Combination of a loudspeaking telephone set and a hand set for soft speaking

摘要

Loudspeaking telephone set having a hand set connected thereto for soft speaking. When the hand set is placed on the telephone set, the microphone and the loudspeaker being in the switched-on condition, there is a risk that oscillations or singing will occur due to acoustic feedback. In order to avoid this risk, the hand set is provided with an ultrasonic transmitting device and the telephone set is provided with an ultrasonic receiving device 13. The receiving device comprises means to determine the propagation time of the ultrasonic signals and means for affecting the gain of the loudspeaking channel of the telephone set in dependence on the propagation time.