4, 8, 16 ohm - how?

[johnaec]

OK, maybe I've overlooked the obvious here, but just how exactly are voice coils wound differently for different ohm ratings? Say you've got an 8 ohm coil, logic says you should have half the windings for 4 ohms, or twice for 16 ohms, but I'm sure that's not the case. 'Different gauge wire? Could somebody clarify this for me, or point me to an article? And of course, along with this, is how it also relates to power ratings and efficiency for any given model, (I never see different efficiency or power ratings vs. ohm rating) - I'm just trying to make sense of the variables involved.

Thanks for helping the simple-minded to understand...:o:

John

[Zilch]

Different gauge wire. 16-Ohm has more turns of smaller wire. While the current is less, the total electromagnetic (motive) force generated is therefore the same....

[johnaec]

Different gauge wire. 16-Ohm has more turns of smaller wire. While the current is less, the total electromagnetic (motive) force generated is therefore the same....I guess that somehow means it can disippate the same power? I'd have thunk the thinner wire couldn't handle as much. Or do the extra turns cancel that out?

John

[Zilch]

The total thermal dissipation area is the same, as the voice coil length is the same. The smaller wire is carrying less current, but the total voltage drop across the voice coil is increased. Power is the product of the voltage and current, so the power is the same, as well....

[johnaec]

Thanks, Zilch - I have a much better understanding now!

John

[Steve Schell]

I have been immersed to an extent in voice coil calculations and winding for the past year or so. The calculations get fairly complex, and I'm not a complex guy!

Eight and 16 ohm coil can be wound to have roughly the same BLI properties (flux density in gap X length of conductor in gap X current through coil). These taken together represent the motor strength. Mass and coil size may also be made about the same through selection of wire size and number of turns. The higher impedance coil will have a greater number of turns filling the same space, using finer wire. It will have more L due to the greater conductor length, but less I due to the lower current drawn by the higher impedance. Inductance, which tends to cause high frequency rolloff, will generally be greater in the higher impedance coil due to the greater number of turns.

Then there is the selection of materials to think about. Aluminum wire has about 70 percent the conductivity of copper,, but only 1/3rd the weight. So, aluminum is usually chosen for high frequency drivers. Terminating aluminum wire is a bitch because it oxidizes almost instantly with exposure to air. Aluminum wire coated with copper (about 10 percent of cross section) is often used to simplify the terminations, but now the calculations are further complicated. Turns out the copper clad aluminum wire has nearly the same conductivity as pure copper, so now it takes more turns of finer wire to get the impedance back up, then the inductance grows from the greater number of turns!

Power handling has many factors... the mass of coil; ability of coil to dissipate heat into surrounding structures; materials used for coil, glues, former, and their ability to withstand heat. One of the best ways to increase driver longevity in high output situations is to design for very high BLI- such a driver has less heat to dissipate (often much less) than one of lower efficiency for the same power output.

These days it is a crazy situation. Sound reinforcement SPLs at live venues are often insane, with PA systems run by chimpanzees who crank the system up until everything melts. Drivers have to be built like the Death Star to withstand the power inputs, and the sound quality suffers from all the added moving mass. This is one reason why vintage drivers are in such demand, as a driver made to dissipate 30 watts can sound so much better than one that can sink 1,000 watts.