Kef R300 Midrange Drive Unit Testing

By now you’ve probably seen my testing of the Kef HTS3001SE and Q100 drive units.  I had read the next step in the Kef Uni-Q line are the drivers in the “R” series.  So, a fella let me borrow the 5 inch midrange drive unit from the R300 speaker to test.  The R300 comes with a separate woofer but I was not sent this.  The only way to obtain these drivers individually is to purchase the speakers they come with (such as the R300 or R500 speaker) and remove them.  Which brings up something worth noting.  I am testing these raw drivers as more or less for knowledge purposes.  These kind of tests tell us exactly what the basis for a speaker is (drive units + enclosure + circuit design).  If Kef is starting with a great drive unit then one can logically assume they have likewise extended efforts to use them as a speaker in a manner which reflects their ‘raw’ performance.  In other words, if the drive unit design is great, odds are so is the complete speaker it’s used in.  Now, let’s get on with it!

 

Kef R300_2 Kef R300_1 Kef R300_3

 

Like the Q100, this driver has a very large motor structure and basket.  For a midrange, this is a HUGE drive unit, relative to other mids I’ve used.  You’ll also notice the motor and frame are a bolt together design.  The spider is below the cone on a tier sitting above where conventional speakers’ spiders are.  The surround of this driver has a curved shape to it, I assume to help it act more like a waveguide as with the cone’s shape.  The voice coil diameter is roughly 45-47mm (I had to spitball this so please take it as only an estimate).  OD is approximately 130mm.  Surface area (minus the tweeter assembly/waveguide) is about 98.01cm².  Actual effective surface area, noting the moving portion of the driver’s cone is 25cm²; the tweeter housing/waveguide is roughly 49mm in diameter.

 Tweeter Testing:

Tweeter Small Signal Parameters

 

Electrical Parameters
Re 2.92 Ohm electrical voice coil resistance at DC
Le 0.012 mH frequency independent part of voice coil inductance
L2 0.011 mH para-inductance of voice coil
R2 0.34 Ohm electrical resistance due to eddy current losses
Cmes 120 µF electrical capacitance representing moving mass
Lces 0.26 mH electrical inductance representing driver compliance
Res 0.94 Ohm resistance due to mechanical losses
fs 907.9 Hz driver resonance frequency
Loss factors
Qtp 0.488 total Q-factor considering all losses
Qms 0.644 mechanical Q-factor of driver in free air considering Rms only
Qes 2.005 electrical Q-factor of driver in free air considering Re only
Qts 0.487 total Q-factor considering Re and Rms only

 

tweeter imp

 

Tweeter Frequency Response

0, 30, and 60 degrees.  2.83v/1m; Nearfield & Farfield merged at 1800hz.

Kef R300 Drive Unit Tweeter Frequency Response 0 30 60

Tweeter Harmonic Distortion at 90dB/1m and 96dB/1m

kef r300 tweeter Fundamental + Harmonic distortion components (90dB1m) r300 tweeter Fundamental + Harmonic distortion components (96dB1m)

 

 

 

 Woofer Testing:

Woofer Small Signal Parameters:

Electrical Parameters
Re 3.08 Ohm electrical voice coil resistance at DC
Le 0.155 mH frequency independent part of voice coil inductance
L2 0.234 mH para-inductance of voice coil
R2 1.77 Ohm electrical resistance due to eddy current losses
Cmes 242 µF electrical capacitance representing moving mass
Lces 3.63 mH electrical inductance representing driver compliance
Res 9.29 Ohm resistance due to mechanical losses
fs 169.5 Hz driver resonance frequency
Mechanical Parameters
(using add. mass)
Mms 7.416 g mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd) 6.9 g mechanical mass of voice coil and diaphragm without air load
Rms 3.293 kg/s mechanical resistance of  total-driver losses
Cms 0.119 mm/N mechanical compliance of driver suspension
Kms 8.41 N/mm mechanical stiffness of driver suspension
Bl 5.531 N/A force factor (Bl product)
Loss factors
Qtp 0.603 total Q-factor considering all losses
Qms 2.399 mechanical Q-factor of driver in free air considering Rms only
Qes 0.796 electrical Q-factor of driver in free air considering Re only
Qts 0.598 total Q-factor considering Re and Rms only
Other Parameters
Vas 0.5905 l equivalent air volume of suspension
n0 0.348 % reference efficiency (2 pi-radiation using Re)
Lm 87.61 dB characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom 88.74 dB nominal sensitivity (SPL at 1m for 1W @ Zn)
Sd 59.26 cm² diaphragm area

woofer imp

Woofer Large Signal Parameters:

It may be pretty obvious that I wasn’t able to resolve all the values here. The limiting suspension resolves first at 1.5mm linear xmax, and to resolve Bl and L(x) would mean pushing on the driver harder than I am comfortable with. Suffice it to say, a high pass filter will help lessen the suspension related distortion, but this driver isn’t intended to cover bass frequencies, either.  If memory serves, it’s crossed above 500hz in the R-series towers where it is accompanied by a midwoofer.

