Much of the audio you hear today starts as digital information that passes through a Digital-to-Analog Converter (DAC). This DAC converts a digital signal into an analog electric current that speakers can use to reproduce sound. For devices created before the digital age, however, these electrical signals came straight from the analog source media and passed through an amplifier to the speaker---no digital conversion required. In headphones, like all speakers, oscillating electric currents travel via the wires to the voice coil. When current passes through the coil, it creates an electric field that interacts with the permanent magnet's electromagnetic field. The variance between the two fields causes the voice coil to vibrate. When the voice coil vibrates, the diaphragm moves with it. This movement of the diaphragm causes pressure waves (or sound waves) in the surrounding air. These waves are the sound you hear. For higher-pitched sounds, the diaphragm moves quickly; for lower pitches, the diaphragm vibrates slowly. Overall audio volume depends on the relative strength of the electrical signal.