What audio technique involves fading one sound down while simultaneously fading another sound up?

The technique described is indeed crossfading. Crossfading refers to the process of gradually decreasing the volume of one audio track while simultaneously increasing the volume of another. This creates a smooth transition between the two sounds, allowing for a seamless blend that enhances the overall listening experience.

Crossfading is commonly used in various audio applications, such as music production, sound design for film, and live DJ performances. By employing crossfading, audio engineers can maintain continuity and flow within a piece without the abrupt changes that can disrupt the listener's immersion. This technique is particularly effective in situations where two tracks need to coexist, such as transitioning between songs or mixing different audio elements.

In contrast, layering refers to the technique of adding multiple audio tracks together to create a fuller sound, while ducking involves lowering the volume of one track in response to another—common in dialogue with background music. Panning is the adjustment of sound placement across the stereo field and does not involve any fading. Each of these techniques has its unique purpose, but crossfading specifically is focused on the dynamic interplay of volume between two audio signals.