John at Sinosplice discusses how speakers of Sinitic languages, which are tonal, can understand whispered speech, which is not tonal.

It turns out that when people whisper a tonal language such as Chinese, they naturally compensate for the lack of tones. How? According to one study:

1. the laryngeal sphincter mechanism is found to be a principal contributing physiological maneuver in the production of whisper, emphasizing the vertical rather than the horizontal component of the laryngeal source;
2. two special behavioral maneuvers are also used in whisper: male speakers tend to lengthen vocalic duration and female speakers tend to exaggerate the amplitude contours of Tone 3 and Tone 4;
3. these two special behavioral maneuvers and two temporal envelope parameters contribute to tone recognition in whisper, but the phonetic context is shown to be a distraction;
4. the environments of the target tones cause perceptual differneces, and the ranking of these environments in order of increasing degree of difficulty is: isolation, sentence-final, sentence-medial and sentence-final;
5. the ranking of the four tones in isolation, in order of increasing degree of perceptual difficulty is: Tone 3, Tone 4, Tone 1 and Tone 2.

further reading:

• Tone Deafness and Whispering Doesn’t Stop Tones, Sinosplice, August 22, 2006
• tonal languages and the tone deaf, Pinyin News, October 9, 2005
• Tones in Whispered Chinese: Articulatory Features and Perceptual Cues, by Man Gao