Lets face it.  A road tunnel is a pretty horrible place to be.  They are very inhospitable, not just to you and me but to sound signals too.  The acoustic environment within road tunnels is generally very active.  Due to the internal surfaces of the tunnel, which are hard with very low sound absorption coefficients, sound reflections continue to resonate throughout the tunnel without much attenuation.  This results in extremely long reverberation times and very poor speech intelligibility.  Reverb times can vary from a couple of seconds through to approximately 10 seconds at some frequencies.

A further problem is the issue of background noise from traffic and ventilation fans.  Steady 60mph traffic can produce a sound level of up to 95dBA, while ventilation fans at full output can produce a sound level of 100dBA.

Road Tunnel Audio Systems

But why do we care about this and why do we need an emergency audio system in a road tunnel? Well, tunnels are extremely dangerous places.  Especially when fire is involved.  A fire in a tunnel can have a devastating  effect which has been demonstrated by incidents in the Mont Blanc and Gotthard tunnels which claimed 38 and 11 lives respectively while injuring hundreds more.  Similar to how sound is unable to escape the tunnel, the heat and fumes from even the smallest fire are unable to dissipate causing it to burn hotter and longer creating a fatal environment for anyone trapped within the tunnel.

So, an effective means of communication is essential in road tunnels.  Unfortunately there’s not much that can be done about the traffic noise or the physical makeup of a tunnel.  At least not without the addition of sound absorption material which is rarely a viable option.  So a PAVA solution has to be specially designed to make the best of the environment.

The best of a bad situation

There are a number of methods that are used to maximise performance in a tunnel environment.   To minimise multipath interference it is important that the output from each loudspeaker is time-aligned to the loudspeaker that precedes it.  So each loudspeaker has an audio delay applied to it so that the sound is emitted at the same time as late arriving sound from preceding loudspeakers.  This creates a single ‘wavefront’ that travels through the tunnel originating from the entry portal travelling right through to the exit portal.

The standard objective measure of speech intelligibility is STI (Speech Transmission Index). Which is a measure between 0 and 1 and is segregated into subjective groups of bad, poor, fair, good and excellent.  BS5839 pt 8 states that 0.5(fair) should be the minimum measurement although the minimum STI may be relaxed for highly reverberant areas (such as tunnels) and accepted by the interested parties.  Having said this, 0.45 or higher should be achievable in most situations.

Not all horns are the same

Another method is to use the correct type of speaker.  Horn loudspeakers are the only option as they are most suited to the high output requirements, though conventional horns are not the ideal choice.  Conventional horns are called ‘re-entrant’ horns and are designed to maximise output from the driver in a compact space.  Unfortunately these also have a limited frequency range, high signal distortion and erratic sound dispersion particularly at higher frequencies.  These issues are amplified in highly reverberant environments.

There are specialist tunnel horns which are directly driven.  They are called long-horns or asymmetric boundary horns.  Boundary horns in particular are based on the principle of acoustic mirroring where the ceiling of the tunnel acts as a large boundary plane giving it an acoustic centre very close to the surface of the tunnel.  This is compared to a conventional horn whose centre is away from the surface creating early reflections against the surface producing more reverberant sound, as well as phase differences at certain frequencies.

Redundancy

As with any voice alarm system, continuity of service is paramount and with tunnel systems there is a heavy focus on redundancy.  For most larger tunnels there will be amplification and control equipment located in multiple locations.  This is for two reasons. Firstly it provides diverse redundancy and can continue operation if something were to occur in one location. Secondly, the distances involved can be large and therefore the equipment is distributed to minimise cable losses.

The speakers in voice alarm systems are usually wired in an interleaved A-B configuration so that the failure of one amplifier or circuit does not affect a whole zone.  As mentioned above each loudspeaker will have an audio delay applied to it meaning that each loudspeaker must be wired separately to its own amplifier channel.  This in itself provides its own redundancy in that an amplifier or circuit failure will only affect a single loudspeaker.

Audio Distribution

As the system will be a distributed system with racks of equipment in multiple locations, an effective means of audio distribution is required.  It is unlikely that there will be dedicated cabling for this purpose.  The tunnels will be served by a resilient ethernet network that will provide connectivity between the rack locations and one or more traffic control centres.  Due to the sensitivity of audio delay the chosen audio protocol must provide time-aligned and low latency audio delivery to each location.  Historically, Cobranet has been a popular choice but more modern IP based protocols are employed for newer installations.

For larger tunnels especially there is a requirement for a traffic control centre to monitor and manage the operation of the tunnel.  The control centre will typically monitor multiple tunnels in a certain geographical area.  Each control centre will also have a back-up control point that will require all the controls to be replicated at a second location.  This requires comprehensive engineering to maintain interoperability with not only the VA but with other traffic systems such fire and SCADA systems.

Radio Interfacing

Another feature of emergency tunnel systems is the ability to broadcast messages via the motorist’s car radio.  To achieve radio broadcasts inside the tunnel, various FM channels are re-broadcast by the use of a ‘leaky feeder’ cable that runs the length of the tunnel.  The emergency VA system can interface with the radio system and take-over the audio that is broadcast.  This is known as a ‘Voice Break-In’ system or VBI system.  In the event of an emergency the messages are heard through the car stereo system, and ‘Emergency Broadcast’ will be displayed on the stereo screen (dependant on make of stereo of course).

Relevant Standards

As with all voice alarm systems the prevailing standard in the UK is BS5839 and the equipment must be certified to the complimentary EN54 product standard.  The Highways Agency design guidelines for tunnel construction BD 78/99, does not specifically address the need for a voice alarm system but does reference BS5839 within the sections regarding Fire Safety Engineering.  Furthermore, some tunnels over 500m long which are situated within the Trans-European Road Network (TERN) must conform to the minimum safety guidelines for road tunnels as defined by the EU directive 2004/54/EC.

We at B.L.Acoustics are well placed to deal with any requirement for emergency tunnel audio systems.  As acoustic consultants and system integrators we possess all the expertise in-house to design, install and maintain emergency tunnel broadcast systems.