How does the duct in a recorder direct air to create sound?

In a recorder, the duct—also referred to as a windway—plays a critical role in sound production by shaping and directing the player's breath. As the player blows into the mouthpiece, the air is forced through this narrow passage, which is formed between the instrument's wall and a plug known as the block or fipple.12

The duct directs this shaped airstream precisely against the sharp edge of an aperture located farther down the body of the instrument. By striking this edge, the airstream sets the internal air column within the tube into regular pulsations, which creates the instrument's sound. Because the duct handles the task of shaping and directing the air, the recorder is considered a whistle flute, making it simpler to play than "true" flutes, where the player must use their own lips to direct the air against the edge.2