Sound propagation and speech transmission in a branching underground tunnel

The characteristics of sound propagation and speech transmission along a tu nnel with a "T" intersection were investigated. At receivers within sight o f the sound source, low frequencies were mainly attenuated around the inter section than high frequencies. At receivers out of sight of the source, hig h frequencies were extensively attenuated. The overall pattern of sound att enuation along the different sections of tunnel, which was calculated by th e conical beam method, agreed well with the measurements in this study. Num erical calculations of reflected and diffracted waves with minimum transmis sion paths in a two-dimensional plane showed that reflected waves were the primary contributors to sound fields out of sight Of the source. The articu lation scores measured at receivers within sight of the source were high, a nd most of the confusion concerned syllables that could easily be misheard, even if there were a high signal-to-noise ratio. The types of syllabic con fusions observed at the receivers out of sight of the source appeared to ha ve been caused by the greater deterioration in speech signals along this pa rt of the tunnel, especially at high frequencies. The evaluation by rapid s peech transmission indices (RASTI) appeared to be overestimated at the rece ivers out of sight of the source. Taking into account the early decay times of impulsive sound and the calculation procedures used in RASTI, it is con cluded that speech intelligibility may not have been evaluated correctly by RASTI. (C) 2000 Acoustical Society of America. [S0001-4966(00)02508-X].