I have done the calculations on the RCF L18 P300, L18 P200N and the L18
851 KN here are the results.
Drivers have what is called a mid band thermal limit, which is how loud
the driver will go before the coil will burn up. This figure only
applies down to the point where the upper xmax limit starts to impede
the displacement. So, the RCF L18 P 200 will produce a maximum of 132.7
dB before it's coil will burn up. The L18 P 300 will produce 132 dB and
the L18 851 KN will produce 135 db, but remember that these are only
maximum SPL figures for frequencies above those which start to impede
the output at low frequencies. So it looks like the 851 KN is a bit
louder than the other two and in the mid band it is, but note it will
also take a lot more power to achieve this 2.3 dB gain over the P 200.
The next thing to look at is where the xmax displacement limit starts.
With the 851 KN it is 135 Hz, so the driver will start to get quieter
below this frequency due to it's limited xmax (excursion). The P 200 has
a figure of 75 Hz which is better but still well in the bass band. The P
300 is not effected by it's xmax until 31 Hz, which is amazing and
really what you want in a driver for sub use. I can demonstrate this
more clearly by giving you the maximum output in dB for each driver at
50 Hz. The P 300 will still produce 132 dB, the same as it's mid band
thermal limit, as it is unaffected by xmax at 50 Hz. The P 200 will
produce 128 .3 dB at 50 Hz, and the 851 KN will only produce 122.7 dB at
50 Hz because of it's very small xmax of 4.5 mm. The 851 KN fiqure of
122.7 dB at 50 Hz is very poor, 10 dB less than the P 300.
The last thing I ran by was the maximum displacement, or Vd of each
driver. Vd is how much air a driver can move. This is limited by it's
Sd, which is it's surface area, and it's xmax, or excursion. So the
bigger the surface and the further the driver can move in and out the
higher the Vd and the more air it can move. The 851 KN can move 468 cc
of air, the P 200 can move almost double this at 880 cc and the P 300
can move 1,356 cc of air. So the P 300 can move 2.8 times the amount of
air that the 851 KN can. That is a very big diffrence.
The P 300 also has parameters that are better suited to reflex loading
as well compared to the 851 KN. It has a lower Fs (resonance) and
highish Qts, both these make for a good sound in a reflex box. I have
also used the P 300 in quite a lot of my commercial designs for other
companies and have never had one stop working on me, a very positive
thing as I'm not known for driving speakers with small amounts of power.
Are you starting to see a pattern emerging here. The P 300 is clearly
the better driver for use as a sub driver. It is unaffected by it's
excursion until 31 Hz, so will produce very loud volumes down to and a
bit below this frequency. The 851 KN is 10 dB down at 50 Hz compared
with the P 300 and can move a third less air at low frequencies. The P
200 sits somewhere in between the P 300 and 851 KN, but a bit nearer the
P 300, helped my its quite large xmax figure.
So it's the P 300 in every way. If you were only going to use a driver
down to 150 Hz and had lots of power to hand then I would say go for the
851 KN because it would be 2.3 dB louder. But seeing as this is for bass/sub
use and will probably be crossed over lower than 150 Hz the P 300 cannot
One more thing would have been useful to know to draw a complete picture
of each drivers performance and that is its power compression value. Or
how much sensitivity the driver will lose due to heating effects when
driven with it's full rated input. Most manufactures will not give these
figures because they are awful, some companies like Precision Devices
will because the values are so small. But power compression is not
really the issue here, when a driver will move 3 times as much air at 10
dB higher an SPL at low frequencies it's the last thing that should
decide which driver to go for.