I did say that a driver that would be good for use in a horn usually has
an EBP (efficiency bandwidth product) of greater than 130. This is not
the rule though, it is only a guide. EBP is Fs divided by Qes and gives
a rough approximation of how well the motor system can control the cone.
It doesn't always follow that high EBP figures automatically make the
driver suitable for horn loading, there are so many parameters that
dictate which driver to chose for a particular horn and so many
different horns for different jobs.
Take the 186 horn on this site, the best driver to use for this horn
i.e. the driver it was designed around, is the PD 1850. You have to make
the rear chamber smaller when fitting this driver as the plan shows
measurements for the PD 186 driver. With the PD 1850 fitted the cab
produces one hell of a bang, the transient response is tight and the
sound fast and detailed in the upper bass range, but the sound has a
hard edge that can lead to listener fatigue early on. If you want the
loudest noise from that horn fit the PD 1850. If you want a sound that
is a lot more listenable then fitting a driver with a lowish EBP
improves things. The PD 186 has an EBP of less than 100, so should not
work too well in a horn loaded situation, but by making the rear chamber
slightly larger than is dictated and opening the throat up a bit the PD
186 will make beautiful noises in that box. It will not ever be as loud
and will never have the detail in the upper bass range and will not have
the transient response of the PD 1850 in this horn, this is because the
stronger motor system in the PD 1850 is better able to move the cone
against the column of air in the horn. So the lower EBP PD 186 will not
be as fast because of it's lack of ability to control the cone well, but
certainly makes for a really good sound. You also asked whether the
sound of a driver with an EBP of less than 100 in a horn would sound
less Hi Fi, well it depends what you call Hi Fi. For me the PD 186 with
it's lower EBP in a horn sounds more Hi Fi because the sound is smooth,
warm, doesn't have too big a peak around the horns cutoff point and has
a lower F3 than the driver that should sound better in that horn. The PD
1850 does not sound as Hi Fi to me in the 186 horn, you really now that
you are listening to a horn, it's a more in your face sound and draws
attention to it's self. The PD 186 doesn't do this, it integrates well
with what ever mid top cabs you are using and doesn't sound like a horn.
So, which driver do you chose, if it's for a PA where you want the
biggest noise for the size of enclosure then higher EBP drivers tend to
win out. For the home where 130 dB is not so important, then for me a
lower EBP figure wins out.
With short folded horns, or bandpass horns like the HD15 the effect is
even more pronounced. A short folded horn with horn length of around 85
cm and a mouth area of 3300 sq cm, so a horn with an 85 Hz cutoff (quarter
space) will sound really hard with high EBP drivers in them. The PD 1850
will rip your head of in one of these bins, but lower EBP drivers (though
not as low as the PD 186) will smooth the sound out and make it more
listenable.
So as I said EBP is not the final word on whether you should put a
driver into a certain horn or not, but for a very quick reference will
stop you from choosing a driver with a very weak motor system that will
not drive the air column at all well. For 12" drivers in horns for mid
use an EBP of over
180 is a good starting point. You will not find many 12" drivers that
are this tight, so to save you looking EV has some that are around 180
and there are 3 PD 12" drivers that are above 180. One has an EBP of 235
and it does sound awesome in a horn.
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