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In Reply to: RE: "So for a horn loaded driver, round up the DC resistance to the next higher standard: i.e. 8 or 16 Ohms." posted by Bill Fitzmaurice on July 21, 2016 at 10:01:58
As I mentioned above somewhere the problem is that JBL call the driver with 6.5Ohm DC resistance '16 Ohm' and the version with 3.3Ohm resistance '8 Ohm' which appears out of kilter with every other manufacturer who would call those 8 and 4 Ohm respectively.
The drivers appear to be fine and measure well within spec, it is this manufacturers nomenclature which is confusing until one reads the minimum impedance spec (9 and 5 Ohm at 5kHz).
Follow Ups:
Larger voice coils, like those used by JBL, will have high inductance, which pushes impedance up, as will relatively large diaphragms compared to the throat size, and for that matter the horn itself. I agree that calling the 2426H 8 ohms is a bit of a stretch, 6 ohms would be more like it. It's a bit difficult to make out the impedance value on the data chart because of the odd vertical scale, but it appears to be in the vicinity of 8 ohms at the recommended 800Hz crossover frequency, so that might be what they're basing it on.
44mm (1.75") voice coil and diaphragm are pretty much standard for a 1" driver and is the same as the Beyma driver I mentioned elsewhere and all other Beyma 1" bolt-on cds.
They were also measured on a horn but the difference between Re and minimum impedance is far less: 4.7 and 5.2Ohm giving 0.5Ohm difference while the very similar JBL gives 1.7Ohm for the 8Ohm version and 2.5 for the 16.
That said one of theirs (SMC-225Nd) has an Re that is actually higher then its minimum impedance. Not sure what I should make of that, some kind of resonance may be.
Not that it would really matter much to me since I'd run them active and then the exact impedance matters very little except for roughly working out headroom/SPL with any given amp. ;-)
Some of the specs for the JBL 2426H/J linked by DJK here were derived with the driver driving a 25 mm (1 in.) diameter terminated "plane wave" tube, which would have a different acoustical resistance (as seen by the driver) compared to some of the other specs derived with the driver on a horn. The plane wave tube standard was set so that different drivers performance could be compared with a standard load separate from whatever horn the driver could be attached to which was not necessarily standard. "It pays to read the instructions" as my wife sometimes reminds me.
Paul
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