Brian Elliott wrote:

> Firstly, the crossover network divides the signal from the amplifier to 
> the woofer and the tweeter drivers.  It must be tuned to the right 
> frequency and shape the frequency falloff curves and phase so that the 
> sound levels from both drivers are balanced without dips or humps or 
> comb-filtering at the crossover band where both drivers transmit part of 
> the signal.  It is also supposed to keep the signal phase true between 
> the two drivers over the frequency range so they operate in a unison 
> making the sound wavefront from both drivers combine and arrive at your 
> ear together.  That is fekking difficult to design, because...
> 
> Crossovers are passive networks of resistors, capacitors and inductors. 
> Networks behave much more complicatedly than isolated components because 
> everything interacts with everything else, not just its immediate 
> neighbours.  Speaker drivers are *also* R-L-C networks, so crossovers 
> must be designed with that driver's electrical properties being integral 
> to it.
> 
> In short, the whole thing is interbalanced, so if one resistor, 
> capacitor or inductor changes, everything goes out of whack -- crossover 
> frequency, frequency response, phase response, impedance response, 
> resonances and ringing -- and the speaker sounds like crap.
> 
> For predictability, a speaker and its crossover also rely on the 
> amplifier's output stage being very low impedance. Signal-wise, the 
> amplifier is near to a short-circuit, regardless of the voltage swings 
> it generates.  A speaker is a motor and when moving, it generates 
> back-EMF through its crossover.  It has mechanical inertia and wants to 
> overshoot.  It  also has natural resonances from driver suspension 
> springiness, cabinet air volume, acoustic transmission line length, 
> tuned acoustic port, etc.  That colouration is NOT part of the original 
> signal.  If the amplifier doesn't soak up that energy (absorb the 
> current), to damp the unwanted motion (overshoot and ringing) it will 
> reflect back into the network and colour the sound you hear.

I read about this kind of thing in a DSP book. Damn, IIR filters sound 
so complicated! Fascinating, but... I wouldn't want to design them for a 
living.

-- 
http://blog.orphi.me.uk/

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