What does the Crossover do?
Like your own brain, the crossover network is a director. In addition to its primary function, the passive crossover in practice is expected to do more than simply split the frequency band in two.
1) The crossover allows you to place two 8 ohm speakers in parallel, one for the highs and one for the lows, and still present an 8 ohm, not a 4 ohm load to the amplifier (which is what the amp would see without the network)
2) The crossover allows you to match two drive units of different efficiencies so that the combination of the two does not result in a large shelf in the frequency response.
3) The crossover allows you to equalize the frequency response of either the woofer or tweeter, making the composite response smoother than the unfiltered curves of either driver (woofer or tweeter) would seem to indicate is possible.
Let’s get back to the basic job of the crossover, and see if we can understand how it accomplishes this by learning a few new but simple concepts. The basic job is to send the high frequency information to the tweeter, and eliminate this same band of high frequencies from the woofer. Next, send the low frequency information to the woofer, and eliminate this same band of low frequencies from the tweeter. A crossover is in essence two frequency filter sections working in parallel. A high pass section and a low pass section. These terms are self explanatory. The high pass passes high frequencies, the low pass passes low frequencies. The high pass is placed in series with the tweeter, and the low pass is in series with the woofer. A normal two way crossover will have six terminals. Two in, and four out. Usually the two sections (high-pass and low-pass) are in parallel and both sections ONLY work properly when the woofer and tweeter are both attached and operational. To understand how the crossover does its job, we must first understand the concept of impedance.
The three main passive elements used in all crossovers are resistors, inductors, and capacitors. Impedance is a measure of how much the resistor, capacitor or inductor impedes the flow of electrons at any given frequency. Resistors are the simplest components, and the easiest to understand. The have a constant resistance to the flow of electrons which does not vary with the frequency of the signal. Resistors do not change the phase of the signal, the time relationship between voltage (electrical pressure) and current (flow of electrons). They only consume power and convert it to heat. That is all they do. In a perfect speaker system there is no need for resistors at all. Of course, while there are no perfect speakers, all the basic crossover theory's we use start with that premise.
The second element we shall consider is a capacitor. The capacitor impedes the flow of electrons through it in a way which is NOT independent of frequency. (It would not be technically correct to use the word resistance, but you can certainly think of it as such to conceptualize this). The capacitor has an impedance which is inversely related to frequency. This is to say when you double the frequency of a signal applied to the capacitors terminals, its impedes that electrical flow by half as much as it did at the lower frequency. If you halve the frequency applied, the capacitors impedance (How much it impedes the flow of electrons) is doubled. This is a very useful characteristic which we use to create networks which are frequency selective (like a crossover).