Frequency is the number of complete repetitions (cycles) of variations in air pressure occurring in a second. The unit of frequency measurement is in Hertz, usually abbreviated as Hz. Each opening and closing of the vocal cords causes a peak of air pressure in the sound wave, and thus the fundamental frequency, so-called pitch, of a sound is estimated by observing the rate of occurrence of the peaks in the waveform. This cyclic pattern occurs at an average of 125 times per second in the average adult male, and approximately twice as fast (250 Hz) in the adult female. Normally, all the accent or intonation curves can be represented with the pitch[14].
However, estimating the pitch of voiced sounds has been described as the hardy perennial among speech-processing problems, so there exist several estimation methods. We have extracted fundamental frequency patterns with a method of correlation coefficients in connection with the polarity of the speech signal. The pitch of the sentence, as described in the previous subsection, is plotted at the second line of Figure 2.3, which signifies the sentence intonation. As the speech is characterized as voiced with the indicated pitch frequency, a pitch estimation is also used to indicate the presence or absence of phonation.
