield output (fig.3, 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 LS50's overall response, averaged across a 30° horizontal window centered on the tweeter axis, is commendably even, with some small dips balanced by small peaks. The tweeter's output remains at full level out to the 30kHz limit of this graph, and unlike in some earlier generations of the Uni-Q concept, no discontinuities are visible 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.

Fig.3 KEF LS50, 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's horizontal and vertical dispersion, referenced to the tweeter-axis response, are shown in figs. 4 and 5, respectively. The radiation pattern in both planes is very uniform, with the usual but well-controlled narrowing of the pattern in the top octaves, though the vertical dispersion is wider than I had expected from my auditioning.

Fig.4 KEF LS50, lateral response family at 50", normalized to response on HF axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.5 KEF LS50, vertical response family at 50", normalized to response on HF axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

What matters most is the result of this quasi-anechoic behavior in the listening room. I performed my usual spatially averaged response measurements of the LS50s in my listening room. Using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, I average 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. This 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.

The red trace in fig.6 shows the KEF LS50's spatially averaged response. It is commendably even from the lower midrange through the mid-treble, though with a slight emphasis in the upper midrange and low treble. The low frequencies are boosted a little by the mode in my room at around 63Hz, but the output has dropped sufficiently below that frequency to get no significant reinforcement from the 31Hz mode. The top two audio octaves slope down a little, due to the increased absorption of the room furnishings in this region, and the in-room spectrum drops sharply above the audible range, presumably due to the tweeter's limited dispersion at ultrasonic frequencies.

The blue trace in fig.6 shows the spatially averaged response of my 1978 pair of Rogers LS3/5a's, taken under identical circumstances, but with a 3dB-higher level at 1kHz to equalize the two pairs of speakers' midrange outputs. The peak between 1 and 2kHz that lends this classic speaker its characteristic slightly nasal coloration can be seen, coinciding with a small dip in the LS50's response. The LS3/5a has noticeably more energy in-room in the top three octaves, this audible as extra "air" in direct comparisons with the KEF. At the low end, the LS3/5a has a bigger response "bump" in the upper bass, and the slower rolloff offered by its sealed-box alignment allows it to excite the lowest-frequency mode of my room enough to usefully extend the bass.

Fig.6 KEF LS50, spatially averaged, 1/6-octave response in JA's listening room (red); and of BBC/Rogers LS3/5a (blue).

The red trace in fig.7 again shows the spatially averaged in-room response of the KEF LS50s, this time compared with the spatially averaged response of the Bowers & Wilkins CM5 speakers (green trace). The two speakers' in-room behavior is very similar in the lower midrange and bass, but the CM5 excites the lowest-frequency room mode more than does the LS50, resulting in better low-frequency extension. Though the KEF has less energy apparent in the upper midrange, the B&W is more laid-back in the low treble. However, the extra energy above 5kHz produced in-room by the CM5 lent it a lighter balance overall. The B&W's high-amplitude tweeter resonance just below 30kHz makes its presence known in this graph, but this is well above what anyone can hear.

Fig.7 KEF LS50, spatially averaged, 1/6-octave response in JA's listening room (red); and of B&W CM5 (green).

Turning to the time domain, the LS50's step response on its tweeter axis (fig.8) indicates that the tweeter is connected in negative acoustic polarity, the woofer in positive polarity. However, the smooth integration of the decay of the tweeter step into the start of the woofer step implies optimal crossover design and correlates with the excellent integration of their outputs in the frequency domain seen in fig.3. The cumulative spectral-decay plot on the tweeter axis (fig.9) shows an 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.)

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

Fig.9 KEF LS50, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).

The KEF LS50 may be relatively affordable, but it offers superb measured performance.—John Atkinson

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

Audio Asylum Bruce from DC's picture

Being a dedicated "stand-mount" guy and something of a cheapskate, this review got me really excited.  Maybe I should replace my 10-year old Joseph Audio RM7si Signatures!

