Crackles can be characterized as abnormal, discontinuous, "rattling" sounds heard best during inspiration in lungs with respiratory disease. Crackles can be either fine or coarse. Fine crackles are soft, high-pitched, and brief while coarse crackles are louder, more low-pitched, and longer in duration than fine crackles.
Fine crackles are usually caused by interstitial lung disease, such as pulmonary fibrosis and congestive heart failure. Coarse crackles are caused by airway disease, such as COPD and bronchitis. The excess secretions in the airways cause the small airways and alveoli in the lungs to collapse and then "pop" open during inspiration, making the sound of "crackles."