A free reed aerophone is a musical instrument where sound is produced as air passes a reed in a chamber, causing the reed to vibrate. The reed is fixed in a close-fitting frame, so that the vibrations cause rapid, periodic interruptions of the air flow. These air pulses initiate the audible vibrations perceived by the listener. In the related "fixed reed" instruments, a vibrating reed is used to set a column of air in vibration within the instrument.
In a free-reed instrument, it is the physical characteristics of the reed itself, such as length, cross-sectional area and stiffness, that determine the pitch (frequency) of the musical note produced. This contrasts with the situation in fixed reed instruments where the pitch is determined by the length of the column of air which vibrates in sympathy with the reed.
The oldest known free-reed instrument is the Chinese sheng, widely accepted as the musical ancestor of the harmonica and accordion. Other well-known free-reeds include the concertina, bandoneon, melodica, and harmonium.
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