Imagine my disappointment when I went to KEF's US website and tried to find a dealer.  Even at maximum range from my location -- Washington DC -- I came up empty.  There is a local dealer, but it stocks only KEF's low-line stuff, not any of the fancy products (including the Reference Series).

Assuming that one would agree that Washington DC is not the sticks, it might be worthwhile for Stereophile to enquire of KEF exactly how many US dealers the company has which stock this product -- or the Reference series for that matter.

Sergei's picture

The reason you can't find a pair Audio Asylum is because KEF in their wisdom decided to insult all of their loyal dealers by offering this product ONLINE ONLY!

I am a Canadian Dealer(soon to be ex-dealer now) for KEF. We had a clent want to hear a pair of these and most likely purchase due to being a long time KEF fan. To both of our surprise when I contacted my KEF rep he explained this was ONLY available through the KEF online store.

We played him a pair of other bookshelf speakers he happily purchased instead.

There is really no logic as a business man and Hi-Fi enthusiats that KEF would want to snub their dealer base with an "Anniversary" product. We were even considering displaying the Blade's in our showroom...


hcie95he's picture

In the review the LS50 is compared with the B&W CM5 and the LS3/5a. In the past John reviewed also the Harbeth P3ESR and the Spendor SA1, which are both more expensive. I am wondering if the LS50 is in the same league as the Spendor and the Harbeth?

By the way I am surprised that KEF doesn't sell the LS50 in the US via their dealer network. In Europe you can buy and audition them via their dealers.

norcalhifiguy's picture


Sad to see the previous comments, but as the owner of small audio shop in a very small town I can say with absolute authority that these are indeed available from direct dealers. (I can't speak to the Canadian issue as we are in the US)

We are thrilled to have these on display in our main listening room and must say they live up to the great reviews. These are amazing little speakers.

Please, do seek out and support your local independent audio dealers. 

These are people that love great audio as much as you do.

Hear them in person. Buy them locally!

audiodoctornj's picture

As one of KEF's highest level dealers, Audio Doctor, has the Blades, the entire Reference Series, R Series, Q Series, and T Series products as well as the LS 50, I can tell you they are available at select dealers, I don't know the issue with the Canadian distributor but they should be available to them as well.

WIth that being said, I do feel that is unfortunate that KEF does not have more stocking dealers with the LS 50, or with a wider range of their products.

The current KEF lineup is some of the best products the company has ever produced and in particular the BLADE represents $60,000.00 and above levels of performance for $30k, the BLADES are amazing speakers!

I feel that there are a few issues going on, Stereophile and the Absolute Sound do not really do comparative anaylsis of one product to another in their reviews, if they did and were not affraid to stand up to their advertisers, they  would proclaim that the BLADE is a bargin for the abolute top level of performance that they give and would recommend that anyone looking at a Wilson, Magico or YG etc should give the BLADES a listen before considering spending more on anything else.

We did the High End Audio Show in NYC last year, and our setup with the BLADES and all Chord Electronics sounded as good as any of the other setups at the show including a $300,000.00 plus YG setup and another mega dollar MBL setup and these were $30k speakers standing up agains $110k speakers!

So in summation, KEF makes fantastic products, has a weak dealer network, and needs to revamp their advertising and marketing to get more people excited by their products.

You should hear a pair of LS 50 setup with the really nice little KEF R 400 sub you have a roughly $3k package that sounds remarkable, huge soundstage, shocking dynamics and great transparency!

If anyone wants to visit our shop call 877 428 2873 and make an appointment to hear these incredible little jewels.  

10sephirot's picture

The little KEF sure is getting good reviews.  Lets face it looks count also.  It made me think about what stands would look best.  In my opinion the Usher RSW-708 in walnut would be a very special look:

The KEF plus the stands I subjectively think look best is a $2000 proposition. 

I'll go ahead and match that up to the Vincent Audio SV-236 in black:

another $2000.

I'll go ahead and add the Musical Fidelity MMF-51SE:

Yeah well anyway that is another grand.   $5000 total.

I told all this to Santa and I got thrown out of the shopping mall.

But in all seriousness this is what a good review makes us do: dream. 

