Study on the Application of Time-delay Technique to Public Address System in a Tunnel

To improve the speech intelligibility (easiness of hearing) of public address system for emergency evacuation in highway tunnels, the authors proposed an application of successive time-delay technique to the system and performed experiment in an actual tunnel. In the experiment, five horn-type directional loudspeakers were set at an interval of 150 meters in the tunnel under construction and the time-delaying networks providing the delay-time estimated from the sound speed in the field was applied. From the loudspeakers, announcements for emergency evacuation for the subjective hearing test and a swept-sine signal for the measurements of impulse response were reproduced with/without the time-delay technique. During the experiment, the effect of the time-delay technique was judged subjectively in the tunnel and the reproduced announcements and the impulse responses were recorded using 6-channel recording system for the subjective hearing test performed later in acoustic laboratory. As the psycho-acoustical experiment in the laboratory, three kinds of subjective tests were performed; (1) the effectiveness of the application of the time-delay system on speech intelligibility, (2) the influence of the error of delay-time in the system caused by the change of the atmospheric conditions in the tunnel, and (3) the effect of speech rate of the emergency evacuation announcement. From the results of these experiments, it has been confirmed that the application of the successive time-delay technique is effective to improve speech intelligibility of the emergency evacuation announcement in road tunnels.