IntroductionMonitoring cognitive workload in individuals that are performing safety-critical jobs has huge potential, for example in aviation [1]. The relationship between cognitive workload and performance has been well studied [2] and the connection between cognitive workload and physical health has also been highlighted [3]. Clearly, it is crucial to manage cognitive workload in the modern day work environment, both in terms of performance and health. Speech processing offers the ability to monitor cognitive workload in a non-intrusive way, compared to measures such as blood pressure, heart rate, electroencephalogram or electrocardiogram. Measuring voice is easy and recent work has shown very promising results in classifying cognitive workload levels using the speech signal [4][5][6]. Successful design and implementation of such a method would provide a powerful tool to developers of cognitive infocommunications systems [7].The main objective of this work is to provide an independent verification of whether there is a link between increased cognitive workload and changes in the speech signal and, if this link exists, to characterize what part of voice is mostly affected. To do this we conducted a cognitive workload experiment where the set tasks were primarily solved using speech from 98 participants. Each participant read a standard passage of text on a computer screen and solved Stroop tasks with three difficulty levels [8]. A separate classifier of the three difficulty levels was trained for each speaker based on two sets of voice parameters comprising of the vocal tract (VT) and voice source (VS) features respectively. The conclusion is that task load does affect the speech signal and that vocal tract parameters are a better indicator of task level than voice source parameters. The context of the work is presented in Section 2 and the speech processing approach is described in Section 3. The experimental methodology and detailed results are given in Section 4 and 5. The results are summarized in Section 6 and the discussion is continued in Section 7.