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Physically speaking, what is the difference between crossover frequency and resonant frequency?

As far as I know, the resonant frequency is the frequency at which the system oscillates. And the crossover frequency is just a mathematical description on the bode graph

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  • \$\begingroup\$ The crossover f is as you expect the point where a bandpass and band stop filter have equal effect to the transfer function. Often this applies the passive filters used to select bandwidth for speakers from bass, Midrange and treble. \$\endgroup\$ Jan 30, 2019 at 1:53
  • \$\begingroup\$ Resonant frequency doesn't necessarily produce sustained oscillations in a "system" and, if the "system" is (say) a Colpitts oscillator then the actual oscillation frequency will hardly ever coincide with a resonant peak in the filter circuit. \$\endgroup\$
    – Andy aka
    Jan 30, 2019 at 12:54

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You are correct about resonant frequency. The "crossover" frequency depends on the topic.

As @John D pointed out in the comments, for controls, it's where the amplitude crosses 0dB. In audio, it is more related to where gain lines intersect, such as where mid-range and tweeters "crossover" and their gains are equal. See the image, where the blue and green cross, this is a crossover point in audio. Note this is irrespective of the magnitude of the gain at this point, in fact it could be positive, or very negative. enter image description here

The -3dB point, is where the output signal amplitude is half of the input amplitude.

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    \$\begingroup\$ Not necessarily. In controls the crossover frequency can be where the magnitude crosses 0dB. The -3dB point is more often called the corner frequency or the "knee". \$\endgroup\$
    – John D
    Jan 29, 2019 at 22:08

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