Sound Moderators reduce the sound of a firearm enough to make it safe. Often, traditional hearing protection is still recommended in addition to a Sound Moderator.
This Study by the Health & Safety Laboratory in the UK published in 2004 looked at the noise levels encountered at various positions around both moderated and un-moderated firearms and made recommendations on hearing protection to stay safe.
Interestingly, the study found that while using un-moderated firearms, traditional hearing protectors which were predicted to provide adequate protection according to standardized methods, were found to be inadequate as they did not always reduce the peak exposure below 140dB.
It also found that Sound Moderators do NOT give any reduction to the noise of a bullet in flight and therefore gave little to no noise reduction forward of the firearm.
Read the whole study below: