a public address system having a microphone , an amplifier which amplifies the output from the microphone and a speaker assembly which receives the output from the amplifier and reproduces a sound , in which the microphone is provided with a transmitter generating an electromagnetic wave and the speaker assembly has two receiving coils disposed perpendicular to each other . the outputs from the receiving coils are used to control the level of an audio signal applied to the speaker assembly and thereby to avoid the occurrence of howling .