For the UK the MoT will test the car against rated factory settings.
Not Quite....
From May 2018
Noise suppression system
The Tester will inspect:
exhaust silencers
under-bonnet noise deadening material fitted as original equipment.
Exhaust noise will be assessed during the emissions test for the vehicle or by revving the engine to around 2,500rpm or half the maximum engine speed if this is lower on vehicles not subject to an emissions test
Exhaust noise from the vehicle must not be unreasonably above the noise level normally expected from a similar vehicle with a standard silencer in average condition.
No mention of using a sound pressure meter.
Also, to become a MOT Tester you do not need a hearing test.
