
234 Classical Mechanics
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similar porous material, energy is dissipated because of the viscosity of the air. Second, the motion
of the air at the opening causes the sound to radiate into the medium outside, thus dissipating
acoustical energy. Similar to electric resistance
(Ohm’s law), we define the acous-
tical resistance R
a
as
X
a
=
3
/
, where p
3
is the pressure needed to derive the volume current
through the dissipative element.
If the sound waves of a given frequency approach the opening of the resonator at a steady state,
we have an alternating applied pressure difference that can be represented by ...