Sound gradually rises from silence over milliseconds or seconds — typically 0.5–2s ramp. Less jarring than hard audio cut, signals arrival of soundscape.
You're in the editing room, the scene is playing silently – and suddenly you have to decide how the sound comes in. Not with a bang, but gently: the audio fade-in does exactly that. The sound starts from silence (or a very low level) and gradually increases to normal volume. This usually happens over half a second to two seconds, depending on the rhythm of the sequence and the emotional intention. In contrast to a hard cut – where sound is simply there – a fade-in creates a bridge. The viewer isn't ripped out of their calm.
You don't need to worry about this on set. That's an editing and mixing decision. But as a DoP, you quickly notice: a long silence that is faded in has a different effect than an abrupt sound cut. You typically see this when a new scene begins – perhaps from black or over a slow establishing shot. The ambient sound of the new location doesn't arrive like a shock, but like a presence that establishes itself. This also works with music: instead of a song suddenly playing, you fade it in. The viewer has time to register the audio information without it being disruptive.
Practically, it's used for transitions between scenes, for dream or flashback sequences, or when you consciously want to build an introspective quality. For example, when a character wakes up: the ambient sounds could be faded in to emphasize their dawning awareness. In contrast to the audio fade-out – where sound disappears – a fade-in signals entry, arrival, taking presence.
The length of the fade is crucial. 0.5 seconds feels fast, almost nervous. Two seconds or longer becomes meditative, but can also feel slow. In science fiction films, longer fades are often used to build atmosphere and technical ambience. In action cinema, it's shorter. This is a creative choice in sound mixing, closely intertwined with picture editing, pacing, and what you show visually. The synchronization between image and sound makes the difference.