Kef Reference 207/2 10″ Woofer Driver

 Note: Above picture linked from Kef’s site.


IMG_6602 IMG_6600


A fellow sent me a set of the 10″ woofer drivers from Kef’s “flagship” 207/2 speaker for me to test recently.  One 207/2 costs $9,999 via Kef direct.  So, whoever was brave enough to dismantle… thank you.  😉

Below are my results.

Thiele-Small Parameters and Impedance

Electrical Parameters
Re 6.82 Ohm electrical voice coil resistance at DC
Le 0.662 mH frequency independent part of voice coil inductance
L2 1.014 mH para-inductance of voice coil
R2 4.3 Ohm electrical resistance due to eddy current losses
Cmes 428 µF electrical capacitance representing moving mass
Lces 64.93 mH electrical inductance representing driver compliance
Res 108.13 Ohm resistance due to mechanical losses
fs 30.2 Hz driver resonance frequency
Mechanical Parameters
(using add. mass)
Mms 48.44 g mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd) 40.801 g mechanical mass of voice coil and diaphragm without air load
Rms 1.047 kg/s mechanical resistance of  total-driver losses
Cms 0.573 mm/N mechanical compliance of driver suspension
Kms 1.74 N/mm mechanical stiffness of driver suspension
Bl 10.642 N/A force factor (Bl product)
Loss factors
Qtp 0.521 total Q-factor considering all losses
Qms 8.776 mechanical Q-factor of driver in free air considering Rms only
Qes 0.553 electrical Q-factor of driver in free air considering Re only
Qts 0.52 total Q-factor considering Re and Rms only
Other Parameters
Vas 103.7157 l equivalent air volume of suspension
n0 0.496 % reference efficiency (2 pi-radiation using Re)
Lm 89.16 dB characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom 89.85 dB nominal sensitivity (SPL at 1m for 1W @ Zn)
Sd 357.53 cm² diaphragm area

207_2  woofer Magnitude of electric impedance Z(f)


Large Signal Analysis with Klippel’s LSI Suite

Displacement Limits thresholds can be changed in Processing property page
X Bl @ Bl min=70% 8.2 mm Displacement limit due to force factor variation
X C @ C min=50% 8.2 mm Displacement limit due to compliance variation
X L @ Z max=17 % >8.9 mm Displacement limit due to inductance variation
X d @ d2=10% 39.3 mm Displacement limit due to IM distortion (Doppler)
Asymmetry (IEC 62458)
Ak 31.5 % Stiffness asymmetry Ak(Xpeak)
Xsym 0.57 mm Symmetry point of Bl(x) at maximal excursion

kef 207_2 woofer  Force factor Bl (X) kef 207_2 woofer  Bl Symmetry Range kef 207_2 woofer  Mechanical compliance Cms (X) kef 207_2 woofer  Stiffness of suspension Kms (X) kef 207_2 woofer  Kms Symmetry Range kef 207_2 woofer  Electrical inductance L(X, I=0) kef 207_2 woofer  Inductance over current L(X=0, I)


One nice plus is the relatively high sensitivity at just above 89dB @ 1w/1m.  The results show the woofer’s 20% THD limit is met by both motor and suspension at 8.2mm.  There is a near constant offset in Bl approximately 0.56mm forward.  There is a forward offset in suspension off approximately 1.0mm.  A shorting ring is used to keep inductance over excursion and inductance over current low, which is used to mitigate higher frequency distortion via intermodulation.