# AI-Based Classification of Pediatric Breath Sounds: Toward a Tool for Early Respiratory Screening
## 2 Breath Sound and Respiratory Diseases in Young Children  
Breath sounds are created in the large airways as a result of vibrations that are generated from air velocity and turbulence [3]. Breath sounds can be heard through a stethoscope and are regarded as one of the most significant bio-signals used to diagnose certain respiratory abnormalities and are widely used by physicians in their practice. Breath sounds are typically categorized into two groups: normal breath sounds, which occur in the absence of respiratory issues, and abnormal or adventitious sounds. Normal breath sounds include both inspiratory and expiratory sounds, which can be heard in the airways during inhalation and exhalation. When auscultating clear lungs--free from swelling, mucus, or blockages--breath sounds are smooth and soft. Normal lung sounds create a smooth, soft sound that can be heard when breathing in and out and are referred to as vesicular lung sounds; i.e., nothing is blocking the airways and they are not narrowed or swollen. Normal breath sounds, as shown in Figure 1, have the longest duration compared to abnormal breath sounds. They consist of two distinguishable phases: inhalation and exhalation. The highest peak in frequency occurs around 100 Hz, with additional dominant frequency groups at approximately 500 Hz, 1600 Hz, and 1800 Hz. In the spectrogram, both inhalation and exhalation are clearly recognizable, with frequencies around 100 Hz and 1800 Hz present throughout the entire duration. Abnormal breath sounds include crackles, wheezes, rhonchi, stridor, and pleural rub. Wheezes are continuous musical adventitious lung sounds, typically with a dominant frequency over 100 Hz and a duration exceeding 100 ms [42]. They are associated with partial airway obstruction, and their presence during breathing often reflects the severity of conditions such as children's nocturnal asthma. Wheezing in children is also used to assess asthma predisposition. Crackles, in contrast, are discontinuous and explosive sounds, usually occurring during inspiration and lasting less than 100 ms [42]. They feature a rapid initial pressure deflection followed by a short oscillation and are classified by duration: fine crackles last under 10 ms, while coarse crackles last over 10 ms [42]. Pneumonia is an inflammatory lung condition affecting the alveoli, with symptoms such as cough, chest pain, fever, and difficulty breathing. It is the leading cause of hospitalization for children in the United States [43]. Pediatric patients with pneumonia, malignancy, or other pulmonary conditions may exhibit a pleural rub in their breath sounds. Pleural rub is characterized by a coarse grating sound resulting from the pleural surfaces rubbing against each other. It can be heard during both inhalation and exhalation [44]. Pleural friction rubs are distinctive sounds akin to \"the sound made by walking on fresh snow\" when the pleural surfaces become rough or inflamed due to conditions like pleural effusion, pleurisy, or serositis [44]. From the sample shown in Figure 2, the dominant frequency of pneumonia-related breath sounds is approximately 2000 Hz, with harmonics around 4 kHz and 6 kHz. The duration of the sound is shorter than the normal breath sound in the time domain. There is no significant difference in amplitude between inhalation and exhalation. However, the harmonic power decreases between phases, particularly in the higher-order harmonics. Asthma is a chronic condition that causes inflammation and narrowing of the airways, leading to wheezing, chest tightness, and shortness of breath. It typically begins in child  
Figure 1: Normal breath sound analysis. (**upper**) Time-domain waveform. (**middle**) Frequency spectrum. (**lower**) Spectrogram. hood, affecting approximately 6% of children, and ranks as the third leading cause of death from respiratory diseases [45]. Wheezing, a high-pitched whistling sound produced during breathing, is commonly associated with asthma.