Crossovers filters are basically Frequency Traffic Cops.They start and stop frequencies and direct them where to go. Crossovers are used to insure that specific range speakers (tweeters, midranges, woofers) do not receive a frequency range greater than their design limits.

Crossover Points - The frequency range allows full power to pass through is called a Passband. The frequency at which a filter starts reducing power is called a Cutoff Point. the range between cutoff points is referred to as the Crossover Area. Starting at the cutoff point, as you move away from the passband area signal attenuation increases. When it drops the signal 3dB, the Crossover Point (or crossover frequency, also referred to as half power point)) has been reached.

There are two main types of crossover filters, Low Pass and High Pass. Low Pass filters only allow low a certain frequency and below to pass, High Pass starts at certain frequency and goes up. You combine the two together to get a Bandpass filter. A bandpass filter only allows a certain ranger frequencies to pass, while rolling off both low and high frequencies.

Slopes - Crossovers have various rates that they attenuate or roll off the signal. These are called Slopes. The are determined by the drop in decibels one octave above (low pass) or below (high pass) the crossover point. An octave up is the frequency doubled and an octave down is frequency halved. The most common slopes are 6dB, 12dB or 18dB.

Active and Passive Crossovers - Crossovers come in two styles - Active and Passive. An active crossover requires a power source (12v+ in car audio). They use a full range, line level preamp output for an input signal. It will split the line level signal into appropriate frequency ranges so it can be sent to separate power amplifiers for the woofers, mids and tweeters. You can set the crossover points wherever you need to, even if they overlap, since the signals are going to separate amps.

Passive Crossovers take a full range output from an amp and split it into appropriate frequency ranges so it can be sent to speaker components (woofers, mids and tweeters). Even though technically you are hooking up speakers in parallel when you use a passive crossover, the overall speaker system impedance will stay the same (if the crossover is designed right). This is because impedance is relative to frequency. For example, let's say we have a 4 ohm woofer and a 4 ohm tweeter in a speaker system. As long as the crossover rolls off the woofer below where it's letting the tweeter kick in, the overall impedance of that speaker system will be 4 Ohms. Because of this, when these are designed the crossover points must be spaced apart from each other (how far depends on the slope) so they don't overlap. To see how to design a passive crossover, visit the Passive Crossover Network Schematics page.