A Revelation – Intelligible Tunnel PA Systems

The Australian Acoustical Society Conference 2011 was held between the 2nd and 4th of November 2011 on the Gold Coast, Queensland. The conference theme was Breaking New Ground.

Papers were presented on areas of acoustics associated with major infrastructure, transportation, industry and mining. Other major topics for the conference included; underwater acoustics/marine bioacoustics, architectural and building acoustics, music acoustics, wind turbines and low-frequency noise.

Peter Ridley and David Spearritt presented a paper on behalf of Stacey Agnew Pty Ltd, 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.  Experimental measurements of sound attenuation and reverberation time in the Sydney Harbour Tunnel were used to calibrate a mathematical model used to estimate “speech transmission index”.  This study forms a basis for future work by Stacey Agnew in the evaluation of acoustic design of 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.