Open-plan offices are the most common office layout in tertiary sector. Despite a noise level below the regulations (<60 dBA), occupants of these offices complain about noise. Among the various noise sources, co-worker’s voices and conversations seem to be the most annoying one, as employee surveys have shown. Moreover, not all employees working in open-plan offices are young people with normal hearing. They can be older (up to 60 yrs.) and have any level of hearing loss. So, the purpose of this study is to investigate the effects of mild hearing loss (onset of presbycusis) on performance in an open-plan office – particularly under the influence of the irrelevant speech (the Irrelevant Speech Effect). An analysis of the decrease in performance in the accomplishment of a cognitive task (a serial recall test) regarding the speech intelligibility level was performed with young normal-hearing participants under two hearing-conditions: with and without a hearing loss simulator, as well as with elderly hearing-impaired participants. During the task, the participants were exposed to five noise-conditions, with five different levels of intelligibility (ranging from STI 0.35 to 0.75), and silence. Afterwards, a subjective intelligibility measurement was performed to compare the signals’ intelligibility level for each hearing-condition.