The whistle works by causing the smooth flow of air to be split by a narrow blade, sometimes called a
fipple, creating a
turbulent vortex which causes the air to vibrate. By attaching a
resonant chamber to the basic whistle, it may be tuned to a particular note and made louder. The length of the chamber typically defines the resonance
frequency. A whistle may also contain a small light ball, usually called the
pea, which rattles around inside, creating a
chaotic vibrato effect that intensifies the sound.
Japanese bird whistles use several small balls and are half filled with water in order to reproduce the sound of a
bird song.