A question on area function of vocal tract

[xinhui zhou <zxinhui@xxxxxxxxxxxx>]

Dear List,

 I am having a question about area function of vocal tract,

 Area function is used to describe the vocal tract shape. It is
assumed that the bent shape of vocal tract can be regarded as a
straight tube with varying area along the axis,and only planar wave
propagation exists with limitation to certain frequency range.

I only undertand it intuitively .  Is there any book or paper which
formuates the idea of area function , i.e. why a bent 3D shape can be
simplied as a tube with area function?

Thanks in adavance,

Xinhui