Use of the speech transmission Index for the assessment of sound annoyance in open-plan offices

In open-plan offices, employees often have to work despite being distracted by multiple sources of speech close to them. This disturbance is even more severe when the surrounding conversations are intelligible. For this reason the disruptive power of such babble-like noises can be
quantified by measuring speech intelligibility. As such, the Speech Transmission Index is usually used to assess the quality of open-plan offices. In this article, the STI is used to evaluate the nuisance potential of sound environments in an open-plan office. 57 subjects were confronted with a serial memory task in four STI conditions (from 0.25 to 0.65). Noise annoyance was assessed objectively by measuring performance, and subjectively using the NASA-TLX questionnaire for measuring cognitive load. A very strong inter-individual variability appeared in performance measurements. Approximately half of the panel was made of high performing subjects, who proved to be insensitive to speech intelligibility. For the other half, performance was reduced when STI increased, as shown in previous studies, with a plateau for STI greater than 0.45. This strong inter-individual variability can explain the discrepancy between results observed in previously published studies. The NASA-TLX questionnaire proved to give useful complementary results. For example, high-performing subjects related a higher workload in the condition of maximum speech intelligibility.