Posted by Wayne Parham [ 65.214.102.163 ] on February 01, 2007 at 10:37:11:
In Reply to: converting active crossover values to passive posted by akhilesh on February 01, 2007 at 07:45:07:
Speaker impedance is complex, in that it is a mixture of voice coil inductance, voice coil resistance, mechanical impedance from the moving mass and suspension and acoustic impedance from the cabinet. Acoustic impedance is transformed to mechanical impedance by presenting a pneumatic load on the cone. Mechanical impedance is then transformed to an electrical impedance by presenting a mechanical load on the cone, which in turn, becomes a back-EMF when the voice coil is presented an electrical signal. The end result is a complex impedance curve. See the documents at the links below for an overview:
While all this sounds complicated, it isn't really all that bad. One thing that makes passive crossover design a little easier is that you don't have to concern yourself with the whole impedance curve of a driver, just the range near the crossover frequency. The impedance for about an octave above and an octave below (depending on crossover slope) is all you really need to worry about. This is the frequency range where driver impedance will have the most effect on the crossover.
In most cases, a damping resistor is all that's needed to tame a tweeter's impedance curve, and sometimes that's not even necessary. A midrange is usually tamed with a Zobel, if needed. Woofers sometimes need Zobels too. Even though the impedance curve of a woofer varies quite a bit (particularly in some cabinets like transmission lines and horns), the fluxuations are mostly in the bass range, well below the crossover point. In that case, the impedance fluxuations won't affect the crossover. Again, it's the frequency range where crossover is needed that's most important.
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