Acoustic imaging of the respiratory system demonstrates regional changes of lung sounds that correspond to pulmonary ventilation. We investigated volume-dependent variations of lung sound phase and amplitude between two closely spaced sensors in five adults. Lung sounds were recorded at the posterior right upper, right lower, and left lower lobes during targeted breathing (1.2 +/- 0.2 l/s; volume = 20-50 and 50-80% of vital capacity) and passive sound transmission (< or =0.2 l/s; volumes as above). Average sound amplitudes were obtained after band-pass filtering to 75-150, 150-300, and 300-600 Hz. Cross correlation established the phase relation of sound between sensors. Volume-dependent variations in phase (< or =1.5 ms) and amplitude (< or =11 dB) were observed at the lower lobes in the 150- to 300-Hz band. During inspiration, increasing delay and amplitude of sound at the caudal relative to the cranial sensor were also observed during passive transmission in several subjects. This previously unrecognized behavior of lung sounds over short distances might reflect spatial variations of airways and diaphragms during breathing.