<P> In the Hornbostel - Sachs scheme of musical instrument classification, wind instruments are classed as aerophones . </P> <P> Sound production in all wind instruments depends on the entry of air into a flow - control valve attached to a resonant chamber (resonator). The resonator is typically a long cylindrical or conical tube, open at the far end . A pulse of high pressure from the valve will travel down the tube at the speed of sound . It will be reflected from the open end as a return pulse of low pressure . Under suitable conditions, the valve will reflect the pulse back, with increased energy, until a standing wave forms in the tube . </P> <P> Reed instruments such as the clarinet or oboe have a flexible reed or reeds at the mouthpiece, forming a pressure - controlled valve . An increase in pressure inside the chamber will decrease the pressure differential across the reed; the reed will open more, increasing the flow of air. The increased flow of air will increase the internal pressure further, so a pulse of high pressure arriving at the mouthpiece will reflect as a higher - pressure pulse back down the tube . Standing waves inside the tube will be odd multiples of a quarter - wavelength, with a pressure anti-node at the mouthpiece, and a pressure node at the open end . The reed vibrates at a rate determined by the resonator . </P> <P> For Lip Reed (brass) instruments, the player controls the tension in their lips so that they vibrate under the influence of the air flow through them . They adjust the vibration so that the lips are most closed, and the air flow is lowest, when a low - pressure pulse arrives at the mouthpiece, to reflect a low - pressure pulse back down the tube . Standing waves inside the tube will be odd multiples of a quarter - wavelength, with a pressure anti-node at the mouthpiece, and a pressure node at the open end . </P>

Instruments in this category produce sound by using air