General principles
The most "useful" acoustic load of a loudspeaker is its radiation, that is to say a coupling between its membrane and ambient air, represented by an impedance Zar provided by the fluid to the membrane. This coupling leads to two phenomena:
At long distances (known as "far field"), pressure is a result of acoustic propagation between the loudspeaker and the point of listening. This propagation takes place only if a work is provided by the membrane to the fluid, so if it is compressed at the moment when the membrane is moving: reaction of the fluid must then be in phase with the vibration of the membrane , which corresponds to the real part of Zar .
At very short distances (known as "near field"),movement of the fluid follows that of the membrane. Inertial force then largely dominates, and the reaction on the fluid corresponds essentially to the mass of the fluid thus driven, called "radiation mass". This corresponds to the imaginary part of Zar .
