Vowel Displays as a Function of Time and Frequency

The examination of the distinguishable sound quality or timbre of vowel sounds is aided by two different types of displays of the sound. The display of the sound image as a function of time shows that the waveforms of the three vowel sounds are distinctly different. This is the most common type of display as captured by an oscilloscope or a computer display. The second type of display shows the sound image as a function of frequency. This is an example of Fourier analysis, and these displays that show the amount of signal associated with each harmonic of the sound signal are called fast Fourier transforms, or FFTs. The FFT provides a way to quantify the differences in the harmonic content of the vowel sounds, and provides some insight into how the ear distinguishes these vowel sounds. With the insight provided by the place theory of pitch perception, it can be seen that the human ear's analysis of sound quality bears some resemblance to an FFT.

These displays were created with MacScope2 by Elisha Huggins.

Vowel Displays as a Function of Time and Frequency

Thanks to Katherine Marks for her permission to use vowel sounds she produced to illustrate how the ear detects the difference.

The inner ear acts as a sophisticated sound analyzer, spreading out the energy content of the sound as a function of frequency. As shown here for three vowel sounds, the result is a distinguishable pattern of excitation on the basilar membrane of the cochlea. This permits the clear distinguishing of vocal sounds even if they have identical pitch and loudness.

The examination of the distinguishable sound quality or timbre of vowel sounds is aided by two different types of displays of the sound. The display of the sound image as a function of time shows that the waveforms of the three vowel sounds are distinctly different. This is the most common type of display as captured by an oscilloscope or a computer display.

The second type of display shows the sound image as a function of frequency. This is an example of Fourier analysis, and these displays that show the amount of signal associated with each harmonic of the sound signal are called fast Fourier transforms, or FFTs. The FFT provides a way to quantify the differences in the harmonic content of the vowel sounds, and provides some insight into how the ear distinguishes these vowel sounds. With the insight provided by the place theory of pitch perception, it can be seen that the human ear's analysis of sound quality bears some resemblance to an FFT.