Operation
The following illustrations depict the type of reed typical of harmonicas, pitch pipes, accordions and reed organs as it goes through a cycle of vibration. One side of the reed frame is omitted from the images for clarity; in reality, the frame completely encloses the reed. Airflow over one side of the reed creates an area of low pressure on that side (see the Bernoulli's principle article for details), causing the reed to flex towards the low-pressure side. The reed frame is constructed so that the flexing of the reed obstructs the airflow, which reduces or eliminates the low pressure area and allows the reed to flex back.
| A reed is fixed by one end in a close-fitting frame. The loose end has a slight rising bend. | |
| Air depression is applied under the reed; the reed prevents air flow, except for a small, high-velocity flow at the tip. | |
| The reed is sucked through the opening, allowing the air to pass. | |
| The elasticity of the reed forces it back through the frame. |
Each time the reed passes through the frame, it interrupts air flow. These rapid, periodic interruptions of the air flow create the audible vibrations perceived by the listener.
In a free-reed instrument, it is the physical characteristics of the reed itself, such as mass, length, cross-sectional area, and stiffness, which primarily determine the pitch (frequency) of the musical note produced. Of secondary importance to the pitch are the physical dimensions of the chamber in which the reed is fitted, and of the air flow.
Read more about this topic: Free Reed Aerophone
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