Design and Optimisation Of Voice Alarm Systems for Underground Stations

Voice Alarm systems (VA) are an essential part of subsurface underground station emergency and evacuation systems. Their main purpose is to assist in the management of emergency situations and evacuation procedures by providing key verbal instructions to the occupants. However these life-critical systems will be ineffective if the messages broadcast are unintelligible. Unfortunately, in most London underground subsurface areas the announcements broadcast by the VA system are not adequately intelligible and often do not reach a minimum specified performance target. The performance of VA relating to its electro-acoustic characteristics is relatively complex and depends on multiple interrelated factors and operational constraints. Underground stations present complex geometrical and architectural features which severely challenge the achievement of satisfactory performance. Despite the importance of VA system, there are few works in the literature providing practical and applicable design knowledge in the context of real world underground spaces. Moreover contractual performance requirements are not suitably laid out and this can lead to ineffective designs. This research aims to provide practical design knowledge and understanding for the improvement of VA speech intelligibility performance in underground spaces. Research results were derived from measurements and designs undertaken for real scenarios. A specific knowledge base is provided on the acoustics of underground spaces, speech intelligibility and VA systems. A critical review of relevant research and performance specifications and standards is undertaken and a new performance design parameter is proposed. An empirical prediction model tool based on a large pool of measured survey data is developed for the prediction of the Speech Transmission Index of VA on platforms. A validating and comparative study is undertaken for two widely used commercial acoustic simulation programs to assess their suitability as design tools for VA systems on platforms, CATT-Acoustic and Odeon. The impact on VA performance of design variables are investigated using a computer simulation of a representative platform. A novel acoustic treatment design concept is proposed. The Yang quasi diffuse sound field theory for platforms is verified and derived knowledge expanded. Practical design recommendations are provided as well as suggestion for further work.