What Causes Dullness To Percussion?

What Causes Dullness To Percussion? Dullness replaces resonance when fluid or solid tissue replaces air-containing lung tissues, such as occurs with pneumonia, pleural effusions, or tumors. Hyperresonant sounds that are louder and lower pitched than resonant sounds are normally heard when percussing the chests of children and very thin adults. What causes muffled lung sounds?