linguistic:paraphrasing
Basic sound production

The recorder produces sound in the manner of a whistle or an organ flue pipe. In normal play, the player blows into the windway (B), a narrow channel in the head joint, which directs a stream of air across a gap called the window, at a sharp edge called the labium (C). The air stream alternately travels above and below the labium, exciting standing waves in the bore of the recorder, and producing sound waves that emanate away from the window. Feedback from the resonance of the tube regulates the pitch of the sound.

In recorders, as in all woodwind instruments, the air column inside the instrument behaves like a vibrating string, to use a musical analogy, and has multiple modes of vibration. These waves produced inside the instrument are not travelling waves, like those the ear perceives as sound, but rather stationary standing waves consisting of areas of high pressure and low pressure inside the tube, called nodes. The perceived pitch is the lowest, and typically loudest, mode of vibration in the air column. The other pitches are harmonics, or overtones. Players typically describe recorder pitches by the number of nodes in the air column. Notes with a single node are in the first register, notes with two nodes in the second register, etc. As the number of nodes in the tube increases, the number of notes a player can produce in a given register decreases because of the physical constraint of the spacing of the nodes in the bore. On a Baroque recorder, the first, second, and third registers span about a major ninth, a major sixth, and a minor third respectively.