An audio crossover is a stereo component that divides frequencies between speakers. Different types of speakers are meant to handle different frequency ranges. The crossover filters the signal before the speaker so that the speaker only "hears" the sounds it is meant to. There are two types of crossovers: active and passive. A passive crossover is an unpowered circuit that comes between the amplifier and speaker, while an active crossover is powered and comes before the amplifier. Active crossovers are useful if you have separate amplifiers for the bass, midrange and tweeters.
Set a Neutral Reference
If you have passive crossovers, start by setting any switches or jumper setting to the neutral, or reference, position. If you have active crossovers as well, set both the lowpass filter for the subwoofer amp and the highpass filter for the midrange/tweeter to the same frequency, between 70 and 90 Hz. If you have a separate set of amplifiers and speakers for the tweeter and midrange, set the crossover points to the manufacturer's recommendations in the manual. Set the crossover slopes to the same value, either 12 or 24 dB per octave. The crossover slope is the steepness of the response curve of the filter. Higher values separate the high and low frequencies more sharply, and usually give better results if you have well-matched components. On the amplifiers, neutralise any equalisation controls.
Listen Before You Tune
Listening to the system before adjusting the crossover will give you a reference to start with and help you hear what adjustments you should make to the crossover. Listen to a cleanly recorded, high-quality CD with a lot of dynamic range, whose sound you are already familiar with. Heavily compressed MP3 files or synth and distortion heavy pop music are not ideal for testing your stereo, nor is heavy hip-hop or dance music, which often emphasises the sub-bass. When listening to the music, take note of which frequency ranges your stereo seems to over- or underemphasise. In tuning your crossover, you are aiming for an even response across the whole frequency spectrum.
Start With Gain Controls
Before tuning the crossovers, adjust the amplifier gain controls to match the level of the bass, mid and high-frequency ranges. Set the gain controls so all three ranges have equal volume.
Adjust Crossover Frequency Within Manufacturer's Recommended Range
Adjust the crossover frequencies up and down and see how the sound changes. Your speakers are meant to take a certain range of frequencies, which the manufacturer will specify in the manual. Subwoofers, for example, are meant to receive only the lowest frequencies. Always adjust the crossover frequency within the manufacturer's recommendations. For example, if your tweeter is rated for a minimum frequency of 5 kHz, do not set the highpass filter to 4.5 kHz. Likewise, avoid overlapping frequencies. If you set the lowpass for the subwoofer to 90 Hz, and the highpass for the midrange/tweeter to 80 Hz, the sound may come out muddy and garbled.
Splitting Crossover Points
You don't always have to set the crossovers to the exact same frequency. According to Car Audio Magazine, sometimes the acoustic resonance of the interior car creates a boost somewhere in the 50 to 100 Hz range. Leaving a gap between your crossover points in this range may account for the natural resonance of the car and smooth out your stereo's frequency response.
Big, Booming Bass
Setting the subwoofer lowpass point above 100 Hz will direct more of the sound to the subwoofer, creating a big, round bass boom that hip-hop or dance music fans may be looking for.
There is no "right" way to tune a crossover. Experiment until it sounds best to you.