There is less tolerance with drivers that are suitable for midrange horn
loading. You can get away with a little variance in drivers for bass
horns, but put the wrong one in a mid horn and it will sound horrible.
Because you hear the sound of a mid horn and not feel it like in a bass
horn the right driver is crucial.
The best drivers for mid horns have a high Fs (resonance) and a low Qts.
A high BL factor is also important. Basically this means that you want a
driver that has a stiff suspension, is well damped and has a strong
motor. A ‘tight’ driver some would say. You can do a quick sum to see if
a driver is suitable for horn use. Take the Fs, on the PD 122 12” driver
it is 42.9 Hz, then divide the Fs into the Qes. The PD 122 has a Qes of
0.197, so 42.9 divided by 0.197 = 217. This is called the drivers
efficiency bandwidth product, for 12” drivers and smaller for use in
horns a figure above 180 is good. Figures above 200, like with the PD
122 are very good and should work well in horns. Bigger drivers for low
midrange like 15” units should have EBP’s above 130. There are some EV
15” drivers that have EBP of around 180, these would be very suitable
for low mid horn use and were used a lot for this application when 15”
drivers in 4560 type cabinets were all the rage.
The EBP sum should help you decide which are the more suitable drivers
for use in horns, but there are other considerations as well. If I had
to choose between a 12” driver with a EBP of 210 that only had a BL of
12, or a driver that had a EBP of 170 but had a BL of 29, then I would
want to test out the one with the highest BL first. If the driver with
the BL of 29 had an EBP of only 110, then I would want to hear the other
driver first. Another parameter that will determine the highest
frequency the horn driver combination will produce is the inductance
value of the coil (Le). This is given in mH, (millionths of a Henry).
Drivers that have an inductance of around 0.75mH (for a 12” driver) will
usually have a more extended high frequency range than a driver with a
Le of 1.50mH.
You should really only use the above ideas as guides, they will help you
make a shortlist of drivers that you should addition, but don’t use the
above to decide on one driver alone. Sometimes a driver that you think
could never work in an application turns out to be the best driver for
the job, so experimentation is called for. But most of the time the
above rules are quite accurate. Take for instance a 12” mid horn I
designed for a company once. I turned to the spec sheets and found a 12”
Precision Devices driver that has an EBP of 233, has a high BL and a
lowish Le. I also tied most other 12” drivers in this horn and the PD
sounded the best. At the other side of spectrum I tried an Eminence
Kappa 12. This is not a bad driver for use in a reflex cabinet, but
sounded unlistenable in the horn I had designed. The EBP for the Kappa
12 is 160 and the BL is a lot less than the PD’s. So from that test the
above rules would seem to work, it’s when you get to the middle ground
with drivers that are suitable for mid horn loading that the differences
are less. Sometimes it just comes down to which driver you like the
sound of best.