The normal standard used when measuring impulse noise is called MIL-STD-1474D.
The measurement is done with the microphone placed 160cm above ground and at a distance of 1.0m at a 90 degree angle from the barrel muzzle. A .308Win without a silencer is normally measured at 168dB (Peak SPL). A silencer is only reducing the noise from the exploding gases by reducing the pressure sequentially through expansion in a number of chambers.
A bullet in supersonic speed creates a supersonic “bang” at approx. 140dB on 1 meter distance throughout the whole trajectory. This supersonic “bang” cannot be reduced!
The working environmental law limits stated by the EU has a peak noise level of impulse noise of 137dB which is not supposed to create permanent hearing damage. Measurement shall be done at the shooters left ear for a right hand shooter.
Example of how you can re-calculate to MIL-STD-1474D when measuring at the shooters left ear:
The noise level is approx. 8dB lower at the shooters ear when firing thanks to the shooter being behind the rifle. For a .308Win this means 160dB non-reduced at ear (168dB – 8dB) and if we reduce the noise on that .308Win with a TRITON No.5 we will measure 130dB at the ear. This gives us a noise reduction of approx. 30dB (168dB – 8dB – 130dB