The present study aimed to investigate the effect of tonal noise and task difficulty on electroencephalography (EEG) and cognitive performance. Twelve healthy volunteers participated in the present study. Four noise signals were generated by four prominence tone levels (0, 2, 5 and 9) at background noise levels of 55 dBA and frequency of 500 Hz using the Test Tone Generator from Esser Audio (USA). The participants were asked to perform the tasks with low, moderate and high levels of difficulty while exposed to the noises in an acoustics laboratory. The values of reaction time, correct rate and missed numbers were recorded during each step. Moreover, the EEG signals were measured. The results showed that higher tone level and more task difficulty significantly decreased the correct rate, and increased the miss numbers. However, no significant effect was observed on reaction times. Furthermore, tone level and task difficulty significantly increased activity of the θ and β bands and decreased activity of the α band. Task difficulty and tone level could significantly affect the parameters of performance and the activity of EEG bands. Therefore, noise control can help sustain appropriate performance.