Laryngeal sounds reflect turbulence of air in the upper airway and are harsher and higher in pitch than normal lung sounds. A linear relationship between tracheal sounds and flow has been observed, and the frequency characteristics are associated with body height and length of the trachea in children.
Stridor is characterised by a high-pitched musical sound, and its presence suggests significant obstruction of the large airway. Inspiratory stridor indicates an obstruction in the extrathoracic airway, and obstruction in the intrathoracic airway results in an expiratory or biphasic sound.
Acute and chronic stridor indicate a range of underlying pathologies, some of which have been characterised further using digital sound analysis. Many studies are descriptive rather than quantitative, and there is little specific research that uses prospective and formal outcome measures before and after interventions. Acoustic analysis may have useful applications in treatment monitoring, surgical planning, voice quality analysis after laryngeal reconstruction, speech development, voice training, therapy and rehabilitation, and these potential applications require further evaluation.
The gold standard diagnostic tool for upper airway lesions in children is upper airway endoscopy, often requiring general anaesthesia. Paradoxical vocal cord dysfunction is a form of inducible laryngeal obstruction which can co-exist with asthma and is currently diagnosed using laryngoscopy during exercise. The use of acoustic analysis to study the relationship between the acoustic characteristics of stridor and localisation of the obstructive lesion has the potential to reduce the need for invasive procedures; however normative data are needed in order to understand the significance of changes in acoustic parameters that would indicate any specific pathology.
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