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In the IoT environment, many terminal devices are deployed in unattended areas. If these devices are moved elsewhere by an attacker, the wrong environmental sensing values are obtained, which causes a major disaster. In this paper, we propose an ambient authentication mechanism based on audio to be used in multi-factor authentication by using the ambient sensors equipped with a smart phone. An ultrasonic signal that is not detectable by the human ear was transmitted by the authenticator, and the attenuated signal received by the user being authenticated was transmitted back to the authentication server. The transmitted and received decoded symbol sequences of the audio signal are used to calculated the bit error rate, which is used to measure the relative distance. Our proposed method can narrow the authentication distance to less than 0.5 meters, which can greatly improve the security of the authentication system.