AST Magazine February 2018 final-feb-2018 | Page 16

Volume 20
Tunnel Voice Alert & Radio Re-broadcast
• ATI installed voice alert and radio re-broadcast systems at the
Ohio River Bridge East End Crossing Tunnel.
These systems use IP communication to provide safety,
evacuation messaging, and emergency services radio
re-broadcast systems in the event of an emergency.
February 2018 Edition
A tunnel environment is highly susceptible to ambient noise and
echo, which can significantly affect the performance of a PA system.
Ideally, PA speakers should have a wide bandwidth of 125 Hz to 8
kHz sound wave frequency and sufficient speaker power to provide
sound levels 15 dB above the complex tunnel background noise.
This performance level is considered to be the minimum required
for adequate speech transmission according to NFPA 72.
During emergencies, this system will initially allow for
intelligible live PA. The speakers project sound longitudinally down the tunnel. Each
speaker can provide up to 400 watts of acoustic power in order to
overcome the traffic and ventilation noises.
Achieving reasonable and effective speech intelligibility in
a tunnel presents a clear challenge to the design of an ade-
quate PA system. Speakers are positioned along the centerline of the tunnel and
aimed opposite to the traffic direction.
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