Models with vocal-tract losses

Rabiner and Schafer [368] give an extensive review of this issue and summarise that there are three main ways in which losses can occur. First, the passage of the wave will cause the walls of the vocal tract (e.g. the cheeks) to vibrate. This can be modelled by refinements to the transfer function. The effect is to raise the formant centre frequencies slightly, dampen the formants and especially the lower formants we should expect this as it the mass of the vocal tract walls will prevent motion at the higher frequencies. The effects of friction and thermal conduction can also be modelled by adding resistive terms to the transfer function. The overall effect of these is the opposite of above the formant centre frequencies are somewhat lowered, and the effects of this are more pronounced at higher frequencies. 

Apart from the overall contribution to damping, these effects tend to cancel each other out, so the overall contribution of these resistive terms is quite small. Since the degree 

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The second problem concerns just how accurate our model of articulation should be. As we saw in our discussion on tube models, there is always a balance between the desire to mimic the phenomenon accurately and with being able to do so with a simple and tractable model. The earliest models, were more or less those described in Chapter 11, but since then a wide number of improvements have been made many along the lines described in Section 11.5. These have included modelling vocal tract losses, source-filter interaction, radiation from the lips, and of course improved glottal source characteristics REFS. In addition many of these have attempted to be models of both the vocal tract itself and the controls within it, such that many have models for muscle movement and motor control. 

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