Lung sounds normally peak at frequencies below 100 Hz, with a sharp drop of sound energy occurring between 100 and 200 Hz, but it can still be detected at or above 800 Hz with sensitive microphones. Therefore, turbulence generated during expiration moves away from the chest wall and become fainter. The inspiratory component originates in the lobar and segmental airways, whereas the expiratory component arises from more central airways.

The origin of both phases of respiration is also indifferent sites. The short expiratory phase is due to the passive nature of expiration resulting in generation of less turbulent airflow. The expiratory sound is audible only in the early phase. However, breath sounds cannot be generated at the alveolar level since airflow is laminar within the alveoli. Vesicular breath sound is a misnomer as vesicles means alveoli, and this gives the impression that the breath sound is originating at the alveolar level. Turbulency occurs when Reynaud's number exceeds 2,000. Whether airflow becomes laminar or turbulent depends on Reynaud's number. Turbulency produces noise as the air molecules collide with each other and with the airway wall. Inhalation of a lighter gas mixture for example, helium reduces the turbulent flow and makes laminar flow more likely. It depends on the density of air more than viscosity. Turbulent flow is disorganized and chaotic in nature. Unlike laminar flow, it does not have high axial flow velocity. Turbulent flow occurs when high velocity of flow passes through a large diameter airway, especially through an airway with irregular walls, for example, the trachea and bronchi or in the airway with sudden branching. Small airways (<2 mm) are not the site of breath sound production as flow here is laminar in nature, and therefore silent. Laminar flow is directly proportional to the driving pressure. Where Q is the volume flow rate, P is the driving pressure, r the radius, n the viscosity, and l depicts length. We will discuss the various types of breath sound, adventitious sounds, and vocal resonance and their clinical importance and pathogenesis. Auscultation of the lungs includes breath sounds-its character and intensity, vocal resonance, and adventitious sounds. Modern stethoscope had undergone several modifications before being molded into the current shape. Initially he used rolled paper cone, and later on a wooden tube. Laënnec published his seminal work in 1819 in his masterpiece, “A Treatise on the Diseases of the Chest”. However, with the invention of stethoscope by Rene Theophile Hyac in the Laënnec in 1816 the art of auscultation not only became popular worldwide, but also comfortable for patients and physicians. He described this as a method of direct auscultation. It was Hippocrates who began the concept of auscultation by applying ear to the patient's chest to hear transmitted breath sounds and called this procedure as “immediate auscultation”. Technologic advancement has led to erosion in the bedside teaching due to overreliance on laboratory testing therefore, the clinical relevance of auscultation has receded significantly in recent years. History taking and a detailed physical examination, including the time-honored sequence of inspection, palpation, percussion, and auscultation should be considered an essential part of clinical examination, even in 21 st century with explosive advancement in technology related to health sciences. The auscultation of the respiratory system is an inexpensive, noninvasive, safe, easy-to-perform, and one of the oldest diagnostic techniques used by the physicians to diagnose various pulmonary diseases.

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