In order that a modulated wave is audible, it is necessary to change the nature of modulated wave. This is accomplished by a circuit called detector. A detector circuit performs the following two functions.

(1) It rectifies the modulated wave i.e. negative half of the modulated wave is eliminated,. a modulated wave has positive and negative halves exactly equal. Therefore, average current is zero and speaker can not respond. If the negative half of this modulated wave is eliminated, the average value of this wave will not be zero since the resultant pulses  are now all in one direction.

(2) It separates the audio signal from the carrier. The rectified  modulated wave contains the audio signal from the carrier. It is desired to recover the audio signal. this is achieved by a filter circuit which removes the carrier frequency and allows the audio signal to reach the load i.e. speaker.