When the attenuation of a single hearing protection device is no longer enough to ensure the hearing protection of its wearer.
Remember that hearing protection devices, as well as double hearing protection, are only a recommended solution, as a last resort when all other means to reduce noise at the source have been exhausted.
We will see how to determine the sound level at which it becomes necessary to use double hearing protection.
We will also see how to calculate the attenuation provided by this double protection and which hearing protection device is predominant in the calculation, the attenuation of the two combined systems being much lower than the sum of the combined attenuation.