Prediction of the Effectiveness of a Sound-Masking System in an Open-Plan Office Including the Lombard Effect

Sound masking can improve speech privacy in rooms by increasing background-noise levels that mask distracting speech sounds. The Lombard Effect indicates that an increase in background-noise level can increase talker voice levels, reducing speech privacy and the benefit of a sound-masking system. To investigate this, a model of an existing open-plan office was created in CATT-Acoustics and validated. The model was used to predict speech-transmission index (STI) and the effectiveness of a sound-masking system, without and with the Lombard Effect, described by the existing Lombard Voice Model. Predictions were made for ambient-noise levels of 30, 40 and 45 dBA, at various distances from a primary talker, and for 0–4 secondary talkers. With 30-dBA ambient noise, STIs at 1 and 4 m varied with the number of talkers from 0.67–0.91 and 0.23–0.62 without the sound-masking system, from 0.58–0.70 and 0.13–0.25 with it but ignoring the Lombard Effect, and from 0.64–0.73 and 0.18–0.27 with the Lombard Effect. The Lombard Effect reduced the benefit of the sound-masking system by 14–67% and 5–50%. With higher ambient noise, the system is less effective; the Lombard Effect can cancel its benefit, resulting in increased STI (decreased privacy) with the system operating.