Irrelevant speech noise is one of the main sources of noise that negatively affect health, well-being, comfort and performance in densely occupied environments. The use of devices that provide lighting feedback based on noise levels generated by occupants could promote occupants change in their behaviour and therefore an improvement of acoustic conditions. In the present work, a long-term monitoring campaign was performed with the S3EM (Speech and Sound SEMaphore) device, that monitors noise levels and visualises their variation through an integrated coloured lighting feedback (i.e., red, yellow and green) based on the results of an advanced algorithm. Six S3EM devices were used in two Intesa Sanpaolo headquarters offices in Milan (Italy) for a total of 4 weeks. In the first and fourth weeks the lighting feedback was off, in the second and third weeks it was on. This procedure allows to evaluate the lighting feedback effect on occupants. Before the in-field campaign, the S3EM devices were calibrated and the correct settings involved in the algorithm were defined for their optimization to the sonic environment profile. The S3EM monitoring was integrated with the ethnographic observation method to describe interactions of occupants with the task, the work environment and with the S3EM.