Audio signals are extremely difficult to reproduce with precision due to the limitations of speakers. Each speaker is designed and sized for reproducing specific frequencies and timbres. The driver for each speaker must interact with other drivers so that the electronic signal, which is converted into sound waves by the speaker, is split up for the optimal sound output that is most true to the original recording. Crossovers help to reduce the peaks and valleys of the sound wave, making the sound more smooth and true to the original recorded electronic signal. Put plainly, crossovers help to isolate and separate frequencies that are pumped into an audio system.
Write down on a piece of paper which type of crossover you are using for your audio system: Linkwitz-Riley, Butterworth or Bessel. Linkwitz-Riley crossovers are able to match attenuation slops, producing a signal that is fairly flat at the crossover point. Butterworth crossovers are able to yield to a peak at the crossover frequency while Bessel crossovers have a frequency response that is between the first two types of crossovers. Also write down the capacitance value of the crossover that you are using, measured in Farads. The crossover component's cap will usually have a Farad's value found inside of it, although it may give the measurement in UF, or million-Farads. To convert UF to F, divide the UF numeric value by one-million. Below the figure for Farads, write down the impedance of the speaker that you are using.
Use your calculator to multiply the crossover's capacitance by the impedance. Write down the number on your sheet of paper. Punch in 0.159 on your calculator and divide it by the product from the capacitance and the impedance, which will give you the crossover frequency in Hertz.
Go to a website with online crossover design calculators, such as the one found at ccs.exl.info, which will allow you to calculate two-way crossovers at various decibel levels. The calculator also allows you to plug in units for high and low pass impedances. Alternatively, if you are using a Zobel circuit configuration for impedance stabilisation, there is a specific calculator for that as well. Other useful online tools include the12volt, which has first, second and third order capacitors and can help calculate the coils (Millihenries).
Passive crossovers differ from electronic crossovers, which have active filters for the frequencies. Passive crossovers use inductor-coils and capacitors, which causes "roll-off" of audio level, and hence the signal sent to each speaker. Capacitors reduce lower frequencies while inductors reduce higher frequencies.