That's odd. dB are additive (or subtractive) in other gain and insertion loss networks. What is the explanation for the exception here? Could it be that the result is measured at the eardrum where the sound coming through the hearing protection is added to sound getting in through alternate paths; i.e., through the bones of the skull? That would certainly set a lower limit below which anything over the ears could not improve suppression further.Rocky Raab said:. . . The NRR or protection level is NOT additive. . .