Achieving crisp, clear speech reproduction in a tunnel is vital for safety, but it can also present a unique audio challenge. Traffic noise and fans create high levels of background noise, while the tunnel’s shape and hard surfaces result in very high reverberation times. These factors make it impossible to understand a conventional Public Address (PA) system.

In the event of a fire, collision or other emergency, the tunnel PA system performs a key role in self-evacuation and is a vital communication link between the tunnel operators and passengers. If passengers are unable to understand instructions during an emergency, it can result in a dangerous situation. In less drastic scenarios, an effective PA allows tunnel operators to communicate with tunnel users to prevent minor incidents from escalating into more precarious situations.

Keeping passengers safe and on the move are key priorities for tunnel managers and operators. Optimally, a PA system is able to deliver clear, understandable announcements the first time they’re made and maintain a reliable avenue of communication with travelers.

Tunnels, however, are complex acoustic environments and pose a significant challenge for sound system designers. PA systems built for other kinds of indoor and outdoor spaces don’t easily adapt to the unique acoustics of tunnels.

In the late 1990s, HARMAN Professional Solutions embarked on a research project to design a loudspeaker that could meet the challenges of tunnel environments. The engineering team at Duran Audio, a HARMAN brand based in the Netherlands, was tasked with developing a solution that would achieve superior speech intelligibility and sound quality, high directivity, high power and low distortion, while controlling installation and maintenance costs.

Ultimately, Duran Audio deployed its patented Asymmetric Boundary Flare (ABF), which is essentially half of a loudspeaker horn. The technology was developed specifically for use in tunnels and is based on the principle of acoustical mirroring. In effect, mirroring creates the other half of the horn by reflecting sound off of a surface (in this case, the tunnel ceiling).

The AYXS® ABF-260 Tunnel Horn is mounted on the ceiling and aimed down the length of the tunnel between lanes of traffic. Using the ceiling as a waveguide helps direct the sound in a controlled manner, while reducing lateral (side-to-side) reflections. The result is the delivery of clear, intelligible announcements for better tunnel safety.

Since it’s completion, the system has been installed in tunnel sites throughout Europe, the United Kingdom, Australia and the U.S., enabling a safer tunnel experience for millions of travelers each year.

Do you have experience with installing PA systems in acoustically challenging environments? Share your insights, and tips and tricks in the comments.

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