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I'm reading about transducers and have a related question.

Is the audio signal flowing through the voice coil of a loud speaker AC or DC?

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    \$\begingroup\$ There are two different ways of defining DC: 1. "Always either positive or always negative, but possibly changing"; or 2. "Never changing". Which one do you mean? \$\endgroup\$ – The Photon Nov 16 '16 at 16:40
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WE try to never put DC through a speaker coil. The design intent is for only AC to flow through a speaker and never DC. See: Why are DC signals bad for loud speakers?

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If there is DC bias, the bias will pass to the transducer coil, which makes heat in the speaker consuming more power, so analog audio should be pure AC coupled. That's the why a series capacitor used in the audio path.

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Pure AC is a time varying electrical signal which periodically reverses it's polarity, crossing zero twice per cycle. Pure DC is a constant, unvarying signal. Fluctuating DC is more appropriately termed AC with a DC bias; if you remove the DC bias (by passing the signal through a capacitor, for example), you are left with a pure AC signal. Thus, an analog audio signal can be either pure AC or AC with a DC bias.

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