To be sure that a custom earpiece for hearing protection shows optimal behaviour, one can perform a fast leak test.
A proper test provides certainty about a perfect fit of the earpiece. When there is a perfect fit, noise can only enter the ear canal through the noise filter and not outside to the earpiece.
For that purpose the room in the ear canal, between eardrum and otoplastic, is brought to a small overpressure of 5mB (0,073 psi), by means of air.