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Electric Bell converts electrical energy into mechanical (vibrational) energy and then that into sound energy.

Loud speaker converts electrical energy into mechanical vibrational energy and then into sound energy.

Radio converts electromagnetic energy (in waves from atmosphere) into electrical energy in the circuit, which is converted in to mechanical vibrations (of diaphragm)  and then into sound energy.

Micro phone converts sound energy into vibrations of diaphragm (mechanical energy) and then that into electromagnetic energy (induced emf and current). 

Microphone does opposite function to that of the other three devices.
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Micro phone is the odd one of the following