1. A proximity detector for use in a mobile telephone having at least a microphone and a loudspeaker operatively connected to a signal processor, the proximity detector comprising: data processing and control means including means for controlling the signal processor for activating the loudspeaker to reproduce an acoustic control signal; correlating means for correlating a control signal received directly by the microphone and a control signal reflected from a user of the telephone and then received by the microphone for determining a distance between the telephone and the user based on a known direct distance between the microphone and the loudspeaker, wherein a difference between a time of receipt of the directly received control signal and a time of receipt of the reflected control signal corresponds to the determined distance between the telephone and the user; and signal level control means for controlling the signal processor for varying the signal level of an audible signal reproduced by the loudspeaker depending on the determined distance.