Abstract: The invention provides a circuit for preventing howling responsive to an acoustic feedback between a microphone and a loudspeaker in a hands free telephone or the like. Both a transmit path and a receive path include a variable loss circuit and a level detector circuit which compares the level of a signal picked-up by a microphone with a level of a signal received from another party. Responsive to the comparison, a normal gain is set in one of the communication paths at a level which is higher than the signal level in the path. A loss is inserted in the other communication path. A control circuit compares outputs of the level detector circuits to control the losses of the variable loss circuits. To measure a loss due to the acoustic coupling and to measure a reverberation time, a manual switch is operated to drive a microprocessor type control circuit responsive to an in-band noise which produces a voice-band noise signal from the loudspeaker.