Offstage instrument or choir part in classical music

An offstage instrument or choir part in classical music is a sound effect used in orchestral and opera which is created by having one or more instrumentalists (trumpet players, also called an "offstage trumpet call", horn players, woodwind players, percussionists, other instrumentalists) from a symphony orchestra or opera orchestra play a note, melody, or rhythm from behind the stage, or having a choir of singers sing a melody from behind the stage. This creates a distant, muted effect which composers use to suggest "celestial voices", melancholy, or nostalgia, or to create a haunting or mysterious effect.

Source: Wikipedia — Offstage instrument or choir part in classical music (CC BY-SA 4.0)