Anita Andersson, Anders Eriksson and Hartmut Traunmüller (1996) "Cries and whispers: Acoustic effects of variations in vocal effort." TMH-QPSR 2/1996: 127 - 130.
This is a preliminary report of an ongoing investigation examining the acoustic effects of the adjustment in vocal effort that is required when the distance between the speaker and the listener is varied over a large range (0.3 m - 187.5 m). Five kinds of characteristics have been studied in the speech of men and women: segment durations, F0, formant frequencies, sound pressure level, and spectral emphasis. When the distance increased, the following effects were observed: The durations of vowel like segments increased successively, as distinct from that of most consonants. The mean value of F0 increased by a factor close to two, while its SD in semitones remained roughly constant. The levels of voiced segments increased by about 30 dB, but less for unvoiced segments. The increase in level was much more pronounced in the upper than in the lower part of the spectrum.
There is also a more elaborated final report:
Hartmut Traunmüller and Anders Eriksson (2000) "Acoustic effects of variation in vocal effort by men, women, and children", J. Acoust Soc. Am. 107: 3438 - 3451. (Abstract).