Audio signals are AC (alternating current) signals. They oscillate back and forth between positive and negative voltage values over time, creating the varying waveforms that represent sound. In contrast, DC (direct current) is steady and unidirectional—it flows in one direction at a constant voltage.
Sound itself is produced by vibrating objects that create pressure waves in air. When a microphone converts these pressure waves into electrical signals, it produces voltage fluctuations that mirror the original sound's variations. These fluctuations cross zero repeatedly as the waveform oscillates, which is the defining characteristic of AC. For example, a 1 kHz audio tone would show voltage oscillating 1,000 times per second.
This AC nature is fundamental to how audio works in speakers, amplifiers, and recording equipment. When an audio signal reaches a speaker, the AC voltage drives the speaker cone back and forth, reproducing the original sound waves.
That said, audio systems do use DC in supporting roles. Power supplies convert wall current to DC to power amplifiers and electronics, but the actual audio signal traveling through cables and circuits is AC. Some audio equipment also uses a small DC bias or offset, but the meaningful information—the sound content—is encoded in the AC component of the signal.