Thanks Stereophile.  Another afternoon wasted daydreaming:)

JRT's picture

In the measurements, the waterfall diagram showing polar response in the vertical plane passing thru the voicecoil centerline appears to be much smoother than the polar response in the horizontal plane.   So I think the first tweak for improved performance, clearly demonstrated in the measurements, would be to lay the speaker on its side.  

prof's picture

In the measurements, the waterfall diagram showing polar response in the vertical plane passing thru the voicecoil centerline appears to be much smoother than the polar response in the horizontal plane.

Umh, no. You've been fooled by the difference in scales. The scale on the vertical waterfall plot is ±45°. The scale on the horizontal waterfall plot is ±90°. At ±45°, the horizontal dispersion is as-smooth or smoother than the vertical dispersion.

techblogpool's picture

Hey guys, I just made an unboxing of these:

ryebread's picture

Quest For Sound in Bensalem, PA

bought them last week, epic speakers

wgb113's picture


Curious what amplification both QFS and you are using.  I might try to head over there for an audition.


Gradofan's picture


The sound I'm getting from this set up is "stupendous!"  Don't think it can get any / much better - certainly not for less than 10X the price!


Using the Sony  XA5400ES SACD/CDP (the best there is), into the Anti-Mode 2.0 as a pre and DSP to eliminate the "room boom," into a tube buffer, into the Class D Audio amp into the LS50's and R400b produces real "you are there" sound.  Incredible detail, resolution, tone, tenure, texture, stage, image, bass, mids and treble... etc., etc...  


Hard to believe... really... 


And just an incredible value!


wgb113's picture

Everything I've read about the LS50s indicate that they're great for a small room.  My room is REALLY small @ 10'x12'x9'.  It's been acoustically treated but placement of the speakers and listening position results in a near-field setup.  Are the KEFs going to work well in such a setup?  It seems their manual recommends 6'-10' between the inner sides of the cabinets before toeing in.


Azani's picture

I've just purchased the pair after searching high and low for floor standers to replace Rogers LS2a/2. Auditioned Harbeth P3ESR, like it but thought it sounded like big voice coming out from small mouth. Haven't got the chance to audition any Spendor monitors tho. 

Anyway, my room is also small (8x13)' with acoustic treatment but still able to handle LS50 quite well. Generally, a slight enhancement from Rogers LS2a/2, livelier that is.

just yesterday, started to experience sudden annoying jerks and "tone switching" (for the lack of better word) on the right speaker whenever CD is played. It was fine the day before. Yet to figure that one out. Anyone experience it?


dc_bruce's picture

and could not be happier. I concur completely with the comments of all 3 Stereophile reviewers. The only advantage of the long-out of production Josephs is that they go solidly down to the low 40 Hz range, reproducing the fundamentals of string and electric bass pretty well. Other than that, the little KEFs have it all over, and I really can't fault them, within reason. I say "within reason" because its unreasonable to expect a speaker of this size to reproduce the weight and scale of a symphony orchestra at full power. That said, these come much closer than you would expect; and they are not as bass-deficient as I had expected.

I am running them (eegads!) on a sort of bookshelf (actually a built-in cabinet) with about a foot clearance from the wall behind them. I decided to use the partial foam plug to attenuate the bass for the most natural sounding response. Of course, I'm using an old REL subwoofer, so getting the last few Hz of bass extension from the KEFs is not necessary.

If you're not trying to fill a large room and are not looking for stomach churning loudness, it's hard to go wrong with these. They seem to impart no particular characteristic of their own to the sound. I running them with a BAT VK-300x integrated (150 wpch @ 8 ohms/ 300 wpch @ 4 ohms) which seems more than enough power.

Ali's picture

Is there any differences between anniversary and non anniversary model?

John Atkinson's picture
Ali wrote:
Is there any differences between anniversary and non anniversary model?

Cosmetics only. The sound is the same. (I bought a pair of regular LS50s after reviewing the "Anniversary" edition.)

John Atkinson
Technical Editor, Stereophile