Acoustic design of voice alarm systems in an occupied underground platform

Voice Alarm systems (VA) in underground railway spaces are life critical communication systems used to instruct occupants in case of emergency or direct them to safety. The effectiveness of these systems depends on the performance quality of the entire electro-acoustic communication chain, which speech intelligibility is the main parameter. VA are designed with the assistance of field measurements and prediction computer models. Relevant standards and industry best practice require the design and performance assessment of VA systems in unoccupied spaces. This is the most practical state to undertake acoustic measurement and is generally considered acoustically the worst-case scenario. However, the influence of occupancy on the performance of the VA remains unknown to the designer. This paper investigates the impact that occupancy can have on the acoustic performance of an underground platform VA. The sound absorption coefficients of varying densities of standing people were determined and implemented in acoustic modelling software to predict the performance of a hypothetical VA on a real underground station platform. The results showed that for a representative level of ambient noise, speech intelligibility increased notably with occupancy density.