Humming
Wordless singing with a closed mouth, producing a muffled, nasal tone.
What it means
Humming refers to wordless singing with the mouth closed, called a bocca chiusa in Italian. The tone is directed through the nose, sounding muffled, soft, and slightly nasal, and is noticeably quieter and more closed in timbre than open vowel singing.
Musical use
As a compositional device, humming by a choir or soloist creates a distant, floating, or instrument-like sound texture without the text coming to the fore. It is well known from the Humming Chorus in Giacomo Puccini's opera Madama Butterfly (premiered in 1904), where the closed choral sound underscores a quiet night scene.
Humming should be distinguished from vocalise, in which open vowels are sung instead of a closed mouth, producing a fuller, brighter tone.