Final answer:
Air displacement plethysmography is the method that measures body volume in the air, unlike other methods such as DEXA and BIA which use X-ray beams and electrical current, respectively.
Step-by-step explanation:
The method of body composition assessment that measures body volume in the air as opposed to underwater is known as air displacement plethysmography. The reference techniques such as underwater weighing involve submerging a subject in water and using Archimedes' principle to determine body density. However, air displacement plethysmography relies on a similar principle but in a different medium; it measures the amount of air displaced by the person's body in a chamber, known as the Bod Pod, to calculate body volume and, subsequently, body composition. Unlike dual-energy X-ray absorptiometry (DEXA), which uses X-ray beams to determine the body's composition, or bioelectrical impedance analysis (BIA), which measures the resistance to a small electrical current passed through the body, air displacement plethysmography directly measures volume to calculate body density. Ultrasound is not typically used for assessing overall body composition in terms of body fat percentage and body volume.