**4. Passive control at the receivers**

It refers to actions aimed at improving acoustic insulation in recipients' homes, through constructive solutions, materials with better acoustic performance, windows sealers, special openings, or with double glass. When none of these options seems to be a solution, the use of personal protection equipment (PPE) is the last option. PPE are not only used by workers: many people who are very sensitive to noise have to wear these devices to sleep, or at least to fall asleep.

Even though, there is another way to improve sound quality for the recipients: it consists of masking the undesirable signal. Even if it is closer to an active method for noise control—it consists of adding acoustic energy to the system—it may not be considered as an ANC technique. Masking is a natural human auditive function: some signals can lift the hearing threshold levels at certain frequencies. With basis on this concept, the common idea of masking solutions attempts to mask high frequencies for making speech incomprehensible to adjacent rooms, for example, between a waiting room and a doctor's office, or between a meeting room and a reception area. In all these cases, the most common technique is music masking. Other options and applications are available as well, for example, the use of white noise or a water fountain sound for enhancing acoustic quality to the receivers [40], which has been studied for a long time to make soundscapes healthier [41].
