  It seems now accepted that speech noise in open plan offices is the main
source of discomfort for employees. This work follows a series of studies
conducted at INRS France and INSA Lyon based on Hongisto's theoretical model
(2005) linking the Decrease in Performance (DP) and the Speech Transmission
Index (STI). This model predicts that for STI values between 0.7 and 1, which
means a speech signal close to 100% of intelligibility, the DP remains constant
at about 7%. The experiment that we carried out aimed to gather more
information about the relation between DP and STI, varying the STI value up to
0.9. Fifty-five subjects between 25-59 years old participated in the
experiment. First, some psychological parameters were observed in order to
better characterize the inter-subjects variability. Then, subjects performed a
Working-Memory (WM) task in silence and in four different sound conditions (STI
from 0.25 to 0.9). This task was customized by an initial measure of mnemonic
span so that two different cognitive loads (low/high) were equally defined for
each subject around their span value. Subjects also subjectively evaluated
their mental load and discomfort at the end of each WM task, for each noise
condition. Results show a significant effect of the STI on the DP, the mental
load and the discomfort. Furthermore, a significant correlation was found
between the age of subjects and their performance during the WM task. This
result was confirmed by a cluster analysis that enabled us to separate the
subjects on two different groups, one group of younger and more efficient
subjects and one group of older and less efficient subjects. General results
did not show any increase of DP for the highest STI values, so the "plateau"
hypothesis of Hongisto's model cannot be rejected on the basis of this
experiment.
