Voice Alarm for Road Tunnels

ASL Underground

Road tunnels are used by millions of people without incident every day. But on the rare occasions when something goes wrong, perhaps with a vehicle fire or collision, they can quickly become extremely hazardous and frightening environments, and one where you need to evacuate people as quickly as possible, or guide them to safe areas and keep them calm.

Mostly, tunnels are unmanned and therefore automatic or remote announcements are critical to ensure safety. In the event of a problem, fire or smoke will rapidly fill a tunnel so the evacuation needs to occur very quickly. This differs from rail tunnels, where the train is designed to keep going through an incident, so the sound system is a last resort.

As demonstrated in many other places used by the public, people tend to ignore a simple alarm but respond and react to a clear voice message telling them what to do.

All new and major refurbishment tunnel projects are installing voice alarm systems to address everyone inside, ensuring they remain calm and informed. These environments create unique challenges for system designers from a physical perspective, as well as conforming with all the relevant standards.

ASL Voice Alarm for Road Tunnels  first image

Challenging Environments  

Tunnels are possibly one of the worst acoustic environments in which to try and reproduce intelligible speech; full of hard surfaces and with curved roofs, they reflect sound in all directions. The more reflections, the more difficult it is to understand any announcements.

The tunnel environment is also quite loud. Substantial noise is generated by the traffic and ventilation fans, which must be considered along with the direction of air flow and traffic when trying to deliver high speech intelligibility.

To counter this, designers use high power and highly directional loudspeakers to send the sound along the tunnel, reducing how much the sound bounces off the sides, to deliver clearer speech. Additionally, to manage the acoustic energy considerations must be incorporated within the architecture, in the form of sound apportion and loudspeaker orientation.

On a system level, intelligibility can be improved by equalising out particular frequencies.

It is not always good to run the system at maximum power, as this adds to the reverb. Ambient noise control can be implemented to automatically monitor the background level and adjust the output accordingly to filter it out.

Another challenge is the long distances involved. Loudspeakers are spread apart along the tunnel but need to be time-aligned to each other to provide coherent sound as it travels along the tunnel.

However, there can be issues with latency variation between distributed hardware, which might interfere with local delay timings. To combat this, ASL use Dante-enabled technology, which ensures the near-zero latency that is essential for time alignment in tunnels.

Dealing with Distance  

In normal conditions, sound takes about 1ms to travel 1ft (30cm). For example, if there was a loudspeaker 50m away from another, the sound from the first loudspeaker would take 150ms to reach the second one. The listener 50m past the second speaker would hear a huge echo, making the speech difficult to understand. By adding a delay of 150ms to the second loudspeaker, the sound from both loudspeakers reaches the listener at the same time, and so will be much clearer. Each loudspeaker needs to be driven by a separate amplifier output to allow for this. The longer the tunnel, the longer the delay needs to be at the last speaker to time-align the message.  

Distance also means that to reduce speaker cable runs it’s beneficial to have control equipment and amplifiers distributed at different points along the tunnel, often far apart from each other and the main control room, requiring fibre connections which can cope with such distances. Wall-mounted options help to reduce the need for rack space, and can be efficiently fit into small service cupboards.

Today there are a number of high SPL loudspeakers on the market with high directivity specifically designed for use in tunnels. Using acoustic simulation software, it is possible to determine the most efficient horn design for optimum intelligibility in a tunnel. 
They are also built to withstand corrosive atmospheric contaminants, high moisture levels, extreme temperatures, and even to resist the high pressure caused by travelling vehicles.

Safe and Compliant  

As a life-safety critical environment, the system must be designed and installed as a voice alarm system following national guidelines and international product standards.

IIt is critical that any voice alarm remains operational, meaning that there is a focus on redundancy. The solution for larger tunnels is to distribute the amplification and control equipment across multiple locations, providing diverse redundancy should problems occur with one section. It is recommended that voice alarm loudspeakers are wired using an interleaved A-B configuration so that the entire area is not impacted by an individual fault. When combined with the fact that each loudspeaker is wired separately to its own amplifier channel, you have a system that is extremely robust.

Nevertheless, Voice Alarm systems need to be continuously monitored to ensure that the system is working and faults are easily identified. In tunnels, getting to some equipment locations can be a challenge, so the ability to relay information about any problem back to the control room is essential.

Voice Alarm equipment must be certified to EN 54 product standards to ensure consistency and reliability in performance, in turn assuring safety.

Tunnel systems need redundancy built in so they carry on working, even in the event of partial failure. EN 54 compliance includes options for redundancy for maximum resilience.

Read about a recent road tunnel project here

The ASL difference

ASL delivers technology for a variety of industries including tunnels. ASL Voice Alarm systems are designed to work together over long distances and powerful signal processing technology allows extremely long time alignment delays to be set and ASL’s system architecture can support large and complex sites with hundreds of audio controllers. All of this can be controlled and monitored using an ASL centralised control system or via an open API protocol, so users can always be confident if there is a critical situation.  
ASL has high-power amplifiers capable of driving high-power loudspeakers as well as the loudspeakers themselves being specifically designed for use in tunnels. Additionally, the systems have a modular design to allow the amplifier racks to be filled with just the right number of amplifiers to match the solution, improving the ease of integration and future scaling. As market leaders, ASL offer complete, turnkey solutions that even include a system build and test.
All systems are extremely low latency, ensuring that time alignment delays are accurate across the long distances presented by tunnels. Simple layer 3 IP connectivity is utilised to allow easy distribution over an existing or dedicated IP network. ASL systems support Dante networking, which ensures time synchronisation between distributed equipment.
ASL systems possess remote fault monitoring that can be programmed to function across multiple separate tunnels, allowing easy remote control. For consistency, ASL control systems are designed to monitor other sub systems, and API is provided to allow seamless integration into third party control systems.
ASL can offer a number of services to support the design and delivery of the Voice Alarm system ranging from initial estimates of types and quantities of speakers and amplifiers to full rack build and Factory Acceptance Testing in a facility in Lewes, UK. 
Since its introduction, ASL has ensured that all products that need to be are compliant with EN 54 industry standards. When working with ASL, clients can rest assured that the technology is functional, reliable and in accordance with guidelines.   

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