Irrelevant speech sound level effects on sustained attention with electrodermal and electroencephalogram monitoring

Speech is a dominant distractor in open-plan offices, yet quantitative evidence on sound-level effects using objective indicators remains limited. This study investigated how the sound pressure level of irrelevant speech influences sustained attention performance and psychophysiological responses in a simulated open-plan office. Twenty-four participants completed the Sustained Attention to Response Task under four acoustic conditions: a quiet control (35–40 dB(A)) and irrelevant speech presented at 40–45, 45–50, and 50–55 dB(A). Behavioural performance was characterised by correct rate, reaction capacity, and a composite performance index, while electrodermal activity and prefrontal electroencephalogram (EEG) were recorded (valid EEG n = 16). Linear mixed-effects models showed no significant main effect of speech level on mean behavioural outcomes, and reaction capacity did not exhibit a significant time-on-task by level interaction. In contrast, the relative change in skin conductance level differed significantly across conditions, with higher speech (50–55 dB(A)) producing greater arousal than lower speech (40–45 dB(A)) (planned contrast p = 0.014, dz = 0.54). Mean EEG task engagement did not differ significantly between sound levels. However, task engagement increased over time only under 50–55 dB(A) (p = 0.002), and the time-by-level interaction approached significance (p = 0.054). The findings suggest that within common open-plan office sound-level ranges, increasing irrelevant speech level may impose measurable physiological costs and increasing engagement demands even when sustained attention performance is maintained, supporting the need for acoustic control of speech levels in open-plan office design and management.