How is the NRR calculated for dual hearing protection?

When calculating the Noise Reduction Rating (NRR) for dual hearing protection, it is necessary to consider both the effectiveness of each hearing protection device to ensure an accurate representation of the noise reduction in a given environment. The method of calculation involves subtracting 7 dB from the highest NRR and then adding 5 dB from the second hearing protection device.

This approach recognizes that wearing two hearing protection devices does not provide a straightforward additive effect. The reasoning is based on empirical data which indicate that the overall effectiveness can be modeled as such to reflect the diminishing returns of additional protection. By subtracting 7 dB from the highest NRR, we account for the initial level of protection provided by the better-performing device. Then, adding 5 dB from the secondary device accounts for the additional but limited benefit that it provides, thereby yielding a more accurate total NRR.

This method ensures that the resulting value still represents an achievable level of noise reduction while recognizing the practical limitations of using dual hearing protection in a workplace setting.