Public address systems in road tunnels are an essential component of safety infrastructure. Proper planning and execution ensure speech intelligibility in acoustically extreme environments. Their design must meet both regulatory requirements and very specific physical and operational constraints.
Acoustic environment and technical requirements
Road tunnels present a series of acoustic challenges that make them one of the most complex environments for voice reproduction. Prolonged reverberation caused by hard surfaces such as concrete or asphalt, combined with constant noise from traffic and ventilation systems, hinders clear message transmission. In emergency situations, when fans operate at full power to evacuate smoke, noise levels can reach up to 94 dB(A), requiring public address systems to emit messages at sound pressure levels above 100 dB to maintain a proper signal-to-noise ratio.
Message intelligibility, a critical factor in any public address system, is typically measured using the Speech Transmission Index (STI). The minimum recommended value in tunnels is 0.45, although general voice evacuation standards usually require 0.50. Achieving these values in tunnels is especially difficult due to the penalty introduced by IEC 60268-16 when sound pressure levels exceed 80 dB(A). Therefore, system design must optimize not only speaker power and directivity but also equalization, which can improve STI by up to 0.1 points when properly adjusted to background noise.
learn more: physical acoustics and intelligibility
Applicable regulations
From a regulatory standpoint, in Spain, Royal Decree 635/2006 sets the minimum safety requirements for road tunnels. This decree states that urban tunnels over 200 meters and interurban tunnels over 500 meters must include public address systems. If these systems are not integrated with the fire detection and alarm system, they must comply with UNE-EN 50849, which governs emergency sound systems. If they are part of the fire evacuation system, they must comply with UNE EN-54 standards, which include specific requirements for control equipment (EN 54-16), loudspeakers (EN 54-24), and power supplies (EN 54-4).
Acoustic delay and synchronization
One of the most relevant technical aspects in the design of these systems is acoustic delay control. Due to the long distances between loudspeakers and between speakers and listeners, sound may arrive with time offsets that negatively affect intelligibility. To correct this, digital signal processing (DSP) systems are used to apply specific delays to each speaker, synchronizing sound arrival at every point in the tunnel. This technique requires each speaker to be powered by an independent amplification channel and all speakers to be oriented in the same direction, allowing a coherent wavefront to be generated along the tunnel.
Sound points and acoustic solutions
Regarding sound points, the traditional solution has been the use of horn loudspeakers, capable of generating high sound pressure levels and covering long distances. These loudspeakers, when regulatory compliance is required, must be certified according to EN 54-24. However, in well-designed projects, it is recommended to use loudspeakers specifically designed for tunnel use, with high directivity and low distortion. These help reduce wall reflections and significantly improve the ratio between direct and reverberant sound, resulting in greater intelligibility.
In short, public address systems in road tunnels cannot be treated as conventional installations. They require a rigorous technical approach based on precise acoustic simulations (preferably using ray-tracing software), careful component selection, and functional integration with the tunnel’s safety systems. Only then can emergency messages be delivered clearly, intelligibly, and effectively to all tunnel users.
horn loudspeaker for tunnels AET-100
Intercom systems in evacuation zones
In addition to public address systems, many tunnels include intercom systems at strategic points such as service galleries, evacuation zones, and shelters. These devices enable direct communication between maintenance personnel and the control center, and provide an immediate contact option for people evacuating the tunnel in case of fire or other emergencies. Their presence is especially useful in situations of panic or disorientation, as they facilitate personalized assistance and coordination of rescue teams. To ensure effectiveness, these systems must be resistant to extreme conditions and integrated into the tunnel’s safety architecture.
Some examples of tunnels with OPTIMUS equipment
OPTIMAX platform
The Northwest motorway crosses the Sierra de Guadarrama through three tunnels excavated between 1963 and 2007, the longest of which reaches 3.34 km. Managing all three as a single system, combined with the length of the infrastructure and the dense traffic in peak periods, set a high bar in both power output and system architecture.
At that scale, time synchronisation between emission zones is the central problem: keeping a coherent wavefront across more than three kilometres calls for a heavily distributed architecture, with amplifiers placed along the route and DSP delays calculated for each point. The configuration installed (17 IF-8P4/0E modules distributed across the system) follows exactly that logic: the goal isn't to centralise power but to deliver it where it's needed, in sync with the rest of the system.
Installed system:
- OPTIMAX3, 3 × IF-8P4 + 17 × IF-8P4/0E, 28.520 W, DC-800ETH
The Northwest motorway crosses the Sierra de Guadarrama through three tunnels excavated between 1963 and 2007, the longest of which reaches 3.34 km. Managing all three as a single system, combined with the length of the infrastructure and the dense traffic in peak periods, set a high bar in both power output and system architecture.
