Kef LS50 Speaker Drive Unit Testing

  • Saturday, Feb 1, 2014
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Up for test is the raw drive unit from Kef’s flagship LS50 monitor. This bookshelf speaker set has gotten a whole lot of praise over the years and as of this writing (02/03/2020) is currently priced at $899 on Amazon (affiliate link here), if you feel so inclined to purchase them.

About a year ago I ran the gamut on some Kef drivers: the HTS3001SE concetric, (LS50 5.25″ concentric), and the R-series concentric. Knowing the lineage points to the LS50 drive unit as a mix of the Q-series concetric with it’s Z-flex surround and the R-series concetric midrange with it’s ribbed cone for reducing mid-high frequency breakup, I was curious to see the measured performance. I finally was provided a drive unit to test, so here we go.

Let’s get on to the testing, but first … the obligatory pictures …

KefLS50 Speaker

(^Borrowed that one from Kef’s site)

kefLS50

kefLS50

kefLS50

Recalling from memory, this driver physically is roughly the same size as the Q100 drive unit I tested previously in all aspects.



Test Results

To make things a bit easier to manage, I’ve broken down the test results in to two sections:

  1. Woofer Testing
  2. Tweeter Testing

Part I: Woofer Testing

Woofer Thiele-Small Parameters and Impedance

Note: When determining the full suite of T/S parameters, the effective diameter of the driver is needed to calculate Vas, Bl, etc. Most of the time this can simply be measured by measuring the diameter of the driver from half-surround to half-surround since the motor must control the entire cone area. However, in this case, the entire cone does not move. Therefore, the effective diameter (and resulting Sd) is not the entire diameter of the driver. The effective diameter here is determined by subtracting the static tweeter assembly from the overall effective diameter of the woofer. See physical measurements section above for all values.

Electrical Parameters
Re 3.01 Ohm electrical voice coil resistance at DC
Le 0.197 mH frequency independent part of voice coil inductance
L2 0.474 mH para-inductance of voice coil
R2 2.46 Ohm electrical resistance due to eddy current losses
Cmes 332 µF electrical capacitance representing moving mass
Lces 9.72 mH electrical inductance representing driver compliance
Res 27.62 Ohm resistance due to mechanical losses
fs 88.6 Hz driver resonance frequency
Mechanical Parameters
(using add. mass)
Mms 10.432 g mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd) 9.706 g mechanical mass of voice coil and diaphragm without air load
Rms 1.137 kg/s mechanical resistance of total-driver losses
Cms 0.309 mm/N mechanical compliance of driver suspension
Kms 3.23 N/mm mechanical stiffness of driver suspension
Bl 5.604 N/A force factor (Bl product)
Loss factors
Qtp 0.507 total Q-factor considering all losses
Qms 5.107 mechanical Q-factor of driver in free air considering Rms only
Qes 0.557 electrical Q-factor of driver in free air considering Re only
Qts 0.502 total Q-factor considering Re and Rms only
Other Parameters
Vas 2.4278 l equivalent air volume of suspension
n0 0.291 % reference efficiency (2 pi-radiation using Re)
Lm 86.84 dB characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom 88.07 dB nominal sensitivity (SPL at 1m for 1W @ Zn)
Madd 9.79 g additional mass
Sd 74.46 cm² diaphragm area



kefLS50 imp



Displacement Limits thresholds can be changed in Processing property page
X Bl @ Bl min=82% 6.7 mm Displacement limit due to force factor variation
X C @ C min=75% 2.9 mm Displacement limit due to compliance variation
X L @ Z max=10 % >7.0 mm Displacement limit due to inductance variation
X d @ d2=10% 12.7 mm Displacement limit due to IM distortion (Doppler)
Asymmetry (IEC 62458)
Ak 7.59 % Stiffness asymmetry Ak(Xpeak)
Xsym 0.19 mm Symmetry point of Bl(x) at maximal excursion



kefLS50 bl

kefLS50 bl-sym

kefLS50 cms

kefLS50 kms

kefLS50 kms-sym

kefLS50 indx

kefLS50 indi



Woofer Frequency Response

Measured at 2.83v/1m. Stitched with a nearfield measurement at approximately 500hz.

Note: Due to the rather tall surround and the fact I don’t have the trim ring used to flush mount the driver in a baffle, this driver was not flush mounted. This will effect the high frequency response to some degree.

  • Black = 0 Deg
  • Orange = 30 Deg
  • Blue = 60 Deg

kefLS50 FR03060WOOFER



Woofer Harmonic Distortion



Harmonic Distortion below given at 90dB/1m and 96dB/1m equivalents.

kefLS50 FRHDWOOFER90

kefLS50 FRHDWOOFER96



Part II: Tweeter Testing

Small Signal Parameters
Electrical Parameters
Re 3.14 Ohm electrical voice coil resistance at DC
Le 0.013 mH frequency independent part of voice coil inductance
L2 0.01 mH para-inductance of voice coil
R2 0.41 Ohm electrical resistance due to eddy current losses
Cmes 92 µF electrical capacitance representing moving mass
Lces 0.19 mH electrical inductance representing driver compliance
Res 1.02 Ohm resistance due to mechanical losses
fs 1205 Hz driver resonance frequency
Loss factors
Qtp 0.536 total Q-factor considering all losses
Qms 0.709 mechanical Q-factor of driver in free air considering Rms only
Qes 2.185 electrical Q-factor of driver in free air considering Re only
Qts 0.535 total Q-factor considering Re and Rms only



Tweeter Frequency Response

Measurement taken at 2.83v/1m.

Note: Due to the rather tall surround and the fact I don’t have the trim ring used to flush mount the driver in a baffle, this driver was not flush mounted. This will effect the high frequency response to some degree.

  • Black = 0 Deg
  • Orange = 30 Deg
  • Blue = 60 Deg



kefLS50 FRTWEETER



Tweeter Harmonic Distortion



Harmonic Distortion below given at 90dB/1m and 96dB/1m equivalents. kefLS50 FRHDTWEETER90

kefLS50 FRHDTWEETER96

End

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wargames