The appended transformations in speaker age and sex have been obtained by analyzing a natural utterance using linear predictive coding and by re-synthesis after recalculation of the parameter values descriptive of the speech signal . The recalculations of F0 and the formant frequencies were based on the values listed in , Table 2, except for an error in this table: The value of kF0 in the transformation from women to girl, 12-14 years, should be 1.03 (instead of 1.27). Speech rate has also been modified . Q-values have been conserved.
Age rating experiments with speech manipulated in this way  show some bias towards the original age of the speaker. This can be attributed to the conserved 'verbal maturity' in the transformed versions.
In order to
a noise source with the right spectrum has to be substituted for the buzz source, and the formant frequencies have to be increased .
It is also possible to modify the speaker in sex.
Not quite convincing? With a better knowledge of the female - male differences in the acoustic properties of speech, it could probably be done in a more convincing way.
Now you may listen to a table conversation by a synthetic Swedish family.
 H. Traunmüller, P. Branderud and A. Bigestans (1989) "Paralinguistic speech signal transformations" PERILUS X: 47–64 (Dept. of Linguistics, Stockholm University).
 H. Traunmüller (1988) "Paralinguistic variation and invariance in the characteristic frequencies of vowels", Phonetica 45: 1–29. doi
 G.J.T. Haselager, I.H. Slis and A.C.M. Rietveld (1991) "An alternative method of studying the development of speech rate" Clinical Linguistics and Phonetics 5: 53-63.
 H. Traunmüller and R. van Bezooijen (1994) "The auditory perception of children's age and sex", in Proceedings ICSLP-94, vol. 3 : 1171-1174. The Acoustical Society of Japan. Abstract | scanned
 I. Eklund and H. Traunmüller (1997) "Comparative study of male and female whispered and phonated versions of the long vowels of Swedish." Phonetica 54: 1-21. doi