Research On Sound Environment In Oversize Waiting Halls Of High-speed Railway Elevated Stations

For the high-speed railway station mega-projects newly completed in China, the waiting halls are usually oversize spaces in volumes of millions of cubic meters. So far the theoretical acoustic researches on and design experiences of oversize spaces are still quite limited, and more in-depth studies on sound field characteristics of the oversize space are much needed and represent one of the new research direction in the field of room acoustics.To explore the sound environment of an oversize waiting hall, studies on the following three aspects are made and some preliminary conclusions reached in this paper.1. Subjective investigations on sound environment were carried out and the sound pressure levels of environment noise measured in 8 waiting halls. The results indicate that there is a significant correlation between the passengers’ overall evaluation of a sound environment and the quietness of the space and the speech intelligibility of the public address system. An ideal sound environment, in addition to clear public addressing, requires a low noise level. Human activity and broadcast of public address system are regarded as the main noise sources in waiting halls. There is a highly positive correlation between the subjective evaluation of speech intelligibility of the public address system and(S/N)’, which is the sound level difference between broadcast noise and environmental noise. For the sound comfort of passengers, the(S/N)’ needs to be at a proper level.Besides, the subjective evaluation of passengers differed distinctively from that of staff, where the latter tended to give negative comments on sound environment.2. Sound field characteristics of oversize waiting halls have been studied through in-situ measurements and computer simulation. The results show that:(1) Where the sound field is stimulated by one single omni source, the sound pressure level(SPL) and sound transmission index(STI) decrease with source-receiver distance in different degrees, which are affected by acoustic treatment and space height. It may produce large error when Sabine formula and Eyring formula apply to reverberation time(RT) calculation of the oversize waiting halls. There is a great difference of early decay time(EDT) and reverberation time(RT) among receivers, and EDT correlates positively to the logarithm of source-receiver distance in near field. Even with highly effective sound absorption measures, it has been difficult to reduce RT to the level of a regular space. When the average sound absorption coefficient reaches 0.36, RT is still as high as 5.67 s. With gradually increasing sound absorption level, RT would be greatly reduced and speech intelligibility improved to a certain extent. Moreover, sound absorption treatment at ceiling would significantly improve speech intelligibility. With the same sound absorption treatment at ceiling, the lowered space height could effectively reduce RT. However, speech intelligibility could only be improved when the space is lowered to be less than 21 meters in height.(2) Where the sound field is stimulated by multiple sources, it is easier to realize the even distribution of SPL and RT with a distributed loudspeaker layout.A small number of distributed sound sources could improve speech intelligibility which would otherwise be affected by acoustic delay when loudspeakers share the same projection area. Compared to the sound field stimulated by a single omni source, the sound field stimulated by multiple sources is more diffused and closer to the actual situation. Therefore, it is preferred to use loudspeakers of public address system(PA) as the sound source for sound field measurement of waiting halls.3. The subjective evaluation test of speech intelligibility of waiting halls has been conducted, which involved both the field test in waiting halls and auralization test in the laboratory. As per the study on the relationship between the objective acoustic parameters(including D50, STI, RT and EDT) and the subjective evaluation scores, as well as the effect of signal-to-noise ratio(SN) and test materials on speech intelligibility, STI is most closely related to the speech intelligibility compared with other parameters and could be used as the objective index for evaluating speech intelligibility of waiting halls. The recommended STI value is 0.50 and the preferred value 0.55. EDT is more closely related to speech intelligibility than T20. However, even with the same EDT value, the details of the impulse response construction could greatly affect the subjective evaluation scores. The speech intelligibility could only be effectively guaranteed with carefully integrated architectural acoustic design and electrical acoustic design. When the service broadcast is in service, SN of 9dB could be used under good acoustic conditions and be increased as high as possible under bad acoustic conditions in a waiting hall. In case of emergency broadcast, SN should exceed 15 dB.The findings of the paper will serve as reference for acoustic design and standard formulation of oversize waiting halls of high-speed railway stations.