Motivation from lossless tubes

The transfer function of a lossless tube can be described by an all pole model. This is also a reasonable approximation to speech formed by the excitation of the vocal tract by glottal pulses (although the glottal pulses are not spectrally flat).

  
Figure 37: The lossless tube model of speech production

But:

• The vocal tract is not built of cylinders
• The vocal tract is not lossless
• The vocal tract has a side passage (the nasal cavity)
• fricatives (e.g. /s/ and /sh/) are generated near the lips

Nevertheless, with sufficient parameters the LP model can make a reasonable approximation to the spectral envelope for all speech sounds.