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Single Formant Vowel with Changing Pitch
This sound illustrates one of the sounds used in the study of Cariani and Delgutte (1996) on the coding of pitch in auditory nerve fibers. It is a so-called single-formant vowel, since its spectral envelope has a single peak in frequency (vowels have multiple such 'formants' - see Chapter 4). See Fig. 3-9 in the book.
Here are two consecutive periods, one pair taken from the beginning (green) and one pair from the middle (orange) of the sound:
The blue arrows span one period of the sound (you can see how similar are the two consecutive periods). In the middle of the sound, the periods are about half as long as in the beginning of the sound.
The gray bars indicate one cycle of the 'fine structure' of each period, which is determined by the formant frequency. In contrast with the pitch periods, they are essentially equal to each other.
Thus, the sound has a pitch that changes over about one octave (it is twice as high in the middle as in the beginning and end of the sound), but its formant frequency remains fixed. In consequence, its timbre remains the same throughout its duration.