Nowadays it is hard to imagine a train sta tion without a PA system. Providing that the announcements are audible and clear, the PA system remains the most effective means of public information with regard to train timetables, changes to platforms, delays or updates to your journey. It is not easy to design a system meeting all those criteria as it needs to be equipped with up-to-date components such as sound equalizers, delay lines, sound sensors, audio limiters and acoustic feedback eliminators.
Ensuring the adequate volume and clarity of public announcements is the key requirement for an effective PA sys tem. Audibility of information depends on the actual volume of the announcement as well as the distance to the surround ing noise levels. In areas with consistent noise levels, the required sound level of public announcements can be ensured by use of appropriate loudspeakers, their strength and highly effective amplifiers. It is recommended that the sound level of announcements should not be lower than 70 dB and at least 10 dB higher than the level of surrounding noise in that area.
However, such an approach will not be sufficient in areas where surrounding noise levels vary significantly. For instance, trains approaching the platform gen erate considerable noise – on open platforms the noise may reach 80 – 85 dB; on enclosed ones it may exceed 90 dB. Such noise will easily drown out public address announcements with their vol ume set to average conditions on the platform. On the other hand, setting the system volume permanently high so as to ensure announcements can be heard on the platform during train arrivals could be problematic. Such sound levels will be too high during non arrival of trains; this is likely to cause discomfort to pas sengers; penetrate other zones (e.g. other platforms) and significantly increase noise emission into the environment in case of open spaces. By using properly calibrated background noise sensors, Ambient sys tems provide the answer to this problem. The sensor readings allow regulating the volume levels of announcements in real time. Ambient systems have in-built abil ity to adjust the volume levels of public announcements within 20 dB range.
The other critical issue is the speech intelligibility of public announcements. Ensuring the high quality of messages remains the on-going quest for many designers of PA systems. The added difficulty is the definition of speech intelligibility itself as it is based on rather a subjective judgment. Over the years, speech intelligibility has been the subject of numerous studies aiming to formulate the objective definition of this parameter. Presently, Speech Transmission Index (STI) is commonly used to measure the quality of speech transmission with values varying from 0 (bad) to 1 (excellent). A table below shows STI values and corresponding subjective speech intelligibility. It is recommended that STI value should not be lower than 0.5.
In practice, achieving good speech intelli gibility is difficult as it depends on several factors for instance the size of a given area, acoustic conditions, quality of loudspeak ers, options to correct frequencies and application of delay lines.
With regard to scale and applied mate rials (in particular those enabling noise reverberation); modern architecture poses a real challenge to designers of PA systems. The reverberation within a large train sta tion can last even 5-6 seconds. Ensuring good speech intelligibility in such difficult conditions is possible through application of highly directional loudspeakers with minimal sound distortion; and support units such as digital frequency correction based on DSP processors.
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