plethysmograph (n.)

(from Greek plithismos ‘enlargement’) A device used in ARTICULATORY PHONETICS to measure lung volume; also called a body plethysmograph. The subject is placed in a sealed container with a single mouthpiece which is closed off following a normal expiration of breath. When the subject breathes in, the resulting changes in pressure within the lungs and within the container are calculated. The difference between full and empty lungs can be an important indicator of disease affecting the flow of air in the VOCAL TRACT.