Devices which perform an input function are commonly called Sensors because they "sense" a physical change in some characteristic, for example heat or force and covert that into an electrical signal. Devices which perform an output function are generally called Actuators and are used to control some external device, for example movement. Both sensors and actuators are collectively known as Transducers because they are used to convert energy of one kind into energy of another kind, for example, a microphone (input device) converts sound waves into electrical signals for the amplifier to amplify, and a loudspeaker (output device) converts the electrical signals back into sound waves and an example of this is given below