I don't know...the title sounded cool in my head. ;)

As you may or may not know, most of your multiple driver speakers on the market today contain a device known as a passive crossover, what exactly are they?

To put it in simple terms, they are a collection of passive devices that when used together form a filter.

In more complex terms, they are a combination of capacitors, inductors (coils), and resistors arranged in various fashions to provide frequency control for your speaker arrangement.

Let's look at the basic components:

Capacitors:

On their own, in a crossover, they can operate as a high-pass filter, only allowing frequencies above the setpoint to pass...at a certain level of attenuation, in this case...6dB or First order.

Capacitors are rated in uf or micro farads for crossover use and also have a voltage rating, which is largely negligable for our purposes here.

Generally we will just use a single capacitor that is near the rating required, but if you really needed to you can run them in series or parallel.

In parallel, the total value of the capacitor is the combined total of the capacitors in parallel. OR C1+C2=Ct

In series the total value of the capacitor is as follows: Ct= (C1*C2) / (C1+C2).

Inductors: (Coils)

Inductors are to a low pass crossover what capacitors are to a high pass. They provide a 1st order or 6dB slope lowpass.

Coils are rated in mH for milliHenry.

You generally will not see coils placed in series or parallel because their size generally prohibits such uses in a crossover.

Resistors:

Resistors are used to make attenuation circuits and other filter devices in a crossover.

Slope or Order:

We are going to be really general here because there are several designs that you can get into that all use different values and characteristics.

1st order or 6dB per octave:

This is the simplest and least effective filter. Take apart a cheap bookshelf and you might see a single capacitor soldered to the tweeter leads...that is a 6dB highpass.

The advantages of these are price and simplicity, the disadvantages are lack of real control (6dB is not a big difference) and the possibility of a 90 degree phase shift.

Example drawing (C=capacitor, L=inductor)

-----C--------Tweeter +

--------------Tweeter -

-----L--------Woofer +

--------------Woofer -

An example of the attenuation of a 1st order Lowpass centered at 200hz.

200hz (0)dB
100hz -6dB
50hz -12dB
25hz -18dB

2nd Order or 12dB per octave.

This crossover uses one each of a capacitor and coil in various configurations to create a high or low pass.

This configuration was your most common for a long time due to its relative combination of simplicity and performance.

Example of the same 200hz lowpass:

200hz (0)
100hz -12dB
50hz -24dB
25hz -36dB

You can see that there is almost no chance for a woofer that would bottom out at 50hz would do so with a 12dB/octave crossover whereas it would still be possible with a 6dB slope.

3rd Order (18dB/octave)

This design adds another of the primary passive component to each of your high/low pass...if high then it will have 2 caps and 1 coil, if low 2 coils and one cap. Obviously this design is getting pretty expensive, especially if you are using high quality components. It is also getting rather large...maybe larger than the space you have internally for it!

200hz LP example:

200hz (0)
100hz -18dB
50hz -36dB
25hz -54dB

Yes there are 4th Order or 24dB slopes but these are best suited to electronic or active x-overs than passives.

Other filters:

Zobel Filter:

Basically this filter tries to eliminate impedence rise in a circuit.

RL or RC Contour network:

This filter tries to compensate for natural frequency response change. They use Resistors and coils and capacitors for this.

Notch Filter:

A basic passive form of equalization. A notch filter will provide...imagine this, a notch at a certain amount of attenuation in a specific filter band. This is determined by the values of the components which are a cap, a coil and a resistor all in parallel.

L-Pad:

This filter, which uses 2 resistors, one along the + one between the +/-, is basically a passive volume control. It will attenuate the component it is attached to a certain amount of dB over its operational band. This is handy for tweeters with too much energy as they are generally more efficient than the woofer.