At that scale, time synchronisation between emission zones is the central problem: keeping a coherent wavefront across more than three kilometres calls for a heavily distributed architecture, with amplifiers placed along the route and DSP delays calculated for each point. The configuration installed (17 IF-8P4/0E modules distributed across the system) follows exactly that logic: the goal isn't to centralise power but to deliver it where it's needed, in sync with the rest of the system.
Installed system:
- OPTIMAX3, 5 × IF-8P4, output of 4,520 W and 2,700 W
- 15 × AET-100, 2 × PRO-150B, 1 × P08-EN54
The project combines the demands of a high-traffic infrastructure with the operational constraints of working inside a tunnel still in active service: installation has to be coordinated with phased partial closures without disrupting overall operation.
A 1,300-metre urban tunnel connecting the Baix Guinardó and Carmel neighbourhoods with the Ronda de Dalt ring road, with heavy traffic and continuous use. The acoustic peculiarity of this tunnel is its urban character: a relatively narrow cross-section and medium length, in which reverberation builds up quickly and traffic noise is constant even outside emergencies.
The design of the installation reflects that complexity: distributing loudspeakers in three groups of differing density (46, 30, and 15 AC-630T units) points to a non-uniform acoustic design, tailored to the conditions of each section of the route. The OPTIMAX3 architecture with distributed amplification allows those zones to be managed independently, synchronising announcements and adjusting emission levels to suit each sector.
Installed system:
- OPTIMAX3, 2 × (IF-8P4 + IF-8P4/0E), 4.520 W
- 8 × SP-920EN, 46 + 30 + 15 × AC-630T, 9 × SP-910DEN
Compact System
The Piedrafita pass concentrates several tunnels along the A-6 corridor between kilometres 407 and 464, on the section of the dual carriageway that crosses the boundary between Lugo and León. The La Escrita, Trabadelo, and Villafranca tunnels are located within less than 10 kilometres of one another, with traffic volumes above 2,000 vehicles a day. It's a particularly demanding stretch, given the density of infrastructure in a single corridor and the climate of the Galician mountains.
The distinctive feature of this project is the coordination of four separate installations within a single technical framework. OPTIMUS is also taking part in the modernisation currently underway at these tunnels, which includes a full upgrade of the safety systems under Spain's Recovery Plan.
Installed system:
- 4 × Compact: 3,000 W + 2,000 W + 3,000 W + 3,000 W
- 4 × SP-920EN, 82 × AC-930EN, EN 54-certified
The Pilar de la Horadada tunnel is 794 metres long, located at km 772 of the AP-7, on the Crevillente–Cartagena section, next to the town of Pilar de la Horadada. It has been in service since 2001 and carries around 28,000 vehicles a day. It's a cut-and-cover tunnel with one bore per carriageway, which means managing two structures with similar acoustic conditions but completely separate installations.
Unlike a tunnel cut through rock, a cut-and-cover tunnel has a more regular cross-section and more consistent construction materials, which makes acoustic behaviour easier to predict, though it doesn't remove either the reverberation challenge or the traffic noise. The chosen solution, with 64 AC-730T column loudspeakers distributed along the route, reflects the need to maintain uniform coverage in each bore with an independent, supervised system for each carriageway.
Installed system:
- Compact, 3.000 W, DC-700ETH
- 64 × AC-730T, 2 × AC-615T
A roughly 460-metre cut-and-cover tunnel built on the road approach to Valencia airport along the N-220, where it passes through the town of Manises. It operates in an environment of constant traffic with one notable feature: the dual nature of the flow, mixing urban mobility with airport-bound traffic, which means a level of continuous, sustained use that isn't typical of conventional road tunnels.
The 4,000 W installed across 460 metres reflects an acoustically demanding setting with a permanently elevated background noise floor. The combination of AC-730T loudspeakers with SP-30BR units gives a design with differentiated zones, adapting coverage to the varying acoustic conditions along the route. The DC-700ETH/T controller, integratable via SIP, makes coordination with the airport's management systems straightforward.
Installed system:
- Compact, 4.000 W, DC-700ETH/T
- 47 × AC-730T, 13 × SP-30BR
Compact LITE
An international project on a road infrastructure in Mexico City. The solution had to meet the communication and emergency announcement needs of a road tunnel while adapting to the local regulatory and operational context, which doesn't replicate European EN 54 standards but has its own technical and certification requirements.
Choosing COMPACT LITE as the central platform fits that reality: a stand-alone system with integrated PA/VA matrix, built-in battery charger, and IP connectivity, covering the needs of a medium-scale tunnel without requiring the heavily distributed architecture used in longer infrastructures. Its ability to integrate with external systems via standard protocols also made it easier to adapt to the project's local control environment.
Installed system:
- Compact LITE + DC-700ETH
- 60 × AC-850T