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Page 599
Cochleagrams
The cochlea is the tiny snaillike organ in the inner ear that maps incoming vibrations into nerve impulses that are transmitted to the brain (see chapter 23). Each place along the length of the cochlea responds to vibrations broadly tuned around a center frequency specific to that place. Auditory scientists have measured the average firing rate of the neurons along the length of the cochlea and have determined that they are related to different frequencies perceived by the ear.
A software model of the cochlea's response to incoming signals is called a cochleagram (Slaney and Lyon 1992). Rather than mapping frequency to the vertical axis, like the spectrogram, the cochleagram maps cochlear place to the vertical axis. That is, it represents the response of different parts of the cochlea to incoming sound. When the cochleagram is plotted at a coarse resolution it looks something like a sonogram representation, but with an enhanced representation of onsets. A more important difference between the sonogram and the cochleagram can be seen in figure 13.31. This zoomed-in view on a high-resolution cochleagram image reveals the timing of the individual glottal pulses of a speech signal. Thus the cochleagram provides a way to study both low-level timing (onsets) and spectrum.
0599-01.GIF
Figure 13.31
Expanded cochleagram of the American dipthong "ree." The
horizontal lines indicate the first three formant tracks. The
vertical lines indicate glottal pulses, which are tilted slightly
due to the natural delay through the cochlea.
(After Slaney and Lyon 1992.)

 
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