Deep Down Under the Elbe
The New Elbe Tunnel in Hamburg, which opened for the first time in 1975, isn’t really new anymore. Nevertheless, with a total length of 3,325 metres it numbers amongst the longest underwater road tunnels in the world and, with a daily capacity of more than 120,000 vehicles, is also one of the most important ones. Equipped with a NEXUS-based announcement system, it is also one of the safest and most up-to-date
When driving to the centre of Hamburg from the South, your way will eventually cross the Elbe river – either over or under it. The northbound A7 autobahn, Hamburg’s busy artery, guides the traffic through a four-bore tunnel under the Hanseatic city’s great waterway. Statistics say that a car breaks down or an accident happens every three days, a perfectly normal value for German autobahns. However, a defective vehicle inside a tunnel tube can quickly become a dangerous obstacle. Therefore, stringent safety regulations are in force for autobahn tunnels to ensure rapid evacuation when necessary. Announcement systems are critical components in case of an emergency, so strict guidelines apply. Therefore, using a NEXUS in the New Elbe Tunnel was an obvious choice!
A Difficult Acoustic Environment
A CAS-300 announcement system, based on a NEXUS audio network, has been in use in tube 2 of the Elbe Tunnel since August 2010. It is used for loudspeaker announcements which pass instructions to the drivers in the tunnel in case of emergency.
Such systems usually battle with the tunnel acoustics. If the speakers are lined up along the tube there are unwanted acoustic overlaps which result in echoes. In order to mitigate the consequent lack of intelligibility, the previous announcement system used a total of around 800 speakers installed along the tunnel at a distance of 3.75 metres apart. This ensured that wherever a motorist was inside the tunnel, he or she could hear direct sound since there was always a loudspeaker a speaker nearby. Nevertheless, the large number of speakers stimulated the reverb sound field to such an extent that intelligibility was still extremely poor.
From Weakness Comes Strength
The new approach with the NEXUS announcement system takes advantage of the tunnel acoustics rather than fighting them. A characteristic of this environment is that sound propagation inside the tunnel can be made to behave like a single coherent acoustic wave. The sound-wave fronts do not overlap each other chaotically but in a temporally-synchronised manner. Technically, this is achieved by using the so-called Synchronised Longitudinal Announcement Speaker System (SLASS) concept.
With just 46 speakers, corresponding to one every 64 metres, the new system generates such a wave which propagates along the tunnel tube. After the first, each individual loudspeaker signal is delayed so that the subsequent speaker’s signal is phased precisely with the signal arriving from the previous one. In consequence of this principle the system produces an overall delay of around ten seconds over the entire tunnel length, a delay which is immaterial for this purpose. The loudspeaker of choice was an innovative pressure-zone horn system that uses the tunnel roof as a quasi horn extension. Not least because of their excellent rear attenuation, the speakers excite a strongly bunched wave in exactly the desired direction of sound. Moreover, they are extremely thin – a considerable benefit, given that headroom is at a premium in a tunnel.
Clear and Concise
The tunnel complex includes three ventilation buildings at the northern and southern ends and in the middle together with a tunnel-operations centre. Each location houses a NEXUS Base Device where DSP cards are installed. These are used to delay the signals correctly and to route them to individual power-amplifier channels. To ensure operational reliability the NEXUS includes redundant optical lines and power supplies and constant system-component monitoring. Other reliability features are implemented using CAS-300 components: amplifier monitoring, including backup-amplifiers, supervised announcement playout; and full monitoring of speaker loops and all audio signals back to the microphone capsule. The CAS-300 also supports subscriberunits that can be used for making announcements if necessary. The outcome is convincing: The system achieves a degree of intelligibility unparalleled in tunnel environments.
Oliver Reimann from IfB consulting, a German engineering firm, planned and designed the announcement system installed in bore 2 of the New Elbe Tunnel. In February 2011, Reimann, with his newly established planning and consulting company SPRECH-FABRIK ENGINEERING, were assigned to plan the conversion of the remaining tunnel bores which will follow the successful bore 2 design.
The photo of the Elbe tunnel portal was kindly provided by the company Cegelec. Many thanks!
Cegelec is a specialist in integrating comprehensive technology in tunnels. This includes traffic and operating systems as well as power supply and ventilation / smoke extraction and safety systems. Some of the most important projects in recent years are the Elbe Tunnel (tunnel 4), Hamburg, the Fleet Moor Tunnel, Hamburg, and the Warnow crossing, Rostock. Cegelec also is the general contractor in a major project to upgrade the Elbe tunnel in Hamburg. By 2011, tunnel tubes 1 to 3 will be upgraded with new infrastructure incorporating the latest technology.
In the annual European ADAC tests, several tunnels equipped by Cegelec were rated as »very good«. The »best tunnel in Germany 2007« rated by the ADAC was the tunnel at the Tiergarten in Berlin, which was also equipped by Cegelec.