An access control system , comprising: a radio interface configured to communicate with a portable user device , an electroacoustic transducer , and a control circuit coupled to the radio interface and to the electroacoustic transducer , characterized in that the control circuit is configured to trigger transmission of an acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device in response to a signal (42; 53; 54) received from the portable user device at the radio interface ; and selectively activate an actuator in an access grant procedure based on a propagation time of the acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device , wherein the control circuit is configured to selectively activate the actuator if all of the following conditions are fulfilled: the propagation time indicates that the portable user device is located within a pre-determined range from the access control system , the portable user device is authenticated to be entitled to access, and an amplitude of the acoustic signal (6; 43; 55; 58) indicates that the portable user device is located outside of the area to which access is requested. a radio interface configured to communicate with a portable user device , an electroacoustic transducer , and a control circuit coupled to the radio interface and to the electroacoustic transducer , characterized in that the control circuit is configured to trigger transmission of an acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device in response to a signal (42; 53; 54) received from the portable user device at the radio interface ; and selectively activate an actuator in an access grant procedure based on a propagation time of the acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device , wherein the control circuit is configured to selectively activate the actuator if all of the following conditions are fulfilled: the propagation time indicates that the portable user device is located within a pre-determined range from the access control system , the portable user device is authenticated to be entitled to access, and an amplitude of the acoustic signal (6; 43; 55; 58) indicates that the portable user device is located outside of the area to which access is requested. trigger transmission of an acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device in response to a signal (42; 53; 54) received from the portable user device at the radio interface ; and selectively activate an actuator in an access grant procedure based on a propagation time of the acoustic signal (6; 43; 55; 58) between the electroacoustic transducer and the portable user device , wherein the control circuit is configured to selectively activate the actuator if all of the following conditions are fulfilled: the propagation time indicates that the portable user device is located within a pre-determined range from the access control system , the portable user device is authenticated to be entitled to access, and an amplitude of the acoustic signal (6; 43; 55; 58) indicates that the portable user device is located outside of the area to which access is requested. the propagation time indicates that the portable user device is located within a pre-determined range from the access control system , the portable user device is authenticated to be entitled to access, and an amplitude of the acoustic signal (6; 43; 55; 58) indicates that the portable user device is located outside of the area to which access is requested. <EOS>