Why can adenoids cause hyponasal speech?

Hyponasal speech, or closed nasality, occurs when there is a decrease in nasal resonance, which muffles the three nasal resonants: M, N, and Ng.1

Large adenoids cause this condition by physically obstructing the airway in the nasopharynx. Because the adenoids are located at the back wall of the nasal pharynx, their enlargement—often due to infection or inflammation—blocks the passage of air through the nose. When the nasal airway is obstructed, the sound waves that typically resonate in the nasal cavity during the production of nasal consonants are restricted, resulting in the characteristic muffled quality of hyponasal speech.21

It is important to note that while enlarged adenoids are a common organic cause of hyponasal speech, they should not be removed solely based on the complaint of "talking through the nose" without a thorough evaluation. In cases where a patient also has an incompetent palate, removing the adenoids may alleviate the closed nasality but can worsen an underlying open nasality (hypernasal speech), leading to a significant deterioration in overall speech quality.1