11.4.2 During Operation
Internal noise levels
While the tunnel environment (hard surfaces, fan noise, etc.) makes it difficult to attenuate noise, emergency telephone systems and other communication media need to be intelligible over this background noise. A target of 85 dBA 1.5 m above the tunnel road level may be the acceptable noise level, with the local requirements obtained for each jurisdiction. Where possible, communication devices should be placed in attenuating devices to further shield users from this background noise (telephone cabinets, refuges, etc.).
Ventilation equipment can be a major source of noise in tunnels and therefore limitation of noise emission is an important factor in the choice of fans. Acoustic treatment by means of inlet and outlet silencers and/or casings with sound absorbent lining may be required to reduce the amount of fan noise transmitted to the outside environment. The external noise levels of the ventilation plant should not exceed acceptable standards for nearby noise sensitive development. A similar analysis of the noise generated by other equipment is necessary.
External noise levels
The control of noise to the outside environment will depend on individual circumstances (e.g. ground conditions, proximity of noise sensitive receptors and mechanical and electrical configuration) and the area background noise levels during tunnel operation. It is possible that local environmental conditions can create external noise issues. For example, the existence of cool air in a valley or other atmospheric conditions (e.g. an inversion) can regularly create an unusual acoustic environment. This may result in noise being transmitted over a long distance, for example, to a receptor sitting on the side of a hill some distance away or alternatively make noise a non-issue because the local atmospheric conditions are not conducive to the transmission of sound energy.
In the case of an external ventilation building, it will be appropriate to specify the maximum installed noise and vibration levels at the nearest sensitive receptor.