Are Speakers Passive Devices?

The majority of speakers are, indeed, passive devices.

Most speakers do not need bias DC voltage (as microphones do) in order to function and play out sound. They do not need DC power to operate.

Thus, all speakers need is a sound signal that is large enough to drive them.

Too small of a signal and the signal will not be sufficient enough for the speakers to pick up and play back. The ac sound signals coming in have to be large enough for the speakers to play them back.

This is why many times speakers have preamplifiers in them to make sure that the signal they are to play out is amplified large enough so the speakers can actually drive them.

So speakers are active devices, meaning they have amplifiers in them and, thus, the whole system needs DC bias power in order to work. However, again, most are passive and only need AC sound signal large enough to drive them, and not DC power.

This is why when you plug a headphone speaker jack into a laptop computer, the speaker jack has no DC power coming through the jack. If you take a multimeter and place on the DC voltage setting and measure from the ground of the jack to the tip of it, you will measure no considerable DC voltage. Because speakers do not need them. However, microphone jacks must have DC voltage coming out of them. If you do the same test, you will notice a voltage a a little above 2 volts coming from the microphone jack. Microphones always need DC voltage power applied directly to them.

This is important to know when you design speaker circuits, that as long as the ac signal driving them is large enough, this is all that is needed to drive speakers.