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This is probably the best article for a beginner about impedance with respect to headphones.

I haven't managed to find an explanation of why the impedance of headphones is just a single, real number (like 250 Ohm). I thought impedance is a complex number, so it should have real (resistance) and imaginary (reactance) part. Maybe the imaginary part is just zero. All I know is that impedance varies with frequency.

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  • \$\begingroup\$ Try this: electronics.stackexchange.com/questions/83829/… and note that speaker impedance is normally dominated by the winding resistance of the coil. \$\endgroup\$
    – Andy aka
    Commented Jul 17, 2015 at 12:58
  • \$\begingroup\$ I've seen a Behringer musical bass amp where the "headphone" jack was wired directly to the speaker output. That certainly is under 2Ω! \$\endgroup\$
    – rdtsc
    Commented Jul 17, 2015 at 13:22

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Impedance does vary by frequency. However, for many things it doesn't vary very much by frequency within the expected operating frequency of the equipment, and a single number is much easier to handle than a frequency response graph. So people quote a single average number.

In fact, your linked article admits this:

Headphone impedance can vary by a factor of 10 or more over the audio band. The SuperFi 5 is rated at 21 ohms but varies from 10 ohms to 90 ohms.

That's quite a lot of variation! It also explains why this causes variation in the audio quality when driven from different sources.

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    \$\begingroup\$ Ok, impedance varies with frequency, that's why it's usually specified as a single number at given frequency. My other question was why it's a single real number instead of a complex number (we know impedance is a complex number). \$\endgroup\$
    – user216094
    Commented Jul 17, 2015 at 14:50

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