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].