The isolation of KTX-I trains against pressure variations during tunnel passages was insufficient for some passengers, leading to efforts to reinforce pressurization in newer generations of trains. Pressure variations have been known to cause passengers to experience ringing in their ears; the ventilation systems on the passenger cabins are sealed when the train enters a tunnel in order to reduce the pressure changes. Pressure variations were not the only train cabin-associated complaint; KTX passengers were also known to have been negatively affected by inconsistent speeds of the trains.