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why does a speaker need a crossover?
If a speaker several Chassis then you have to split the frequencies in such a way that each driver only gets the frequencies for which it is used. This is done with the so-called switch. The crossover is usually built into the speaker and is not visible from the outside.
The division is usually done with capacitors, inductors and resistors, in addition there are sometimes fuses and zener diodes for overload protection. These switches are then passive switches. A 3-way loudspeaker then has 3 chassis, a bass chassis, a mid-range chassis and a tweeter chassis (e.g. a dome tweeter, so called because of its construction (dome = hemisphere).
One possibility would be to build with a 3-way crossover and the crossover frequencies (crossover frequencies) 800 Hz, 3000 Hz: 20-800 Hz for the bass, 800-3000 Hz for the bass midrange, 3000 to 20000 Hz for the Tweeter.
You can find more crossover frequencies in the HiFi-Knowledge under the item "Separation frequency information".
Unfortunately, it is technically not possible to build a crossover with just a few components that performs a 100 percent separation at the specified frequencies. Many crossovers attenuate the signals from the crossover frequency, e.g. B. 12 dB each octave, in the above example the bass chassis would reproduce the 1600 Hz frequency at 1/8 the volume, at the same time as the mid-range driver, which reproduces it at full volume. The sound waves are superimposed and result in a wavy frequency curve, with this ripple also having other causes.
There are also loudspeaker boxes of the "bass reflex housing" type, in which case, instead of the bass reflex opening, another bass chassis without voice coil (passively resonating) is used.
Consequently, one tries to achieve the greatest possible edge steepness, the following values per octave are usual: 6dB, 12dB, 18dB, 24dB. There are also Equalizer with slopes of 96dB/octave, which are used as crossovers, but that usually sets several amplifier in advance (bi-amping Etc.). Active loudspeakers also have built-in crossovers and often a separate amplifier for each driver, but it is easier to use steep slopes here - values of around 24 dB are not unusual.