The illustration of the harmonic content of three vowels is just a single example and may not be typical - it just serves to illustrate that there are measurable differences in harmonic content. These differences can be detected by the ear as differences in timbre.

The data was taken with a MacScope and an inexpensive microphone. The harmonic amplitudes were squared to make them proportional to sound intensity and then the base 10 logarithm was taken to more nearly represent the ear's response to that sound intensity.

The place theory of pitch perception provides some indication of how the ear distinguishes the difference between vowel sounds. Since different frequencies excite the basilar membrane of the inner ear at different locations, this model would indicate that the illustrated vowel sounds would indeed produce distinctly different patterns of excitation in the inner ear. The ear is seen to be a sound analyzer which is capable of detecting very subtle differences in sound timbre.

Different vowel sounds, different patterns of excitation in the inner ear.

According to the place theory for pitch perception, different frequency sounds excite the basilar membrane of the inner ear in different places. The high frequencies selectively excite the hair cells near the entry point at the oval window and the low frequencies produce maximum response near the smaller end of the tapering membrane. Since the harmonic content of the different vowel sounds is different, the patterns of excitation of the receptor cells are different.

Harmonic Spectrum Example

Intensity of harmonics as a function of frequency for the same vowel sound produced at different pitches:

The "AH" sounds produced at different pitches have similar spectra. These spectra are influenced by the cavity resonances of the oral and nasal cavities which are determined by the articulation process which forms the vowel.

Harmonic Spectrum Example

Intensity of harmonics as a function of frequency for different vowel sounds produced at the same pitch:

Different vowel sounds have distinctly different harmonic spectra. These spectra are influenced by the cavity resonances of the oral and nasal cavities which are determined by the articulation process which forms the vowels.

A given vowel sound has a recognizable timbre, regardless of its pitch.

Different vowel sounds have distinctly different timbres, even at the same pitch.