High Fidelity Speaker Design

 

First order crossover is simplest with the -6dB per octave slope, second order is -12dB, third -18dB, fourth -24dB... these get more complicated on paper/PC because the components are reactive and produce a phase shift. For best results the higher order ones need to be tested with the real drivers to confirm the results. Even -24dB/octave lets quite a lot through and you'd need digital filters to get very steep rolloffs like -48dB/octave. Lots of web resources on this stuff.

 

First order is simple and good if the speakers allow it. The reason for using steeper filters is to reduce the overlap between the two drivers, partly to control their interaction (lobing) in the pass band where they both produce sound and for mechanical/electrical reasons. The larger one will probably exhibit breakup distortion and narrower dispersion of the sound ('beaming') at higher frequencies and the smaller one will be unable to handle cone excursion and power of lower frequencies.

 

There may also be a difference in driver sensitivity so you'll sometimes find a resistor (potential divider) just before the HF crossover that reduces the tweeter's level.

 

The speaker crossovers must handle higher voltages and currents so the components need to be rated for this use.