Weak breath sounds are generally caused by two factors: 1) the lung disease itself leads to a decrease in the amount of gas entering the alveoli, resulting in a decrease in airflow through the airways, and 2) problems in the chest wall or chest cavity that affect the conduction of gas and sound, resulting in a weakening of the breath sounds. Commonly, the breath sounds are weakened after the alveoli are filled or collapsed, most commonly caused by atelectasis, for example. In addition, problems in the thoracic cavity, such as pleural effusion, can also be caused. In the elderly, the breath sounds can be weakened bilaterally due to old age and respiratory weakness, which is related to the decrease of respiratory dynamics.