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How much is impedance of a typical hands-free speaker and how much power is needed to properly drive it?

A typical hands-free speaker

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    \$\begingroup\$ A typical one these days is BlueTooth. \$\endgroup\$ – Eugene Sh. Jul 31 '17 at 18:31
  • \$\begingroup\$ By typical hands-free i mean hand-frees with 3.5 mm jack \$\endgroup\$ – Hesam Qodsi Jul 31 '17 at 18:35
  • \$\begingroup\$ A common impedance is 32 ohms, but there's no real standard for it. Power is typically measured in tens of milliwatts, but again, that can be design- and application-specific. \$\endgroup\$ – AaronD Jul 31 '17 at 18:41
  • \$\begingroup\$ If you have a specific one in your possession that you want to use, you can just measure the (approximate) impedance with an ohmmeter and then calculate the power by measuring AC voltage while it's running. Watts = Volts^2 / Ohms \$\endgroup\$ – AaronD Jul 31 '17 at 18:42
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Lightweight headphones and earbuds usually have a nominal impedance of 32Ω. A few tens of milliwatts should be enough to drive them properly. Once you get into hundreds of milliwatts, the sound levels can become dangerous.

The efficiency of headphones and speakers varies considerably, so you'd need the specifications to work out how many milliwatts you'd need for a given volume level.

See also What wattage should a headphone amp be?

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