An axiom of great hi-fi is that speaker drivers should always be smaller than the wavelengths of sounds they are reproducing. For that reason, there are drivers of different sizes in a speaker to cover the entire audible realm. The goal of speaker crossover designs is to segregate and direct high and low frequency bands electronically, so each driver is operating in its optimal range.
Determine the impedance of the speaker drivers the crossover will control. For example, a three-way design including woofer, midrange and tweeter will be based on their individual impedances. If you cannot find the impedance of each driver printed on each driver itself or its data sheet, measure it with a multimeter.
Set the multimeter to sense DC (direct current) resistance in ohms. Start with the 30-ohm range. Multiply the reading by 1.3 to approximate the speaker's impedance for audio purposes. A driver measuring 6 ohms is nominally an 8-ohm speaker and 8-ohm data should be used. For this example, assume all drivers are nominally 8-ohm drivers. Divide your design page into sections, so woofer, midrange and tweeter can be separately evaluated. The woofer will need a low-pass filter section, the tweeter will need a high-pass filter section, and the midrange will need both sections.
Check manufacturer's recommendations for woofer-to-midrange-and midrange-to-tweeter frequency transitions. In this example, the woofer will crossover to the midrange at about 500 hertz (cycles-per-second) and the midrange to tweeter transition will take place at about 3,500 hertz. Check the driver's highest frequency to its wavelength figure from an online frequency-wavelength calculator.
Select the appropriate capacitors and coils for each crossover section with an online crossover calculator. This allows you to benefit from decades of experimentation and the experience of top designers with many types of drivers in many types of speaker designs.
Round-off the capacitance values (in microfarads, uF) and reluctance values (in millihenries, mH) to select real components in a vendor catalogue. Be sure that the capacitors and the inductors selected are rated to handle the wattage that your amplifier is capable of sending to it. If your amplifier outputs a maximum of 100-watts-per-channel, use components rated for that output or more.
Lay out a crossover design sheet. The low-pass filter for this woofer will put a 5-mH inductor in series and a 20-uF capacitor in parallel with the woofer between the inductor and the driver to deliver a low-pass filter crossover point of 500 hertz.
Use a high-pass filter for the tweeter that will put a 0.75 uF capacitor in series and a 0.75-mH inductor in parallel with the tweeter between the capacitor and the driver.
Include both a high-pass filter at 500 hertz and a low-pass filter at 3,500 hertz for the midrange to allow it to operate between those two transition points. The high-pass filter should be located between the amplifier-in leads and the midrange speaker with the low-pass filter on it already.
Design the physical layout or construction diagram. Segregate the inductors for the three sections physically as far away as possible from each other. Since this crossover design forces a 180-degree phase-shift at each crossover step, reverse the polarity of the midrange driver (negative-to-positive and positive-to-negative) for the best sound from the three drivers working together.
Reference the wealth of information on speaker design on the web before trying to construct a crossover network.
Make sure the tweeter circuit is correct before using the design to prevent burning out an expensive speaker.