The purpose of this paper was to develop an acoustic beacon signal for triangular surveying method in indoor location. The proposed method is based on the time of flight between the acoustic beacon and the single microphone. The plural amplitude modulated signals were applied as the beacon signal to reduce the effect of disturbing noises, and direct conversion receivers were used to cancel the difference of synchronization between the transmitter and the receiver. In this paper, we evaluated the number of modulated signals included in a beacon signal. The performance was evaluated by moving the microphone between a pair of acoustic beacons which were not synchronized to the receiver.