Evaluation of Speech Transmission in a Road Tunnel

Long reverberation times and high background noise levels associated with jet fans degrade speech transmission in road tunnels. Sound attenuation and reverberation time measurements in the Sydney Harbour Tunnel are used here to calibrate a mathematical model used to estimate speech transmission index STI. This study forms a basis for future work in the evaluation of acoustic design alternatives for public address systems required for communication to motorists who may become stranded within the tunnel during normal operations or tunnel emergencies.

If intelligibility in an existing tunnel is deemed to be too low, the means by which it may be improved are;

Minimise distortion in the loudspeakers by preventing overdriving of source announcement
Select loudspeakers with flatter more uniform response between 125Hz and 8kHz
Use highly directional loudspeakers or line arrays in closer spacing, to maximise the direct sound which reaches the listener
Align speakers so they project sound longitudinally down the tunnel
Move loudspeakers down closer to listener’s ear height, to increase the early unreflected energy reaching the listener

In a tunnel’s design stage it may be possible to make significant reductions to the reverberation time by appropriate selection of surfaces.