Because it blocks the nasal cavity, which is essential for the sound vibrations produced during humming.
How It Works
Humming requires lowering the soft palate (velum) to direct airflow and sound through the nasal cavity, creating a buzzing resonance around 250 Hz from the cavity's natural frequencies.
Pinching the nostrils seals the nares,
preventing this airflow and resonance, so no sound escapes.
Vocal Science Context
This demonstrates nasal resonance, where the nasal tract boosts specific harmonics but also creates anti-resonances that filter certain frequencies.
It's why nasal sounds like "m," "n," or "ng" naturally engage this pathway, but oral sounds do not.