The Bernoulli effect provides that, when a gas such as air flows, its pressure drops. This is the basis for how aircraft wings work: The cross-sectional shape of the wing, called an aerofoil (or airfoil), forces air to follow a longer path over the top of the wing, thereby speeding it up and creating a net upward force called lift.
Figure B.8 illustrates the Bernoulli effect for the case
of a reservoir at constant pressure
(``mouth pressure'') driving
an acoustic tube. Any flow inside the ``mouth'' is neglected. Within the
acoustic channel, there is a flow with constant particle velocity
.
To conserve energy, the pressure within the acoustic channel must drop
down to
. That is, the flow kinetic energy subtracts
from the pressure kinetic energy within the channel.
For a more detailed derivation of the Bernoulli effect, see, e.g., [180]. Further discussion of its relevance in musical acoustics is given in [145,198].