Displacement Limits thresholds can be changed in Processing property page
X Bl @ Bl min=82% >2.5 mm Displacement limit due to force factor variation
X C @ C min=75% 1.5 mm Displacement limit due to compliance variation
X L @ Z max=10 % >2.5 mm Displacement limit due to inductance variation
X d @ d2=10% 7 mm Displacement limit due to IM distortion (Doppler)

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

Woofer Frequency Response

0, 30, and 60 degrees.  2.83v/1m; Nearfield & Farfield merged at 500hz.

Kef R300 Drive Unit Woofer Frequency Response 0 30 60

Woofer Harmonic Distortion at 90dB/1m and 96dB/1m

kef r300 woofer Fundamental + Harmonic distortion components (90dB1m) kef r300 woofer Fundamental + Harmonic distortion components (96dB1m)

Miscellaneous Testing

I took some time to do a bit of additional testing with this driver just for fun using REW software and my calibrated mic.

I used an active crossover with a 3khz/LR2 crossover point between the mid and tweeter.  I then measured the driver at 0, 30, 45, 60, and 90 degrees to see how the response of the driver with this crossover applied measures in all angles.  The results are overlaid below in 1/12 resolution.

NOTE:  SPL is not indicative of any particular test method.  I just applied power to the driver and tested in various axes.  In other words, this is NOT indicative of 1w/1m or 2.83v/1m test standards.  The following is for the sake of seeing the driver’s performance in all axes with a 3khz/LR2 crossover.

drive unit sound power

Now, averaged them together for a single plot average of all the above points:

drive unit average

Same result as above, but in 1/3 octave:

drive unit average one-third

Another bit of testing I did was to see how the peak between 5-6khz could be tamed.  So, I applied some DSP correction and did a comparison.  The result below is a measurement of the raw woofer response vs the EQ’d response taken on axis (0 degrees).

This isn’t to say it’s needed.  It’s just something I did because I had some time and thought I’d share.  😉

r300 woofer raw vs eq

I failed to save the results from the off-axis measurements with the DSP included, but suffice it to say, the EQ cuts added to tame this peak worked and were shown to have diminished greatly in the off-axis measurements as well as the on-axis measurement shown above.

Impulse Response Note

Being this is called a coincident driver and the benefit of these are they are supposed to emanate sound from the sound point, I measured the tweeter and midrange drive units separately and evaluated the arrival of the impulse response.  The two impulse response lined up to a ‘T’.  Unfortunately, I didn’t save this measurement because I simply forgot to before I had to shut down the computer so am unable to post the results.

Parting Thoughts

The benefit of having a coincident design is excellent.  I’ve toyed with a few here and there, though, I felt the companies’ never quite got it right.  So, when I first started testing the Kef drive units I didn’t expect much, to be honest.  However, the previous Kef Uni-Q units I’ve tested (HTS3001SE & Q100) have proven to be very well designed.  My results for the R300 drive unit show the same standard of performance.  I took the time to listen to these along with the Q100 speakers I have and must say that I am now sold on Kef’s coincident driver engineering.  I was extremely impressed by these (two) drive units’ performance in my listening tests.  People tend to get caught up in subjective ‘analysis’ whereas I fall in to a very objective analyzer category.  I tend to ignore subjective reviews in whole and often advise others to use them lightly, unless there is objective data to correlate.  This is why it’s rare I comment on the sound of a speaker/driver.  But, I can unequivocally say, after about 3 weeks of listening to these R-series mids and the Q100 speakers – against my coveted DIY speakers and countless other drive units – these truly are the best drive units I’ve laid ears on.  And, for the price, the Q100 speaker is what I would consider an excellent value for the critical listener on a real-world budget.  In fact, I intended to sell my Q100’s after testing the drive units but I have since decided to use the Q100’s as portable reference system.

One thought on “Kef R300 Midrange Drive Unit Testing

  1. Thanks for posting this detailed look at the R300 midrange-tweeter coaxial driver.

    How would you compare the coaxial from the r300 to the one from the Q100? More specifically, do you see objective advantages that merit using the r300 driver over the q100? I’m considering either building a DIYer with one of the drivers, or ouright purchasing the q100s (r300s being out of my speaker budget right now) is why I ask.

Leave a Reply

Your email address will not be published. Required fields are marked *