Normal breath sounds are broadly defined as breathing heard through the chest wall of a healthy person. They include tracheal, bronchial, vesicular, and bronchovesicular sounds, as well as sounds heard at the mouth. Three variables affect the normal sounds heard during auscultation: the distance between the source of the sound and the chest wall, the path of sound transmission, and the sound’s location. Normal breath sounds are produced by airflow patterns, by associated pressure changes within the airways, and by solid tissue vibrations.

Normal breath sounds heard over large airways are produced by turbulent airflow in those airways. They’re loudest when auscultated over the trachea on the anterior chest wall surface next to the sternum. As air travels through the airways beyond the segmental bronchi, these normally high-pitched sounds become diminished because they’re filtered by the chest wall, pleurae, and air-filled lung tissue. The normal breath sounds heard over most of the chest wall surface are soft and low-pitched; they’re softer and shorter during expiration than during inspiration.