Is a speaker usually driven with AC or DC current?

Answer 1

The amplifier's speaker output is AC.

Answer 2

AC always.

DC, or direct current, is an electrical circuit that does not change current flow direction, therefore DC carries an one-way flow of electricity of a certain amplitude value (the amount of power of the flow of electrons). AC, or alternate current, is an electrical circuit of current that alternates its direction of flow, and frequency is a function of how many cycles the current changes flow directions, expressed in Hz.

A DC circuit will not make any sound; there is no change of direction of electricity current flow, so the driver of a speaker driven by DC will just be moving either in OR out, and stays in or out as long as there is a current, but NOT both in AND out, because the DC is creating force on the motor structure of the speaker driver in only one direction due to the one direction flow of the current.

For the speaker to make sound, the driver needs to BOTH move in AND out by changing directions of the current flow. It is this movement of the speaker driver moving in AND out that is vibrating the air to make sound.

So, sound is made by AC, because music is made up of variations of many different frequencies and loudness, with low frequency as the bass, high frequency as treble, and the amplitude of the current expressed as the loudness variation.

Additional information:

* What comes out of your walls is AC at 110V and 60hz frequency, with most houses build with 15 amp to 20 amp circuit capacity. This 60hz has nothing to do with music reproduction, but everything to do with that annoying, low frequency electrical "hum" noise heard in faulty electrical equipment. Your amplifier has to harvest this electrical energy from your wall sockets by converting AC to DC, then back to AC that is analogous to the sound waveform of your music to drive your speakers. In Europe, the household electricity supply is at 220V and 50hz.

* The inward or backward movement of the speaker driver carries the inverse phase of the electrical signal, making sound waves of inverse phase, which can cancel out the forward sound waves made by the speaker, but this is inverse sound wave is usually blocked by the enclosure of the speaker. This is more an issued with low frequency signals as the wavelengths are very long. This is not as much of an issue with higher frequency signals because the wavelength is shorter than the baffle front panel which the speaker driver is mounted on, so most of the inverse waves are still blocked, but this will still decrease its efficiency, or loudness. This is also why a free air subwoofer driver without an enclosure or at least a large panel blocking the inverse sound waves from the back will not make much sound. These subwoofer bass speakers producing low frequency sounds of long wavelengths need a closed enclosure, or a port in the enclosure that redirects this back sound wave with delays so it is no longer inversely phase, which then creates an additive property to bass sound loudness, instead of cancellation of bass sound.

* With the same principle, active noise cancelling headphones all contain an active external mic that is constantly measuring ambient noise, then generates the exact out of phase or inverse sound waves to that of the ambient noise inside the headphones, cancelling out noise. This works better with lower frequency, drone like ambient noise such as wind and engine noise, and does not work well with higher frequency noise, such as human voices. So these headphones still rely on physical isolation for blocking out sounds of these frequencies.