**2.4 Pulmonary auscultation: sounds in healthy and diseased lungs**

Lung sounds occur because of air turbulence in the larger airways [15, 20]. They are the results of pulmonary vibrations and the respective airways transmitted to the thoracic wall. Sounds that occur during natural breathing differ depending on where they are acquired as well as the moment of the ventilatory cycle [20]. So, controlling where to place wearable devices and their sampling frequency and duration allows obtaining significant data from lung sounds.

Normal pulmonary sounds are classified into:


The anatomical structures may influence the sound heard during normal breathing [21].

Pathological changes in the lungs directly affect the perception of lung sounds from the airways to the thoracic surface. Abnormal lung sounds, also called adventitious noises, are classified into:


There are other sounds and more details about each of them, and wearable systems contribute to distinguishing the different sounds in clinical practice.

The concepts presented in this section are very important for understanding the respiratory system in healthy and unhealthy conditions. Depending on the event one aims to observe, this information helps to identify the best location for sensor placement. It also contributes to a better interpretation of the respiratory signals obtained.

After this brief overview of the main concepts involving respiratory anatomy and physiology, the next section explains how wearable devices for respiratory monitoring have been made.
