Directivity of a dipole
Considering a system of two monopol sources spaced at a distance d and generating volume velocities
and
. The size of the sources and the spacing between them are assumed to be small comparing to the wavelength.
Far field
It is shown that the pressure response in a dipole plane in far field is written :
where pm(r) is a pressure generated by a monopol of volume velocity
:
Near field
On the other hand in near field
, we have :
The expression is proportional to
in far field, but not in near field: this is what is called "proximity effect" (greater presence of low frequencies for a source close to the microphone).
