Free reed

A free reed aerophone is a musical instrument that produces sound as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows.

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.

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 generally the physical characteristics of the reed itself, such as mass, length, cross-sectional area, and stiffness, which 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. As an exception, the pitch of the Chinese bawu and hulusi are determined by fingering recorder-like tone holes along the instrument body.

Various free reed instruments appear to have been invented since antiquity, but were unknown in the West until the early modern period. The most likely precursor to free reed aerophones is the Jew's harp, an instrument known to many cultures throughout the world, and by many names (e.g., k'uang in ancient China). In this instrument, the main sound producer is the vibrating reed tongue itself, rather than the air flow.

...
Wikipedia