The croup is characterized by a high pitch, a musical sound like a wire tremor, a long duration, and a pronounced exhalation that largely disappears during inspiration. These pathological breath sounds can also occur in cases of obstructed ventilation caused by other diseases. In the case of bronchial foreign bodies and endobronchial tuberculosis, croup is often detected during auscultation. However, this croup is usually confined to one side of the lung or to one part of the lung where the obstruction exists, and is rarely widespread on both sides. Rales can also be present during cardiogenic asthma attacks, but they are present during both inspiration and expiration, and the prolongation of expiration time is much less pronounced than in bronchial asthma. The dry rales are characterized by continuity, duration >20ms, and low tone (frequency <200Hz =. The mechanism of their production is partial obstruction of the airway, where the lumen becomes smaller and the airflow through it causes airway vibration and acoustics. Dry rales may present a single tone or multiple tones. It can occur locally, confined to a certain part of the chest, or can be diffused throughout the lung lobes. The dry rales are mostly present during inspiration, and the movement of secretions and coughing often change, disappear or reproduce the nature of the dry rales. The croup is continuous, musical and high pitched (frequency >400Hz) for >20ms. The mechanism of occurrence is similar to that of dry rales, and the pitch of the sound is independent of the size of the original airway internal diameter. The occurrence of croup is determined by the mass of the airway (mass), the elasticity of the airway, the rate of airflow and the change in the internal diameter of the airway. Croup is characterized by a homogeneous monotone and a polytonal tone. It can be localized or heard in the whole lung. It is mostly seen in the expiratory phase, but can also occur in the inspiratory and expiratory-aspiratory phases. It is important to note that: (1) severe airway obstruction may have no croup or reduced croup; (2) the presence of croup in both expiratory and inspiratory phases indicates increased obstruction; (3) decreased croup tone and increased intensity indicate improved bronchospasm; (4) normal lungs can cause croup at the end of maximal expiration; (5) small airway obstruction may not have croup because the air flow through the small airway is too low to cause airway vibration; (6) croup is more likely to be heard in the trachea than in the lung fields; ⑦ abnormal sounds that change in character with coughing are usually caused by secretions. Dry rales and croup often occur in the following clear conditions: ① bronchospasm: asthma, COPD, pulmonary embolism, physico-chemical irritation (including aspiration), cystic pulmonary fibrosis, carcinoid syndrome; ② airway edema: bronchial asthma, COPD, infection caused by fine bronchitis, pulmonary edema, physico-chemical irritation, cystic pulmonary fibrosis; ③ dynamic compression: emphysema, bronchiectasis, cystic pulmonary fibrosis, airway malformation; ④ neoplasia inside and outside the trachea, inhaled foreign bodies; ⑤ pulmonary secretions: bronchitis (acute, chronic), pneumonia, asthma, bronchiectasis.