STI Measurement and Evaluation of Directional Loudspeakers for Emergency Broadcasting in Noise Barrier Tunnels

Noise barrier tunnels are increasingly used to mitigate transportation noise in densely populated areas; however, their enclosed or semi-enclosed acoustic environments can create challenges for emergency evacuation broadcasting during fire incidents. This study investigated the acoustic performance of directional loudspeakers for emergency broadcasting systems through measurements in a semi-anechoic chamber and an actual tunnel. Two directional loudspeakers with diameters of 5 and 15 inches were evaluated under different installation heights, source–receiver distances, and directivity angles. Sound pressure level (SPL) and the Speech Transmission Index (STI) were used as the primary performance indicators. In the semi-anechoic chamber, the 15-inch loudspeaker generally achieved the target SPL of 80 dB(A) or higher, whereas the 5-inch loudspeaker showed greater variation with distance and directivity angle. In the actual tunnel, excessive reverberation substantially reduced speech intelligibility. Reverberation times ranged from 12.4 s at 125 Hz to 2.0 s at 8 kHz, and the maximum STI was 0.49, below the target criterion of 0.60. These results indicate that sufficient SPL alone does not ensure effective emergency communication in highly reverberant tunnels. Therefore, emergency broadcasting design should consider loudspeaker placement and reverberation control through appropriate acoustic treatments.