Accounting for 'real world' factors
It is very likely that under conditions of real use, hearing protectors will give lower protection than predicted by manufacturers' data which is obtained from standardised tests. The difference between manufacturers' data and 'real-world' protection is due to factors such as poor fitting and wearing of spectacles or other personal protective equipment. You should account for this 'real world' protection by 'derating' the protector by 4 dB. This means, having followed one of the standardised methods for calculating the attenuation provided by a hearing protection device, assume that the sound level at the ear when hearing protection is worn will be 4 dB higher than would be predicted by the method. In this way you will get a better indication of the protection that users are most likely to get, and can select appropriate hearing protection accordingly.
The derating does not apply to the assessment of hearing protector performance against peak noise.
The 4 dB derating described above is regarded as an appropriate factor to bridge the gap between manufacturers' data and real-world factors, without introducing further complexity to the prediction of hearing protector performance. You will still be able to select an appropriate hearing protection device for the character of the noise, and hearing protectors that show better repeatability in standardised tests will still be distinguishable.
The use of a derating factor will not necessarily mean that you will need to select a protector with a higher rating than one you currently use. By following all the guidance in Part 5 of L108 you should be able to ensure that even if wearers get better protection than that predicted by these methods, the protected level at the ear will be within the recommended range.
You may wish to demonstrate by means other than relying on manufacturers' data and the methods outlined in this book that the hearing protection you supply meets the requirement to reduce the noise level at the ear to the appropriate level, eg by measurement of noise levels underneath hearing protection devices in conditions of real use.