Robert G. Loudon and Raymond L. H. Murphy

Our knowledge of lung sounds is based largely on empirical observation. Laënnec (1) knew that the sounds heard by applying the ear directly to the chest provided useful information, and he was delighted when he found that a device intended to preserve decorum unexpectedly presented better information. With enormous industry, and with access to a large population of pulmonary patients both clinically and in the autopsy room, he took full advantage of his invention, the stethoscope. He compared what he heard at the bedside with what he subsequently saw in the morgue, and his teaching and writing popularized auscultation. His descriptions of the sounds were by simile (e.g., “comme le cri d’un petit oiseau”), and his understanding of their meaning was based on comparison of auscultatory sounds with gross pathology, and intuition based on analogies with familiar sound-producing mechanisms. Over the next century, isolated efforts at understanding mechanisms of sound production in the respiratory tract were reported. Some of these described ingenious experiments (2 4); some applied known principles of acoustics to the respiratory tract (5,6). Developments in pulmonary physiology allowed the interpretation of lung sounds to be couched in physiological as well as anatomical terms. Forgacs (7—9) led this movement, adding clinical physiological measurements to careful stethoscopic observations. Graphic methods for displaying sound signals were applied to lung sounds by several workers (10,11). Technical advances in recording and analysis of sound signals and the wide availability and appeal of computers have led to an upsurge of interest in lung sounds and their interpretation (12,13). Clinical applications of our better under