Correct, low frequency range only. MGB, certain engine noise ancilliaries are reduced thus making for a quieter working environment and a little way to supporting duty of care in the work place. However the baddy frequencies -engine whine, certain harmonics -these are what cause actual damage to the pilots hearing range (mid range I believe) and one can do very little about it at the mo'. I am told by the manufacturers that it is quite a technical feat trying to conquer the high freq problems