fade in

A fade in is an audio engineering technique in which volume increases gradually from silence or near-silence over a set duration, typically measured in seconds. The curve of this increase can be linear (equal volume change per unit time) or logarithmic (matching perceived loudness more closely to human hearing). In electrical terms, the control signal driving the audio amplifier's gain parameter ramps upward smoothly rather than jumping abruptly to its target level.

Fade ins serve both technical and artistic purposes. Technically, they prevent the click or pop that occurs when an audio signal switches on suddenly from zero. Artistically, they create a gentle introduction that can set mood or draw a listener into a recording. The duration varies widely: a two-second fade in works well for a song intro, while longer fades of five to ten seconds suit documentary soundscapes or background music. The fade curve matters because a linear increase in voltage does not sound like a linear increase in loudness; most professional equipment uses either an equal-power curve or an exponential curve to match human auditory perception.

Implementation and Control

Fade ins are implemented in several ways depending on the context. In analog mixing consoles, an operator physically pushes up a motorized fader, which adjusts a potentiometer that controls the amplitude of the signal. In digital audio workstations and automated mixing systems, automation envelopes define the gain over time; the control surface or software computes the exact voltage or digital gain coefficient at each sample. Many audio devices include dedicated fade buttons or menus that trigger a preset fade rate.

The fade-in curve itself is a critical parameter. Some systems default to a constant-power curve, which maintains constant total energy during the transition and prevents any perceived dip in level. Others use a linear voltage curve, which is simpler to implement in analog circuits but can sound like the volume dips slightly in the middle. Digital systems can implement any mathematically defined curve, including ease-in functions that start slow and accelerate.

Fade in is distinct from other volume transitions. A crossfade involves one signal fading in while another fades out simultaneously, common in DJ mixing and song transitions. A slow attack in a synthesizer or compressor is a related but different control over the sound's envelope. The term fade in originated in film and radio, where tape operators would physically raise playback levels; the term transferred directly to electronic mixing boards and persists in digital audio software.