Wind Instrument - Physics of Sound Production

Physics of Sound Production

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.

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.

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.

For Air Reed (flute and fipple-flute) instruments, the flow of air over the mouth of the instrument forms a flow-controlled valve. Some of the air-stream flows into the instrument's mouth, leading to an increase in internal pressure, while some of the air-stream flows across the top of the mouth—through a Bernoulli effect this reduces the pressure at the mouth, drawing air out of the mouth and leading to a decrease in internal pressure. When the pressure inside the chamber decreases, more of the air-stream will enter the mouth, and less will flow across the top of the mouth. A pulse of high pressure arriving at the mouth will direct more air across the top of the mouth; this will decrease the internal pressure, and send a low-pressure pulse back down the tube. A pulse of low pressure arriving at the mouth will draw more air into the mouth; this will increase the internal pressure, and send a high-pressure pulse back down the tube. Standing waves inside the tube will be multiples of a half-wavelength, with pressure nodes at both ends. The air-stream across the mouth vibrates at a rate determined by the resonator.

To a rough approximation, a tube of about 40 cm. will exhibit resonances near the following points:

• For a reed or lip-reed instrument: 220 Hz (A3), 660 Hz (E5), 1100 Hz (C#6).
• For an air-reed instrument: 440 Hz (A4), 880 Hz (A5), 1320 Hz (E6).

In practice, however, obtaining a range of musically useful tones from a wind instrument depends to a great extent on careful instrument design, and playing technique.