A security apparatus and a method for determining the distance between a computer (72) and a token (70) carried by a user (52) for allowing the user (52) to access the computer (72). The computer (72) is connected to a first transceiver (16) having at least one antenna for emitting and detecting a radio frequency (RF) signal, and to a microphone (76) for detecting an audio signal (24). The computer (72) is also connected to a counter (26) for measuring a time interval (28) between a transmitted RF signal (20) and audio signal (24) from the token (70). The token (70) is connected to a second transceiver (30) having at least one antenna for emitting and detecting an RF signal (20) and a loudspeaker (74) for emitting an audio signal (24). The method is characterized by comparing the measured time interval (28) to a predetermined time range (38) and enabling the computer (72) in response to the time interval (28) being within the predetermined time range (38) such that the user (52) may have access to the enabled computer (72) and disabling the computer (72) in response to the time interval (28) being outside the predetermined time range (38).