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Which specs should I look for in a driver for midrange horn use?

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 EBPs 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 dont 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 PDs. So from that test the above rules would seem to work, its 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.

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