In normal speech, the vocal folds alternate between states of voice and voicelessness. In whispering, only the voicing changes, so that the vocal folds alternate between whisper and voicelessness (though the acoustic difference between the two states is minimal). Because of this circumstance, there is currently no known possibility to use speech recognition successfully on a whispering person, as the characteristic spectral range needed to detect syllables and words is not given through the total absence of tone.
There is no symbol in the IPA for whispered phonation, since it is not used phonemically in any language. However, a sub-dot under phonemically voiced segments is sometimes seen in the literature, as [ʃʊ̣ḍ] for whispered should.
Whispering is generally used quietly, to limit the hearing of speech to listeners who are nearby; for example, to convey secret information without being overheard or to avoid disturbing others in a quiet place such as a library or place of worship. Loud whispering, known as a stage whisper, is generally used only for dramatic or emphatic purposes. Whispering also takes less effort to vocalize than a normal speech pattern. This is because less air needs to be used to vocalize the sound. However, while it takes less effort to produce a whisper, it tires out the vocal folds more quickly.
A number of species of animals have been observed to whisper, particularly cotton-top tamarins, the barbastelle bat, and the female of the fish species, croaking gouramis. The reasons for animal whispering vary, and are not fully understood, but whispering among the tamarins appears to serve a social purpose, while the species of bats appears to whisper in order to evade detection by its particular prey, eared moths.