I understand that formants represent the resonances of the vocal tract and that the frequency of a formant is determined by the shape and size of the vocal tract.

If we speed up the speech waveform, that would be comparable to shortening the vocal tract. As I understand it,the sound itself would stay the same, let's say, an /a/ stays an /a/ but it would sound like it was spoken by a person with a shorter vocal tract, i.e. the frequency of the sound will increase as the higher frequencies are boosted due to the shorter resonance body.

Consider the following problem. We have the fricative /sh/. When we speed up the waveform, there is more energy in the higher formant frequencies of the sound and /sh/ is perceived as /s/. So, why is it the case that /a/ stays /a/ and /sh/ turns into /s/. Is this just an exception to the rule, or am I confusing something here?

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First of all, if you compare the [ʃ] of an adult male with the [ʃ] of a 6-year-old child, you will see that the energy will be lower for the adult male--i.e. the energy does shift a bit with vocal tract/head size. However, you are correct in observing that speeding up the waveform by too great a factor will result in a sound that is likely to be perceived as [s].

The key to understanding why shifting the vowel formants and shifting the fricative noise energy seems to result in different perceptual effects lies in the fact that the noise source of the [ʃ] is created right towards the front of the mouth, while the voicing source for vowels is created way down at the other end of the vocal tract. So, voicing gets filtered through a lot more tract than the [ʃ] noise, which just gets filtered through the front of the mouth. Shifting the vowel formants upward is like shortening the overall tract (making the vowels sound like they are being produced by a smaller head). But shifting the fricative noise upward is shortening the part of the vocal tract that is in front of the point of articulation. This could be interpreted as maintaining the same vocal tract size but moving the point articulation forward (and perhaps unrounding the lips), which results in the perception of a more fronted (and unrounded) fricative.

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