What is the difference between Coupler (Artificial Ear) and Baffle/Free field measurement?

The baffle measurement represents a special case of free field measurement. However, in this case, the sound is propagating through the air as a wave. In case of miniature speakers, the transition between mechanical movement and air pressure is poor, that’s why very high excursions are needed to produce high SPL (e.g. very rough example: 0.5mm of a typical mobile phone receiver result in ~80 dB @ 10 cm).

The coupler simulates the human ear (therefore it is also called “artificial ear”). Typical measurements at the coupler are done with a small sealed connection between speaker and microphone. This small geometry doesn’t allow for wave radiation at low frequencies. Instead, simply the whole air pressure inside the volume will change at once. Because in this case, the coupling between mechanical movement and air pressure is very good, so even small excursions produce high SPL (e.g. very rough example: 10µm of Ganymede results in ~110 dB).

For higher frequencies the tube-connection between the speaker and microphone will produce additional resonances, influencing the original performance of any speaker drastically.
Looking at the SPL curves in the coupler and baffle one can conclude: