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Thread: crossovers

  1. #1


    Sorry for the stupid question, but I never understood what a crossover does. Can someone explain it?
    Ivan Fegundez. Recording Technician, live sound technician, and mastering technician.

  2. #2
    Let's start with a 2-way crossover. The input to a 2-way crossover is a complete audio signal, covering the entire frequency range. The crossover splits this into two signals, sending high frequencies to one output and low frequencies to the other output.

    The reason this is necessary is that a woofer does not handle high frequency signals well and a tweeter cannot handle low frequency signals at all. So you need to send the woofer only low frequencies and the tweeter only high frequencies.

    The frequency at which the divide occurs is called the crossover frequency. For small box-type monitors, everything except the highest frequencies go to the woofer, so the crossover frequency is typically 4kHz. In some systems, the woofer is built to filter out high frequency signals on its own, so the crossover may be just a capacitor to keep low frequencies out of the tweeter. This would usually not be true of the best quality systems - experts agree that for best results, crossover design is as critical or more critical than the actual woofers and tweeters.

    For high-power sound reinforcement systems with large horns, the horns handle both middle and high frequencies and the woofer handles only bass. A typical crossover frequency would be 1kHz to 2kHz.

    A 3-way crossover is the same, except that it divides the signal into three frequency bands which are fed to three different kinds of speakers.

    No crossover can divide the frequency range precisely, perfectly passing everything up to, say, 2kHz and perfectly rejecting 2.001kHz and above. Rather, the response "rolls off" beyond the crossover frequency, exhibiting greater attenuation the farther you get from the crossover frequency. The simplest crossovers have 6dB/octave attenuation, meaning that at one octave removed from the crossover frequency, the response is about 6dB down; at 2 octaves it's about 12dB down etc. There is also some attenuation at the crossover frequency itself, as the response transitions from flat to rolloff. This point is usually 3dB down. When you hear someone refer to the "3dB down point" that's what they mean - the frequency where response transitions from flat to rolloff.

    Of course, the whole system should wind up "flat." While the response of the signal sent to the woofer is flat below the crossover frequency, the signal sent to the tweeter is flat above it, so when the outputs of the two speakers are summed (acoustically, in the air) the overall response is flat.

    (I am ignoring the fact that few real-world speakers are actually flat, and crossovers usually include some non-flat characteristics to improve them.)

    If the entire frequency range comes out of the power amp and goes into the speaker, and the crossover is inside the speaker, this is called a "passive" crossover because there is no power source except the signal itself.

    A crossover can also be placed before the power amp, so it splits the frequency range when it's still at line level. In this case you'd need two power amps, one for high frequencies and one for low frequencies. There would then be no need for another crossover at the speaker. Since a line-level crossover usually requires power from an AC outlet, this is called an "active" crossover.


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