Tunnels are definitely among the most challenging acoustic environments. Yet many countries have legislation that requires a PA system to be present in tunnels, for service announcements as well as for mass evacuation. Meeting speech intelligibility requirements is never easy in traffic tunnels. In general, this can only be achieved with PA systems built from high quality components, which are also tuned for optimal performance.
Problems typically encountered in traffic tunnels are long travel times along the length of the tunnel, high levels of background noise (which vary with the time of day), extremely long reverberation times due to the surface properties of walls and ceilings, and quite often discrete echoes due to ill-placed reflective surfaces.
For a PA system, this means (at least) the following:
- One needs to use efficient, high-power loudspeakers that are capable of withstanding the harsh tunnel environment. In practice, the frequency transfer and non-linear distortion characteristics of such loudspeakers are far from ideal. This implies that the use of EQs is an absolute must – preferably individual ones per loudspeaker.
- The use of delay lines to compensate for travel time difference is inevitable
- In practice, the PA system must adapt to the level of ambient noise – the difference in background noise level between a blocked tunnel (in which traffic is standing still) and a tunnel in which traffic is driving at the usual speeds is considerably.
The Bedrock DSP500 series is capable of all signal processing needs described above. It is easily customized to be controlled remotely by control systems used in tunnels (PLCs or otherwise). Even more power comes with Bedrock Optimizer. Bedrock optimizer uses the hardware platform of the DSP500 series, but features embedded software for optimizing speech intelligibility and logging the STI. This means that the PA system will always function optimally, and that it can be proven that the system meets its requirements – not just at install time, but at any moment.