Approximated expression of a radiation impedance
At low frequencies (when the wavelength is large compared to the radius of the piston - ka << 1), the radiation impedance can be written in general terms:
where
is the characteristic impedance in pressure/volume velocity analogy.
is density of the air (1,2 kg/m3 at 20 deg C), c is the speed of sound in air at rest (
at 20 deg C) and
is a section of a piston. k is a wave number
.
Resistance and radiation mass can thus be written:
,
where
depends on the radiation conditions (sphere, half-sphere, ...) and
on the mounting conditions of a piston.
