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. Harmonic profile The recorder sound, for the most part, lacks high harmonics and odd harmonics predominate in its sound with the even harmonics being almost entirely absent, although the harmonic profile of the recorder sound varies from recorder to recorder, and from fingering to fingering. As a result of the lack of high harmonics, writers since Praetorius have remarked that it is difficult for the human ear to perceive correctly the sounding octave of the recorder. Air As in organ flue pipes, the sounding pitch of duct type whistles is affected by the velocity of the air stream as it impinges upon the labium. The pitch generally increases with velocity of the airstream, up to a point. Air speed can also be used to influence the number of pressure nodes in a process called over blowing. At higher airstream velocities, lower modes of vibration of the air column become unstable, resulting in a change of register. The air stream is affected by the shaping of the surfaces in the head of the recorder (the "voicing"), and the way the player blows air into the windway. Recorder voicing is determined by physical parameters such as the proportions and curvature of the windway along both the longitudinal and latitudinal axes, the bevelled edges (chamfers) of the windway facing towards the labium, the length of the window, the sharpness of the labium (i.e. the steepness of the ramp) among other parameters. The player is able to control the speed and turbulence of the