Where there is an excess of air in the chest cavity, as in emphysema or in pneumothorax, the percussion sound becomes abnormally loud and clear.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
In pneumothorax of either of the pleural sacs the air compresses the lung, pushes the heart from its normal position, and the space which the air occupies in the pleura yields a clear hollow sound on percussion, whilst, by the ear or stethoscope applied to a corresponding part of the thoracic walls, we discover the absence of the respiratory murmur.
"Surgical Anatomy"
Joseph Maclise