Transducers are found in headphones, loudspeakers and microphones.
A transducer – also called a driver – converts acoustic signals (sound waves) into electrical signals (electrical voltage) or vice versa. Dynamic transducers are most commonly found in headphones, but there are other drivers, such as balanced-armature or electrostatic transducers. For example, a microphone converts sound pressure into electrical voltage, while a loudspeaker or headphones work the other way around.
→ see also: “These are the different types of sound transducers” and “What are the advantages of dynamic transducers?”