The sites where normal breath sounds originate aren’t clearly defined. For example, sounds heard over the trachea and large airways are characterized as high-pitched, harsh, loud, and tubular (hollow) with a long expiratory phase. These sounds are thought to reflect the turbulent airflow patterns in the first divisions of the large airways.

Sounds heard over other chest wall areas are softer and have a shorter expiratory phase. These sounds may also be produced by turbulent airflow in the first few divisions of the large airways; however, the fact that these sounds vary with changing airflow rates and distribution of ventilation in the lungs suggests that they are produced in the peripheral airways. Also, inspiratory sounds may be produced in the peripheral airways, and expiratory sounds may be produced in the larger airways. Breath sounds aren’t produced in the terminal bronchioles because of minimal airflow rates.

Disease processes that alter the airway or airflow dynamics produce abnormal breath sounds or adventitious sounds. These sounds are generated by the vibration of solid structures, by airflow through narrowed airways, or by abrupt changes in airway pressure.

The stability of an airway depends on the interaction of the elastic properties of the lung tissue, the intrapulmonary airways, and the pressures—both internal and external—exerted on the airways. During the respiratory cycle, a pressure gradient exists across the airway wall and between the trachea and alveoli. As the chest expands during inspiration, intrapleural pressure falls. At the same time, the lung’s elastic recoil increases, exerting traction on airway walls. This traction increases the airway’s diameter and decreases the resistance to airflow. The pressure gradient between the alveolar and atmospheric pressures increases, driving air through the airways toward the alveoli.

Intrapleural pressure and alveolar elastic recoil pressures drive expiratory airflow. At the beginning of expiration, intrapleural pressure and the alveolar recoil force cause the alveolar pressures to rise and push the air through the airways and out through the upper airway. Air pressures fall as the air flows through the airways from the alveoli to the upper airway. A point is reached, particularly during forceful expiration, when intrapleural and intrapulmonary pressures are equal. Called the equal pressure point, it can occur anywhere within the thorax and is a function of the mechanical properties of the lung (elastic recoil, airflow resistance, and stiffness of the walls of the airway). Downstream from this point, pressures outside of the airways can compress and further narrow the airways, causing increased resistance to airflow.

Variations in airflow patterns are affected by the intricate network of branching airways, the various airway diameters, and the potential for irregular airway wall surfaces within the tracheobronchial tree.

Breath sounds are also produced by mechanical vibrations of solid tissue. These vibrations travel through the different media of air, fluid, or tissue at varying speeds. Their frequency, intensity, and duration can be measured.